diff --git a/paging/common/src/main/java/android/arch/paging/BoundedDataSource.java b/paging/common/src/main/java/android/arch/paging/BoundedDataSource.java
deleted file mode 100644
index 0656490..0000000
--- a/paging/common/src/main/java/android/arch/paging/BoundedDataSource.java
+++ /dev/null
@@ -1,80 +0,0 @@
-/*
- * Copyright (C) 2017 The Android Open Source Project
- *
- * Licensed under the Apache License, Version 2.0 (the "License");
- * you may not use this file except in compliance with the License.
- * You may obtain a copy of the License at
- *
- *      http://www.apache.org/licenses/LICENSE-2.0
- *
- * Unless required by applicable law or agreed to in writing, software
- * distributed under the License is distributed on an "AS IS" BASIS,
- * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
- * See the License for the specific language governing permissions and
- * limitations under the License.
- */
-
-package android.arch.paging;
-
-import android.support.annotation.Nullable;
-import android.support.annotation.RestrictTo;
-import android.support.annotation.WorkerThread;
-
-import java.util.ArrayList;
-import java.util.List;
-
-/**
- * Simplest data source form that provides all of its data through a single loadRange() method.
- * <p>
- * Requires that your data resides in positions <code>0</code> through <code>N</code>, where
- * <code>N</code> is the value returned from {@link #countItems()}. You must return the exact number
- * requested, so that the data as returned can be safely prepended/appended to what has already
- * been loaded.
- * <p>
- * For more flexibility in how many items to load, or to avoid counting your data source, override
- * {@link PositionalDataSource} directly.
- *
- * @param <Value> Value type returned by the data source.
- *
- * @hide
- */
-@RestrictTo(RestrictTo.Scope.LIBRARY_GROUP)
-public abstract class BoundedDataSource<Value> extends PositionalDataSource<Value> {
-    /**
-     * Called to load items at from the specified position range.
-     *
-     * @param startPosition Index of first item to load.
-     * @param loadCount     Exact number of items to load. Returning a different number will cause
-     *                      an exception to be thrown.
-     * @return List of loaded items. Null if the BoundedDataSource is no longer valid, and should
-     *         not be queried again.
-     */
-    @WorkerThread
-    @Nullable
-    public abstract List<Value> loadRange(int startPosition, int loadCount);
-
-    @WorkerThread
-    @Nullable
-    @Override
-    public List<Value> loadAfter(int startIndex, int pageSize) {
-        return loadRange(startIndex, pageSize);
-    }
-
-    @WorkerThread
-    @Nullable
-    @Override
-    public List<Value> loadBefore(int startIndex, int pageSize) {
-        if (startIndex < 0) {
-            return new ArrayList<>();
-        }
-        int loadSize = Math.min(pageSize, startIndex + 1);
-        startIndex = startIndex - loadSize + 1;
-        List<Value> result = loadRange(startIndex, loadSize);
-        if (result != null) {
-            if (result.size() != loadSize) {
-                throw new IllegalStateException("invalid number of items returned.");
-            }
-        }
-        return result;
-    }
-}
diff --git a/paging/common/src/main/java/android/arch/paging/ContiguousDataSource.java b/paging/common/src/main/java/android/arch/paging/ContiguousDataSource.java
index 414c4ff..03f2e86 100644
--- a/paging/common/src/main/java/android/arch/paging/ContiguousDataSource.java
+++ b/paging/common/src/main/java/android/arch/paging/ContiguousDataSource.java
@@ -19,44 +19,21 @@
 import android.support.annotation.NonNull;
 import android.support.annotation.Nullable;
 
-import java.util.List;
-
 abstract class ContiguousDataSource<Key, Value> extends DataSource<Key, Value> {
     @Override
     boolean isContiguous() {
         return true;
     }
 
-    abstract void loadInitial(Key key, int initialLoadSize, boolean enablePlaceholders,
-            @NonNull PageResult.Receiver<Key, Value> receiver);
+    public abstract void loadInitial(@Nullable Key key, int initialLoadSize,
+            boolean enablePlaceholders,
+            @NonNull InitialLoadCallback<Value> callback);
 
-    void loadAfter(int currentEndIndex, @NonNull Value currentEndItem, int pageSize,
-            @NonNull PageResult.Receiver<Key, Value> receiver) {
-        if (!isInvalid()) {
-            List<Value> list = loadAfterImpl(currentEndIndex, currentEndItem, pageSize);
+    abstract void loadAfter(int currentEndIndex, @NonNull Value currentEndItem, int pageSize,
+            @NonNull LoadCallback<Value> callback);
 
-            if (list != null && !isInvalid()) {
-                receiver.postOnPageResult(new PageResult<>(
-                        PageResult.APPEND, new Page<Key, Value>(list), 0, 0, 0));
-                return;
-            }
-        }
-        receiver.postOnPageResult(new PageResult<Key, Value>(PageResult.APPEND));
-    }
-
-    void loadBefore(int currentBeginIndex, @NonNull Value currentBeginItem, int pageSize,
-            @NonNull PageResult.Receiver<Key, Value> receiver) {
-        if (!isInvalid()) {
-            List<Value> list = loadBeforeImpl(currentBeginIndex, currentBeginItem, pageSize);
-
-            if (list != null && !isInvalid()) {
-                receiver.postOnPageResult(new PageResult<>(
-                        PageResult.PREPEND, new Page<Key, Value>(list), 0, 0, 0));
-                return;
-            }
-        }
-        receiver.postOnPageResult(new PageResult<Key, Value>(PageResult.PREPEND));
-    }
+    abstract void loadBefore(int currentBeginIndex, @NonNull Value currentBeginItem, int pageSize,
+            @NonNull LoadCallback<Value> callback);
 
     /**
      * Get the key from either the position, or item, or null if position/item invalid.
@@ -65,12 +42,4 @@
      * that isn't yet loaded, a fallback item (last loaded item accessed) will be passed.
      */
     abstract Key getKey(int position, Value item);
-
-    @Nullable
-    abstract List<Value> loadAfterImpl(int currentEndIndex,
-            @NonNull Value currentEndItem, int pageSize);
-
-    @Nullable
-    abstract List<Value> loadBeforeImpl(int currentBeginIndex,
-            @NonNull Value currentBeginItem, int pageSize);
 }
diff --git a/paging/common/src/main/java/android/arch/paging/ContiguousPagedList.java b/paging/common/src/main/java/android/arch/paging/ContiguousPagedList.java
index cdff391..bd1bb37 100644
--- a/paging/common/src/main/java/android/arch/paging/ContiguousPagedList.java
+++ b/paging/common/src/main/java/android/arch/paging/ContiguousPagedList.java
@@ -21,6 +21,7 @@
 import android.support.annotation.NonNull;
 import android.support.annotation.Nullable;
 
+import java.util.List;
 import java.util.concurrent.Executor;
 
 class ContiguousPagedList<K, V> extends PagedList<V> implements PagedStorage.Callback {
@@ -31,28 +32,13 @@
     private int mPrependItemsRequested = 0;
     private int mAppendItemsRequested = 0;
 
-    @SuppressWarnings("unchecked")
-    private final PagedStorage<K, V> mKeyedStorage = (PagedStorage<K, V>) mStorage;
-
-    private final PageResult.Receiver<K, V> mReceiver = new PageResult.Receiver<K, V>() {
-        @AnyThread
-        @Override
-        public void postOnPageResult(@NonNull final PageResult<K, V> pageResult) {
-            // NOTE: if we're already on main thread, this can delay page receive by a frame
-            mMainThreadExecutor.execute(new Runnable() {
-                @Override
-                public void run() {
-                    onPageResult(pageResult);
-                }
-            });
-        }
-
+    private PageResult.Receiver<V> mReceiver = new PageResult.Receiver<V>() {
         // Creation thread for initial synchronous load, otherwise main thread
         // Safe to access main thread only state - no other thread has reference during construction
         @AnyThread
         @Override
-        public void onPageResult(@NonNull PageResult<K, V> pageResult) {
-            if (pageResult.page == null) {
+        public void onPageResult(int type, @NonNull PageResult<V> pageResult) {
+            if (pageResult.isInvalid()) {
                 detach();
                 return;
             }
@@ -62,25 +48,30 @@
                 return;
             }
 
-            Page<K, V> page = pageResult.page;
-            if (pageResult.type == PageResult.INIT) {
-                mKeyedStorage.init(pageResult.leadingNulls, page, pageResult.trailingNulls,
+            List<V> page = pageResult.page;
+            if (type == PageResult.INIT) {
+                mStorage.init(pageResult.leadingNulls, page, pageResult.trailingNulls,
                         pageResult.positionOffset, ContiguousPagedList.this);
-                notifyInserted(0, mKeyedStorage.size());
-            } else if (pageResult.type == PageResult.APPEND) {
-                mKeyedStorage.appendPage(page, ContiguousPagedList.this);
-            } else if (pageResult.type == PageResult.PREPEND) {
-                mKeyedStorage.prependPage(page, ContiguousPagedList.this);
+
+                // notifyInserted is safe here, since if we're not on main thread, we won't have any
+                // Callbacks registered to listen to the notify.
+                notifyInserted(0, mStorage.size());
+
+                mLastLoad = pageResult.leadingNulls + pageResult.positionOffset + page.size() / 2;
+            } else if (type == PageResult.APPEND) {
+                mStorage.appendPage(page, ContiguousPagedList.this);
+            } else if (type == PageResult.PREPEND) {
+                mStorage.prependPage(page, ContiguousPagedList.this);
             }
 
             if (mBoundaryCallback != null) {
                 boolean deferEmpty = mStorage.size() == 0;
                 boolean deferBegin = !deferEmpty
-                        && pageResult.type == PageResult.PREPEND
-                        && pageResult.page.items.size() == 0;
+                        && type == PageResult.PREPEND
+                        && pageResult.page.size() == 0;
                 boolean deferEnd = !deferEmpty
-                        && pageResult.type == PageResult.APPEND
-                        && pageResult.page.items.size() == 0;
+                        && type == PageResult.APPEND
+                        && pageResult.page.size() == 0;
                 deferBoundaryCallbacks(deferEmpty, deferBegin, deferEnd);
             }
         }
@@ -93,24 +84,32 @@
             @Nullable BoundaryCallback<V> boundaryCallback,
             @NonNull Config config,
             final @Nullable K key) {
-        super(new PagedStorage<K, V>(), mainThreadExecutor, backgroundThreadExecutor,
+        super(new PagedStorage<V>(), mainThreadExecutor, backgroundThreadExecutor,
                 boundaryCallback, config);
         mDataSource = dataSource;
 
-        // blocking init just triggers the initial load on the construction thread -
-        // Could still be posted with callback, if desired.
-        mDataSource.loadInitial(key,
-                mConfig.initialLoadSizeHint,
-                mConfig.enablePlaceholders,
-                mReceiver);
+        if (mDataSource.isInvalid()) {
+            detach();
+        } else {
+            DataSource.InitialLoadCallback<V> callback = new DataSource.InitialLoadCallback<>(
+                    mConfig.enablePlaceholders, mDataSource, mReceiver);
+            mDataSource.loadInitial(key,
+                    mConfig.initialLoadSizeHint,
+                    mConfig.enablePlaceholders,
+                    callback);
+
+            // If initialLoad's callback is not called within the body, we force any following calls
+            // to post to the UI thread. This constructor may be run on a background thread, but
+            // after constructor, mutation must happen on UI thread.
+            callback.setPostExecutor(mMainThreadExecutor);
+        }
     }
 
     @MainThread
     @Override
     void dispatchUpdatesSinceSnapshot(
             @NonNull PagedList<V> pagedListSnapshot, @NonNull Callback callback) {
-
-        final PagedStorage<?, V> snapshot = pagedListSnapshot.mStorage;
+        final PagedStorage<V> snapshot = pagedListSnapshot.mStorage;
 
         final int newlyAppended = mStorage.getNumberAppended() - snapshot.getNumberAppended();
         final int newlyPrepended = mStorage.getNumberPrepended() - snapshot.getNumberPrepended();
@@ -120,7 +119,8 @@
 
         // Validate that the snapshot looks like a previous version of this list - if it's not,
         // we can't be sure we'll dispatch callbacks safely
-        if (newlyAppended < 0
+        if (snapshot.isEmpty()
+                || newlyAppended < 0
                 || newlyPrepended < 0
                 || mStorage.getTrailingNullCount() != Math.max(previousTrailing - newlyAppended, 0)
                 || mStorage.getLeadingNullCount() != Math.max(previousLeading - newlyPrepended, 0)
@@ -190,7 +190,14 @@
                 if (isDetached()) {
                     return;
                 }
-                mDataSource.loadBefore(position, item, mConfig.pageSize, mReceiver);
+                if (mDataSource.isInvalid()) {
+                    detach();
+                } else {
+                    DataSource.LoadCallback<V> callback = new DataSource.LoadCallback<>(
+                            PageResult.PREPEND, mMainThreadExecutor, mDataSource, mReceiver);
+                    mDataSource.loadBefore(position, item, mConfig.pageSize, callback);
+                }
+
             }
         });
     }
@@ -213,7 +220,13 @@
                 if (isDetached()) {
                     return;
                 }
-                mDataSource.loadAfter(position, item, mConfig.pageSize, mReceiver);
+                if (mDataSource.isInvalid()) {
+                    detach();
+                } else {
+                    DataSource.LoadCallback<V> callback = new DataSource.LoadCallback<>(
+                            PageResult.APPEND, mMainThreadExecutor, mDataSource, mReceiver);
+                    mDataSource.loadAfter(position, item, mConfig.pageSize, callback);
+                }
             }
         });
     }
diff --git a/paging/common/src/main/java/android/arch/paging/DataSource.java b/paging/common/src/main/java/android/arch/paging/DataSource.java
index ff44521..994661a 100644
--- a/paging/common/src/main/java/android/arch/paging/DataSource.java
+++ b/paging/common/src/main/java/android/arch/paging/DataSource.java
@@ -20,26 +20,68 @@
 import android.support.annotation.NonNull;
 import android.support.annotation.WorkerThread;
 
+import java.util.List;
 import java.util.concurrent.CopyOnWriteArrayList;
+import java.util.concurrent.Executor;
 import java.util.concurrent.atomic.AtomicBoolean;
 
 /**
- * Base class for incremental data loading, used in list paging. To implement, extend either the
- * {@link KeyedDataSource}, or {@link TiledDataSource} subclass.
+ * Base class for loading pages of snapshot data into a {@link PagedList}.
  * <p>
- * Choose based on whether each load operation is based on the position of the data in the list.
+ * DataSource is queried to load pages of content into a {@link PagedList}. A PagedList can grow as
+ * it loads more data, but the data loaded cannot be updated.
  * <p>
- * Use {@link KeyedDataSource} if you need to use data from item <code>N-1</code> to load item
- * <code>N</code>. For example, if requesting the backend for the next comments in the list
+ * A PagedList / DataSource pair serve as a snapshot of the data set being loaded. If the
+ * underlying data set is modified, a new PagedList / DataSource pair must be created to represent
+ * the new data.
+ * <h4>Loading Pages</h4>
+ * PagedList queries data from its DataSource in response to loading hints. {@link PagedListAdapter}
+ * calls {@link PagedList#loadAround(int)} to load content as the user scrolls in a RecyclerView.
+ * <p>
+ * To control how and when a PagedList queries data from its DataSource, see
+ * {@link PagedList.Config}. The Config object defines things like load sizes and prefetch distance.
+ * <h4>Updating Paged Data</h4>
+ * A PagedList / DataSource pair are a snapshot of the data set. A new pair of
+ * PagedList / DataSource must be created if an update occurs, such as a reorder, insert, delete, or
+ * content update occurs. A DataSource must detect that it cannot continue loading its
+ * snapshot (for instance, when Database query notices a table being invalidated), and call
+ * {@link #invalidate()}. Then a new PagedList / DataSource pair would be created to load data from
+ * the new state of the Database query.
+ * <p>
+ * To page in data that doesn't update, you can create a single DataSource, and pass it to a single
+ * PagedList. For example, loading from network when the network's paging API doesn't provide
+ * updates.
+ * <p>
+ * To page in data from a source that does provide updates, you can create a
+ * {@link DataSource.Factory}, where each DataSource created is invalidated when an update to the
+ * data set occurs that makes the current snapshot invalid. For example, when paging a query from
+ * the Database, and the table being queried inserts or removes items. You can also use a
+ * DataSource.Factory to provide multiple versions of network-paged lists. If reloading all content
+ * (e.g. in response to an action like swipe-to-refresh) is required to get a new version of data,
+ * you can connect an explicit refresh signal to call {@link #invalidate()} on the current
+ * DataSource.
+ * <p>
+ * If you have more granular update signals, such as a network API signaling an update to a single
+ * item in the list, it's recommended to load data from network into memory. Then present that
+ * data to the PagedList via a DataSource that wraps an in-memory snapshot. Each time the in-memory
+ * copy changes, invalidate the previous DataSource, and a new one wrapping the new state of the
+ * snapshot can be created.
+ * <h4>Implementing a DataSource</h4>
+ * To implement, extend either the {@link KeyedDataSource}, or {@link PositionalDataSource}
+ * subclass. Choose based on whether each load operation is based on the position of the data in the
+ * list.
+ * <p>
+ * Use {@link KeyedDataSource} if you need to use data from item {@code N-1} to load item
+ * {@code N}. For example, if requesting the backend for the next comments in the list
  * requires the ID or timestamp of the most recent loaded comment, or if querying the next users
  * from a name-sorted database query requires the name and unique ID of the previous.
  * <p>
- * Use {@link TiledDataSource} if you can load arbitrary pages based solely on position information,
- * and can provide a fixed item count. TiledDataSource supports querying pages at arbitrary
- * positions, so can provide data to PagedLists in arbitrary order.
+ * Use {@link PositionalDataSource} if you can load arbitrary pages based solely on position
+ * information, and can provide a fixed item count. PositionalDataSource supports querying pages at
+ * arbitrary positions, so can provide data to PagedLists in arbitrary order.
  * <p>
- * Because a <code>null</code> item indicates a placeholder in {@link PagedList}, DataSource may not
- * return <code>null</code> items in lists that it loads. This is so that users of the PagedList
+ * Because a {@code null} item indicates a placeholder in {@link PagedList}, DataSource may not
+ * return {@code null} items in lists that it loads. This is so that users of the PagedList
  * can differentiate unloaded placeholder items from content that has been paged in.
  *
  * @param <Key> Input used to trigger initial load from the DataSource. Often an Integer position.
@@ -47,8 +89,36 @@
  */
 @SuppressWarnings("unused") // suppress warning to remove Key/Value, needed for subclass type safety
 public abstract class DataSource<Key, Value> {
-
+    /**
+     * Factory for DataSources.
+     * <p>
+     * Data-loading systems of an application or library can implement this interface to allow
+     * {@code LiveData<PagedList>}s to be created. For example, Room can provide a
+     * DataSource.Factory for a given SQL query:
+     *
+     * <pre>
+     * {@literal @}Dao
+     * interface UserDao {
+     *    {@literal @}Query("SELECT * FROM user ORDER BY lastName ASC")
+     *    public abstract DataSource.Factory&lt;Integer, User> usersByLastName();
+     * }
+     * </pre>
+     * In the above sample, {@code Integer} is used because it is the {@code Key} type of
+     * PositionalDataSource. Currently, Room uses the {@code LIMIT}/{@code OFFSET} SQL keywords to
+     * page a large query with a PositionalDataSource.
+     *
+     * @param <Key> Key identifying items in DataSource.
+     * @param <Value> Type of items in the list loaded by the DataSources.
+     */
     public interface Factory<Key, Value> {
+        /**
+         * Create a DataSource.
+         * <p>
+         * The DataSource should invalidate itself if the snapshot is no longer valid, and a new
+         * DataSource should be queried from the Factory.
+         *
+         * @return the new DataSource.
+         */
         DataSource<Key, Value> create();
     }
 
@@ -58,19 +128,168 @@
     }
 
     /**
-     * If returned by countItems(), indicates an undefined number of items are provided by the data
-     * source. Continued querying in either direction may continue to produce more data.
-     */
-    @SuppressWarnings("WeakerAccess")
-    public static int COUNT_UNDEFINED = -1;
-
-    /**
      * Returns true if the data source guaranteed to produce a contiguous set of items,
      * never producing gaps.
      */
     abstract boolean isContiguous();
 
     /**
+     * Callback for DataSource initial loading methods to return data and position/count
+     * information.
+     * <p>
+     * A callback can be called only once, and will throw if called again.
+     * <p>
+     * It is always valid for a DataSource loading method that takes a callback to stash the
+     * callback and call it later. This enables DataSources to be fully asynchronous, and to handle
+     * temporary, recoverable error states (such as a network error that can be retried).
+     *
+     * @param <T> Type of items being loaded.
+     */
+    public static class InitialLoadCallback<T> extends LoadCallback<T> {
+        InitialLoadCallback(boolean acceptCount,
+                DataSource dataSource, PageResult.Receiver<T> receiver) {
+            super(PageResult.INIT, acceptCount, dataSource, receiver);
+        }
+
+        /**
+         * Called to pass initial load state from a DataSource.
+         * <p>
+         * Call this method from your DataSource's {@code loadInitial} function to return data,
+         * and inform how many placeholders should be shown before and after. If counting is cheap
+         * to compute (for example, if a network load returns the information regardless), it's
+         * recommended to pass data back through this method.
+         *
+         * @param data List of items loaded from the DataSource. If this is empty, the DataSource
+         *             is treated as empty, and no further loads will occur.
+         * @param position Position of the item at the front of the list, relative to the total
+         *                 count. If there are {@code N} items before the items in data that can be
+         *                 loaded from this DataSource, pass {@code N}.
+         * @param totalCount Total number of items that may be returned from this DataSource.
+         *                   Includes the number in the initial {@code data} parameter
+         *                   as well as any items that can be loaded in front or behind of
+         *                   {@code data}.
+         */
+        public void onResult(@NonNull List<T> data, int position, int totalCount) {
+            int trailingUnloadedCount = totalCount - position - data.size();
+            if (mAcceptCount) {
+                dispatchResultToReceiver(new PageResult<>(
+                        data, position, trailingUnloadedCount, 0));
+            } else {
+                dispatchResultToReceiver(new PageResult<>(data, position));
+            }
+        }
+
+        /**
+         * Called to pass initial load state from a DataSource without supporting placeholders.
+         * <p>
+         * Call this method from your DataSource's {@code loadInitial} function to return data,
+         * if position is known but total size is not. If counting is not expensive, consider
+         * calling the three parameter variant: {@link #onResult(List, int, int)}.
+         *
+         * @param data List of items loaded from the DataSource. If this is empty, the DataSource
+         *             is treated as empty, and no further loads will occur.
+         * @param position Position of the item at the front of the list. If there are {@code N}
+         *                 items before the items in data that can be provided by this DataSource,
+         *                 pass {@code N}.
+         */
+        void onResult(@NonNull List<T> data, int position) {
+            // not counting, don't need to check mAcceptCount
+            dispatchResultToReceiver(new PageResult<>(
+                    data, 0, 0, position));
+        }
+    }
+
+    /**
+     * Callback for DataSource loading methods to return data.
+     * <p>
+     * A callback can be called only once, and will throw if called again.
+     * <p>
+     * It is always valid for a DataSource loading method that takes a callback to stash the
+     * callback and call it later. This enables DataSources to be fully asynchronous, and to handle
+     * temporary, recoverable error states (such as a network error that can be retried).
+     *
+     * @param <T> Type of items being loaded.
+     */
+    public static class LoadCallback<T> {
+        final boolean mAcceptCount;
+        final int mType;
+        private final DataSource mDataSource;
+        private final PageResult.Receiver<T> mReceiver;
+
+        private int mPositionOffset = 0;
+
+        // mSignalLock protects mPostExecutor, and mHasSignalled
+        private final Object mSignalLock = new Object();
+        private Executor mPostExecutor = null;
+        private boolean mHasSignalled = false;
+
+        private LoadCallback(int type, boolean acceptCount,
+                DataSource dataSource, PageResult.Receiver<T> receiver) {
+            mType = type;
+            mAcceptCount = acceptCount;
+            mDataSource = dataSource;
+            mReceiver = receiver;
+        }
+
+        LoadCallback(int type, Executor mainThreadExecutor,
+                DataSource dataSource, PageResult.Receiver<T> receiver) {
+            mType = type;
+            mAcceptCount = false;
+            mPostExecutor = mainThreadExecutor;
+            mDataSource = dataSource;
+            mReceiver = receiver;
+        }
+
+        void setPositionOffset(int positionOffset) {
+            mPositionOffset = positionOffset;
+        }
+
+        void setPostExecutor(Executor postExecutor) {
+            synchronized (mSignalLock) {
+                mPostExecutor = postExecutor;
+            }
+        }
+
+        /**
+         * Called to pass loaded data from a DataSource.
+         * <p>
+         * Call this method from your DataSource's {@code load} methods to return data.
+         *
+         * @param data List of items loaded from the DataSource.
+         */
+        public void onResult(@NonNull List<T> data) {
+            dispatchResultToReceiver(new PageResult<>(
+                    data, 0, 0, mPositionOffset));
+        }
+
+        void dispatchResultToReceiver(final @NonNull PageResult<T> result) {
+            Executor executor;
+            synchronized (mSignalLock) {
+                if (mHasSignalled) {
+                    throw new IllegalStateException(
+                            "LoadCallback already dispatched, cannot dispatch again.");
+                }
+                mHasSignalled = true;
+                executor = mPostExecutor;
+            }
+
+            final PageResult<T> resolvedResult =
+                    mDataSource.isInvalid() ? PageResult.<T>getInvalidResult() : result;
+
+            if (executor != null) {
+                executor.execute(new Runnable() {
+                    @Override
+                    public void run() {
+                        mReceiver.onPageResult(mType, result);
+                    }
+                });
+            } else {
+                mReceiver.onPageResult(mType, result);
+            }
+        }
+    }
+
+    /**
      * Invalidation callback for DataSource.
      * <p>
      * Used to signal when a DataSource a data source has become invalid, and that a new data source
diff --git a/paging/common/src/main/java/android/arch/paging/KeyedDataSource.java b/paging/common/src/main/java/android/arch/paging/KeyedDataSource.java
index 3214a4e..b6656f3 100644
--- a/paging/common/src/main/java/android/arch/paging/KeyedDataSource.java
+++ b/paging/common/src/main/java/android/arch/paging/KeyedDataSource.java
@@ -1,5 +1,5 @@
 /*
- * Copyright (C) 2017 The Android Open Source Project
+ * Copyright 2017 The Android Open Source Project
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -16,223 +16,127 @@
 
 package android.arch.paging;
 
-import android.support.annotation.AnyThread;
 import android.support.annotation.NonNull;
 import android.support.annotation.Nullable;
-import android.support.annotation.WorkerThread;
 
-import java.util.ArrayList;
-import java.util.Collections;
 import java.util.List;
-
 /**
  * Incremental data loader for paging keyed content, where loaded content uses previously loaded
  * items as input to future loads.
  * <p>
- * Implement a DataSource using KeyedDataSource if you need to use data from item <code>N-1</code>
- * to load item <code>N</code>. This is common, for example, in sorted database queries where
+ * Implement a DataSource using KeyedDataSource if you need to use data from item {@code N - 1}
+ * to load item {@code N}. This is common, for example, in sorted database queries where
  * attributes of the item such just before the next query define how to execute it.
- * <p>
- * A compute usage pattern with Room SQL queries would look like this (though note, Room plans to
- * provide generation of much of this code in the future):
- * <pre>
- * {@literal @}Dao
- * interface UserDao {
- *     {@literal @}Query("SELECT * from user ORDER BY name DESC LIMIT :limit")
- *     public abstract List&lt;User> userNameInitial(int limit);
- *
- *     {@literal @}Query("SELECT * from user WHERE name &lt; :key ORDER BY name DESC LIMIT :limit")
- *     public abstract List&lt;User> userNameLoadAfter(String key, int limit);
- *
- *     {@literal @}Query("SELECT * from user WHERE name > :key ORDER BY name ASC LIMIT :limit")
- *     public abstract List&lt;User> userNameLoadBefore(String key, int limit);
- * }
- *
- * public class KeyedUserQueryDataSource extends KeyedDataSource&lt;String, User> {
- *     private MyDatabase mDb;
- *     private final UserDao mUserDao;
- *     {@literal @}SuppressWarnings("FieldCanBeLocal")
- *     private final InvalidationTracker.Observer mObserver;
- *
- *     public KeyedUserQueryDataSource(MyDatabase db) {
- *         mDb = db;
- *         mUserDao = db.getUserDao();
- *         mObserver = new InvalidationTracker.Observer("user") {
- *             {@literal @}Override
- *             public void onInvalidated({@literal @}NonNull Set&lt;String> tables) {
- *                 // the user table has been invalidated, invalidate the DataSource
- *                 invalidate();
- *             }
- *         };
- *         db.getInvalidationTracker().addWeakObserver(mObserver);
- *     }
- *
- *     {@literal @}Override
- *     public boolean isInvalid() {
- *         mDb.getInvalidationTracker().refreshVersionsSync();
- *         return super.isInvalid();
- *     }
- *
- *     {@literal @}Override
- *     public String getKey({@literal @}NonNull User item) {
- *         return item.getName();
- *     }
- *
- *     {@literal @}Override
- *     public List&lt;User> loadInitial(int pageSize) {
- *         return mUserDao.userNameInitial(pageSize);
- *     }
- *
- *     {@literal @}Override
- *     public List&lt;User> loadBefore({@literal @}NonNull String userName, int pageSize) {
- *         // Return items adjacent to 'userName' in reverse order
- *         // it's valid to return a different-sized list of items than pageSize, if it's easier
- *         return mUserDao.userNameLoadBefore(userName, pageSize);
- *     }
- *
- *     {@literal @}Override
- *     public List&lt;User> loadAfter({@literal @}Nullable String userName, int pageSize) {
- *         // Return items adjacent to 'userName'
- *         // it's valid to return a different-sized list of items than pageSize, if it's easier
- *         return mUserDao.userNameLoadAfter(userName, pageSize);
- *     }
- * }</pre>
  *
  * @param <Key> Type of data used to query Value types out of the DataSource.
  * @param <Value> Type of items being loaded by the DataSource.
  */
 public abstract class KeyedDataSource<Key, Value> extends ContiguousDataSource<Key, Value> {
-
-    @Nullable
     @Override
-    List<Value> loadAfterImpl(int currentEndIndex, @NonNull Value currentEndItem, int pageSize) {
-        return loadAfter(getKey(currentEndItem), pageSize);
+    final void loadAfter(int currentEndIndex, @NonNull Value currentEndItem, int pageSize,
+            @NonNull LoadCallback<Value> callback) {
+        loadAfter(getKey(currentEndItem), pageSize, callback);
+    }
+
+    @Override
+    final void loadBefore(int currentBeginIndex, @NonNull Value currentBeginItem, int pageSize,
+            @NonNull LoadCallback<Value> callback) {
+        loadBefore(getKey(currentBeginItem), pageSize, callback);
     }
 
     @Nullable
     @Override
-    List<Value> loadBeforeImpl(
-            int currentBeginIndex, @NonNull Value currentBeginItem, int pageSize) {
-        List<Value> list = loadBefore(getKey(currentBeginItem), pageSize);
-
-        if (list != null && list.size() > 1) {
-            // TODO: move out of keyed entirely, into the DB DataSource.
-            list = new ArrayList<>(list);
-            Collections.reverse(list);
+    final Key getKey(int position, Value item) {
+        if (item == null) {
+            return null;
         }
-        return list;
+
+        return getKey(item);
     }
 
 
-    @Override
-    void loadInitial(Key key, int initialLoadSize, boolean enablePlaceholders,
-            @NonNull PageResult.Receiver<Key, Value> receiver) {
-
-        PageResult<Key, Value> pageResult =
-                loadInitialInternal(key, initialLoadSize, enablePlaceholders);
-        if (pageResult == null) {
-            // loading failed, return empty page
-            receiver.onPageResult(new PageResult<Key, Value>(PageResult.INIT));
-        } else {
-            receiver.onPageResult(pageResult);
-        }
-    }
-
     /**
-     * Try initial load, and either return the successful initial load to the receiver,
-     * or null if unsuccessful.
+     * Load initial data.
+     * <p>
+     * This method is called first to initialize a PagedList with data. If it's possible to count
+     * the items that can be loaded by the DataSource, it's recommended to pass the loaded data to
+     * the callback via the three-parameter
+     * {@link DataSource.InitialLoadCallback#onResult(List, int, int)}. This enables PagedLists
+     * presenting data from this source to display placeholders to represent unloaded items.
+     * <p>
+     * {@code initialLoadKey} and {@code requestedLoadSize} are hints, not requirements, so if it is
+     * difficult or impossible to respect them, they may be altered. Note that ignoring the
+     * {@code initialLoadKey} can prevent subsequent PagedList/DataSource pairs from initializing at
+     * the same location. If your data source never invalidates (for example, loading from the
+     * network without the network ever signalling that old data must be reloaded), it's fine to
+     * ignore the {@code initialLoadKey} and always start from the beginning of the data set.
+     *
+     * @param initialLoadKey Load items around this key, or at the beginning of the data set if null
+     *                       is passed.
+     * @param requestedLoadSize Suggested number of items to load.
+     * @param enablePlaceholders Signals whether counting is requested. If false, you can
+     *                           potentially save work by calling the single-parameter variant of
+     *                           {@link DataSource.LoadCallback#onResult(List)} and not counting the
+     *                           number of items in the data set.
+     * @param callback DataSource.LoadCallback that receives initial load data.
      */
-    @Nullable
-    private PageResult<Key, Value> loadInitialInternal(
-            @Nullable Key key, int initialLoadSize, boolean enablePlaceholders) {
-        // check if invalid at beginning, and before returning a valid list
-        if (isInvalid()) {
-            return null;
-        }
+    @Override
+    public abstract void loadInitial(@Nullable Key initialLoadKey, int requestedLoadSize,
+            boolean enablePlaceholders, @NonNull InitialLoadCallback<Value> callback);
 
-        List<Value> list;
-        if (key == null) {
-            // no key, so load initial.
-            list = loadInitial(initialLoadSize);
-            if (list == null) {
-                return null;
-            }
-        } else {
-            List<Value> after = loadAfter(key, initialLoadSize / 2);
-            if (after == null) {
-                return null;
-            }
+    /**
+     * Load list data after the specified item.
+     * <p>
+     * It's valid to return a different list size than the page size, if it's easier for this data
+     * source. It is generally safer to increase the number loaded than reduce.
+     * <p>
+     * Data may be passed synchronously during the loadAfter method, or deferred and called at a
+     * later time. Further loads going down will be blocked until the callback is called.
+     * <p>
+     * If data cannot be loaded (for example, if the request is invalid, or the data would be stale
+     * and inconsistent, it is valid to call {@link #invalidate()} to invalidate the data source,
+     * and prevent further loading.
+     *
+     * @param currentEndKey Load items after this key. May be null on initial load, to indicate load
+     *                      from beginning.
+     * @param pageSize Suggested number of items to load.
+     * @param callback DataSource.LoadCallback that receives loaded data.
+     */
+    public abstract void loadAfter(@NonNull Key currentEndKey, int pageSize,
+            @NonNull LoadCallback<Value> callback);
 
-            Key loadBeforeKey = after.isEmpty() ? key : getKey(after.get(0));
-            List<Value> before = loadBefore(loadBeforeKey, initialLoadSize / 2);
-            if (before == null) {
-                return null;
-            }
-            if (!after.isEmpty() || !before.isEmpty()) {
-                // one of the lists has data
-                if (after.isEmpty()) {
-                    // retry loading after, since it may be that the key passed points to the end of
-                    // the list, so we need to load after the last item in the before list
-                    after = loadAfter(getKey(before.get(0)), initialLoadSize / 2);
-                    if (after == null) {
-                        return null;
-                    }
-                }
-                // assemble full list
-                list = new ArrayList<>();
-                list.addAll(before);
-                // Note - we reverse the list instead of before, in case before is immutable
-                Collections.reverse(list);
-                list.addAll(after);
-            } else {
-                // load before(key) and load after(key) failed - try load initial to be *sure* we
-                // catch the case where there's only one item, which is loaded by the key case
-                list = loadInitial(initialLoadSize);
-                if (list == null) {
-                    return null;
-                }
-            }
-        }
-
-        final Page<Key, Value> page = new Page<>(list);
-
-        if (list.isEmpty()) {
-            if (isInvalid()) {
-                return null;
-            }
-            // wasn't able to load any items, but not invalid - return an empty page.
-            return new PageResult<>(PageResult.INIT, page, 0, 0, 0);
-        }
-
-        int itemsBefore = COUNT_UNDEFINED;
-        int itemsAfter = COUNT_UNDEFINED;
-        if (enablePlaceholders) {
-            itemsBefore = countItemsBefore(getKey(list.get(0)));
-            itemsAfter = countItemsAfter(getKey(list.get(list.size() - 1)));
-        }
-
-        if (isInvalid()) {
-            return null;
-        }
-        if (itemsBefore == COUNT_UNDEFINED || itemsAfter == COUNT_UNDEFINED) {
-            itemsBefore = 0;
-            itemsAfter = 0;
-        }
-        return new PageResult<>(
-                PageResult.INIT,
-                page,
-                itemsBefore,
-                itemsAfter,
-                0);
-    }
+    /**
+     * Load data before the currently loaded content.
+     * <p>
+     * It's valid to return a different list size than the page size, if it's easier for this data
+     * source. It is generally safer to increase the number loaded than reduce. Note that the last
+     * item returned must be directly adjacent to the key passed, so varying size from the pageSize
+     * requested should effectively grow or shrink the list by modifying the beginning, not the end.
+     * <p>
+     * Data may be passed synchronously during the loadBefore method, or deferred and called at a
+     * later time. Further loads going up will be blocked until the callback is called.
+     * <p>
+     * If data cannot be loaded (for example, if the request is invalid, or the data would be stale
+     * and inconsistent, it is valid to call {@link #invalidate()} to invalidate the data source,
+     * and prevent further loading.
+     * <p class="note"><strong>Note:</strong> Data must be returned in the order it will be
+     * presented in the list.
+     *
+     * @param currentBeginKey Load items before this key.
+     * @param pageSize Suggested number of items to load.
+     * @param callback DataSource.LoadCallback that receives loaded data.
+     */
+    public abstract void loadBefore(@NonNull Key currentBeginKey, int pageSize,
+            @NonNull LoadCallback<Value> callback);
 
     /**
      * Return a key associated with the given item.
      * <p>
      * If your KeyedDataSource is loading from a source that is sorted and loaded by a unique
      * integer ID, you would return {@code item.getID()} here. This key can then be passed to
-     * {@link #loadBefore(Key, int)} or {@link #loadAfter(Key, int)} to load additional items
-     * adjacent to the item passed to this function.
+     * {@link #loadBefore(Object, int, LoadCallback)} or
+     * {@link #loadAfter(Object, int, LoadCallback)} to load additional items adjacent to the item
+     * passed to this function.
      * <p>
      * If your key is more complex, such as when you're sorting by name, then resolving collisions
      * with integer ID, you'll need to return both. In such a case you would use a wrapper class,
@@ -243,101 +147,5 @@
      * @return Key associated with given item.
      */
     @NonNull
-    @AnyThread
     public abstract Key getKey(@NonNull Value item);
-
-    /**
-     * Return the number of items that occur before the item uniquely identified by {@code key} in
-     * the data set.
-     * <p>
-     * For example, if you're loading items sorted by ID, then this would return the total number of
-     * items with ID less than {@code key}.
-     * <p>
-     * If you return {@link #COUNT_UNDEFINED} here, or from {@link #countItemsAfter(Key)}, your
-     * data source will not present placeholder null items in place of unloaded data.
-     *
-     * @param key A unique identifier of an item in the data set.
-     * @return Number of items in the data set before the item identified by {@code key}, or
-     *         {@link #COUNT_UNDEFINED}.
-     *
-     * @see #countItemsAfter(Key)
-     */
-    @WorkerThread
-    public int countItemsBefore(@NonNull Key key) {
-        return COUNT_UNDEFINED;
-    }
-
-    /**
-     * Return the number of items that occur after the item uniquely identified by {@code key} in
-     * the data set.
-     * <p>
-     * For example, if you're loading items sorted by ID, then this would return the total number of
-     * items with ID greater than {@code key}.
-     * <p>
-     * If you return {@link #COUNT_UNDEFINED} here, or from {@link #countItemsBefore(Key)}, your
-     * data source will not present placeholder null items in place of unloaded data.
-     *
-     * @param key A unique identifier of an item in the data set.
-     * @return Number of items in the data set after the item identified by {@code key}, or
-     *         {@link #COUNT_UNDEFINED}.
-     *
-     * @see #countItemsBefore(Key)
-     */
-    @WorkerThread
-    public int countItemsAfter(@NonNull Key key) {
-        return COUNT_UNDEFINED;
-    }
-
-    @WorkerThread
-    @Nullable
-    public abstract List<Value> loadInitial(int pageSize);
-
-    /**
-     * Load list data after the specified item.
-     * <p>
-     * It's valid to return a different list size than the page size, if it's easier for this data
-     * source. It is generally safer to increase the number loaded than reduce.
-     *
-     * @param currentEndKey Load items after this key. May be null on initial load, to indicate load
-     *                      from beginning.
-     * @param pageSize      Suggested number of items to load.
-     * @return List of items, starting after the specified item. Null if the data source is
-     * no longer valid, and should not be queried again.
-     */
-    @SuppressWarnings("WeakerAccess")
-    @WorkerThread
-    @Nullable
-    public abstract List<Value> loadAfter(@NonNull Key currentEndKey, int pageSize);
-
-    /**
-     * Load data before the currently loaded content, starting at the provided index,
-     * in reverse-display order.
-     * <p>
-     * It's valid to return a different list size than the page size, if it's easier for this data
-     * source. It is generally safer to increase the number loaded than reduce.
-     * <p class="note"><strong>Note:</strong> Items returned from loadBefore <em>must</em> be in
-     * reverse order from how they will be presented in the list. The first item in the return list
-     * will be prepended immediately before the current beginning of the list. This is so that the
-     * KeyedDataSource may return a different number of items from the requested {@code pageSize} by
-     * shortening or lengthening the return list as it desires.
-     * <p>
-     *
-     * @param currentBeginKey Load items before this key.
-     * @param pageSize         Suggested number of items to load.
-     * @return List of items, in descending order, starting after the specified item. Null if the
-     * data source is no longer valid, and should not be queried again.
-     */
-    @SuppressWarnings("WeakerAccess")
-    @WorkerThread
-    @Nullable
-    public abstract List<Value> loadBefore(@NonNull Key currentBeginKey, int pageSize);
-
-    @Nullable
-    @Override
-    Key getKey(int position, Value item) {
-        if (item == null) {
-            return null;
-        }
-        return getKey(item);
-    }
 }
diff --git a/paging/common/src/main/java/android/arch/paging/ListDataSource.java b/paging/common/src/main/java/android/arch/paging/ListDataSource.java
index d3a171e..b6f366a 100644
--- a/paging/common/src/main/java/android/arch/paging/ListDataSource.java
+++ b/paging/common/src/main/java/android/arch/paging/ListDataSource.java
@@ -16,10 +16,12 @@
 
 package android.arch.paging;
 
+import android.support.annotation.NonNull;
+
 import java.util.ArrayList;
 import java.util.List;
 
-public class ListDataSource<T> extends TiledDataSource<T> {
+class ListDataSource<T> extends PositionalDataSource<T> {
     private final List<T> mList;
 
     public ListDataSource(List<T> list) {
@@ -27,13 +29,22 @@
     }
 
     @Override
-    public int countItems() {
-        return mList.size();
+    public void loadInitial(int requestedStartPosition, int requestedLoadSize, int pageSize,
+            @NonNull InitialLoadCallback<T> callback) {
+        final int totalCount = mList.size();
+
+        final int firstLoadPosition = computeFirstLoadPosition(
+                requestedStartPosition, requestedLoadSize, pageSize, totalCount);
+        final int firstLoadSize = Math.min(totalCount - firstLoadPosition, requestedLoadSize);
+
+        // for simplicity, we could return everything immediately,
+        // but we tile here since it's expected behavior
+        List<T> sublist = mList.subList(firstLoadPosition, firstLoadPosition + firstLoadSize);
+        callback.onResult(sublist, firstLoadPosition, totalCount);
     }
 
     @Override
-    public List<T> loadRange(int startPosition, int count) {
-        int endExclusive = Math.min(mList.size(), startPosition + count);
-        return mList.subList(startPosition, endExclusive);
+    public void loadRange(int startPosition, int count, @NonNull LoadCallback<T> callback) {
+        callback.onResult(mList.subList(startPosition, startPosition + count));
     }
 }
diff --git a/paging/common/src/main/java/android/arch/paging/Page.java b/paging/common/src/main/java/android/arch/paging/Page.java
deleted file mode 100644
index e9890ed..0000000
--- a/paging/common/src/main/java/android/arch/paging/Page.java
+++ /dev/null
@@ -1,51 +0,0 @@
-/*
- * Copyright (C) 2017 The Android Open Source Project
- *
- * Licensed under the Apache License, Version 2.0 (the "License");
- * you may not use this file except in compliance with the License.
- * You may obtain a copy of the License at
- *
- *      http://www.apache.org/licenses/LICENSE-2.0
- *
- * Unless required by applicable law or agreed to in writing, software
- * distributed under the License is distributed on an "AS IS" BASIS,
- * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
- * See the License for the specific language governing permissions and
- * limitations under the License.
- */
-
-package android.arch.paging;
-
-import android.support.annotation.NonNull;
-import android.support.annotation.Nullable;
-
-import java.util.List;
-
-/**
- * Immutable class representing a page of data loaded from a DataSource.
- * <p>
- * Optionally stores before/after keys for cases where they cannot be computed, but the DataSource
- * can provide them as part of loading a page.
- * <p>
- * A page's list must never be modified.
- */
-class Page<K, V> {
-    @SuppressWarnings("WeakerAccess")
-    @Nullable
-    public final K beforeKey;
-    @NonNull
-    public final List<V> items;
-    @SuppressWarnings("WeakerAccess")
-    @Nullable
-    public K afterKey;
-
-    Page(@NonNull List<V> items) {
-        this(null, items, null);
-    }
-
-    Page(@Nullable K beforeKey, @NonNull List<V> items, @Nullable K afterKey) {
-        this.beforeKey = beforeKey;
-        this.items = items;
-        this.afterKey = afterKey;
-    }
-}
diff --git a/paging/common/src/main/java/android/arch/paging/PageResult.java b/paging/common/src/main/java/android/arch/paging/PageResult.java
index 55d5fb7..4384e72 100644
--- a/paging/common/src/main/java/android/arch/paging/PageResult.java
+++ b/paging/common/src/main/java/android/arch/paging/PageResult.java
@@ -16,11 +16,22 @@
 
 package android.arch.paging;
 
-import android.support.annotation.AnyThread;
 import android.support.annotation.MainThread;
 import android.support.annotation.NonNull;
 
-class PageResult<K, V> {
+import java.util.Collections;
+import java.util.List;
+
+class PageResult<T> {
+    @SuppressWarnings("unchecked")
+    private static final PageResult INVALID_RESULT =
+            new PageResult(Collections.EMPTY_LIST, 0);
+
+    @SuppressWarnings("unchecked")
+    static <T> PageResult<T> getInvalidResult() {
+        return INVALID_RESULT;
+    }
+
     static final int INIT = 0;
 
     // contiguous results
@@ -30,8 +41,8 @@
     // non-contiguous, tile result
     static final int TILE = 3;
 
-    public final int type;
-    public final Page<K, V> page;
+    @NonNull
+    public final List<T> page;
     @SuppressWarnings("WeakerAccess")
     public final int leadingNulls;
     @SuppressWarnings("WeakerAccess")
@@ -39,26 +50,34 @@
     @SuppressWarnings("WeakerAccess")
     public final int positionOffset;
 
-    PageResult(int type, Page<K, V> page, int leadingNulls, int trailingNulls, int positionOffset) {
-        this.type = type;
-        this.page = page;
+    PageResult(@NonNull List<T> list, int leadingNulls, int trailingNulls, int positionOffset) {
+        this.page = list;
         this.leadingNulls = leadingNulls;
         this.trailingNulls = trailingNulls;
         this.positionOffset = positionOffset;
     }
 
-    PageResult(int type) {
-        this.type = type;
-        this.page = null;
+    PageResult(@NonNull List<T> list, int positionOffset) {
+        this.page = list;
         this.leadingNulls = 0;
         this.trailingNulls = 0;
-        this.positionOffset = 0;
+        this.positionOffset = positionOffset;
     }
 
-    interface Receiver<K, V> {
-        @AnyThread
-        void postOnPageResult(@NonNull PageResult<K, V> pageResult);
+    @Override
+    public String toString() {
+        return "Result " + leadingNulls
+                + ", " + page
+                + ", " + trailingNulls
+                + ", offset " + positionOffset;
+    }
+
+    public boolean isInvalid() {
+        return this == INVALID_RESULT;
+    }
+
+    abstract static class Receiver<T> {
         @MainThread
-        void onPageResult(@NonNull PageResult<K, V> pageResult);
+        public abstract void onPageResult(int type, @NonNull PageResult<T> pageResult);
     }
 }
diff --git a/paging/common/src/main/java/android/arch/paging/PagedList.java b/paging/common/src/main/java/android/arch/paging/PagedList.java
index f18e108..275684f 100644
--- a/paging/common/src/main/java/android/arch/paging/PagedList.java
+++ b/paging/common/src/main/java/android/arch/paging/PagedList.java
@@ -17,6 +17,7 @@
 package android.arch.paging;
 
 import android.support.annotation.AnyThread;
+import android.support.annotation.MainThread;
 import android.support.annotation.NonNull;
 import android.support.annotation.Nullable;
 import android.support.annotation.RestrictTo;
@@ -51,7 +52,7 @@
  * PagedList can present data for an unbounded, infinite scrolling list, or a very large but
  * countable list. Use {@link Config} to control how many items a PagedList loads, and when.
  * <p>
- * If you use {@link LivePagedListProvider} to get a
+ * If you use {@link LivePagedListBuilder} to get a
  * {@link android.arch.lifecycle.LiveData}&lt;PagedList>, it will initialize PagedLists on a
  * background thread for you.
  * <h4>Placeholders</h4>
@@ -88,9 +89,8 @@
  * </ul>
  * <p>
  * Placeholders are enabled by default, but can be disabled in two ways. They are disabled if the
- * DataSource returns {@link DataSource#COUNT_UNDEFINED} from any item counting method, or if
- * {@code false} is passed to {@link Config.Builder#setEnablePlaceholders(boolean)} when building a
- * {@link Config}.
+ * DataSource does not count its data set in its initial load, or if  {@code false} is passed to
+ * {@link Config.Builder#setEnablePlaceholders(boolean)} when building a {@link Config}.
  *
  * @param <T> The type of the entries in the list.
  */
@@ -104,7 +104,7 @@
     @NonNull
     final Config mConfig;
     @NonNull
-    final PagedStorage<?, T> mStorage;
+    final PagedStorage<T> mStorage;
 
     int mLastLoad = 0;
     T mLastItem = null;
@@ -123,7 +123,7 @@
 
     protected final ArrayList<WeakReference<Callback>> mCallbacks = new ArrayList<>();
 
-    PagedList(@NonNull PagedStorage<?, T> storage,
+    PagedList(@NonNull PagedStorage<T> storage,
             @NonNull Executor mainThreadExecutor,
             @NonNull Executor backgroundThreadExecutor,
             @Nullable BoundaryCallback<T> boundaryCallback,
@@ -162,7 +162,8 @@
         if (dataSource.isContiguous() || !config.enablePlaceholders) {
             if (!dataSource.isContiguous()) {
                 //noinspection unchecked
-                dataSource = (DataSource<K, T>) ((TiledDataSource<T>) dataSource).getAsContiguous();
+                dataSource = (DataSource<K, T>) ((PositionalDataSource<T>) dataSource)
+                        .wrapAsContiguousWithoutPlaceholders();
             }
             ContiguousDataSource<K, T> contigDataSource = (ContiguousDataSource<K, T>) dataSource;
             return new ContiguousPagedList<>(contigDataSource,
@@ -172,7 +173,7 @@
                     config,
                     key);
         } else {
-            return new TiledPagedList<>((TiledDataSource<T>) dataSource,
+            return new TiledPagedList<>((PositionalDataSource<T>) dataSource,
                     mainThreadExecutor,
                     backgroundThreadExecutor,
                     boundaryCallback,
@@ -195,7 +196,7 @@
      * Because PagedLists are initialized with data, PagedLists must be built on a background
      * thread.
      * <p>
-     * {@link LivePagedListProvider} does this creation on a background thread automatically, if you
+     * {@link LivePagedListBuilder} does this creation on a background thread automatically, if you
      * want to receive a {@code LiveData<PagedList<...>>}.
      *
      * @param <Key> Type of key used to load data from the DataSource.
@@ -211,9 +212,9 @@
         private Key mInitialKey;
 
         /**
-         * The source of data that the PagedList should load from.
-         * @param dataSource Source of data for the PagedList.
+         * The source of data that the PagedList will load from.
          *
+         * @param dataSource Source of data for the PagedList.
          * @return this
          */
         @NonNull
@@ -286,8 +287,21 @@
         /**
          * Creates a {@link PagedList} with the given parameters.
          * <p>
-         * This call will initial data and perform any counting needed to initialize the PagedList,
-         * therefore it should only be called on a worker thread.
+         * This call will dispatch the {@link DataSource}'s loadInitial method immediately. If a
+         * DataSource posts all of its work (e.g. to a network thread), the PagedList will
+         * be immediately created as empty, and grow to its initial size when the initial load
+         * completes.
+         * <p>
+         * If the DataSource implements its load synchronously, doing the load work immediately in
+         * the loadInitial method, the PagedList will block on that load before completing
+         * construction. In this case, use a background thread to create a PagedList.
+         * <p>
+         * It's fine to create a PagedList with an async DataSource on the main thread, such as in
+         * the constructor of a ViewModel. An async network load won't block the initialLoad
+         * function. For a synchronous DataSource such as one created from a Room database, a
+         * {@code LiveData<PagedList>} can be safely constructed with {@link LivePagedListBuilder}
+         * on the main thread, since actual construction work is deferred, and done on a background
+         * thread.
          * <p>
          * While build() will always return a PagedList, it's important to note that the PagedList
          * initial load may fail to acquire data from the DataSource. This can happen for example if
@@ -451,13 +465,11 @@
         // safe to deref mBoundaryCallback here, since we only defer if mBoundaryCallback present
         if (begin) {
             //noinspection ConstantConditions
-            mBoundaryCallback.onItemAtFrontLoaded(
-                    snapshot(), mStorage.getFirstLoadedItem(), mStorage.size());
+            mBoundaryCallback.onItemAtFrontLoaded(mStorage.getFirstLoadedItem());
         }
         if (end) {
             //noinspection ConstantConditions
-            mBoundaryCallback.onItemAtEndLoaded(
-                    snapshot(), mStorage.getLastLoadedItem(), mStorage.size());
+            mBoundaryCallback.onItemAtEndLoaded(mStorage.getLastLoadedItem());
         }
     }
 
@@ -501,7 +513,6 @@
         if (isImmutable()) {
             return this;
         }
-
         return new SnapshotPagedList<>(this);
     }
 
@@ -522,7 +533,7 @@
      * <p>
      * When a PagedList is invalidated, you can pass the key returned by this function to initialize
      * the next PagedList. This ensures (depending on load times) that the next PagedList that
-     * arrives will have data that overlaps. If you use {@link LivePagedListProvider}, it will do
+     * arrives will have data that overlaps. If you use {@link LivePagedListBuilder}, it will do
      * this for you.
      *
      * @return Key of position most recently passed to {@link #loadAround(int)}.
@@ -556,8 +567,8 @@
     /**
      * Position offset of the data in the list.
      * <p>
-     * If data is supplied by a {@link TiledDataSource}, the item returned from <code>get(i)</code>
-     * has a position of <code>i + getPositionOffset()</code>.
+     * If data is supplied by a {@link PositionalDataSource}, the item returned from
+     * <code>get(i)</code> has a position of <code>i + getPositionOffset()</code>.
      * <p>
      * If the DataSource is a {@link KeyedDataSource}, and thus doesn't use positions, returns 0.
      */
@@ -583,15 +594,25 @@
      * GC'd.
      *
      * @param previousSnapshot Snapshot previously captured from this List, or null.
-     * @param callback         Callback to dispatch to.
+     * @param callback         LoadCallback to dispatch to.
      * @see #removeWeakCallback(Callback)
      */
     @SuppressWarnings("WeakerAccess")
     public void addWeakCallback(@Nullable List<T> previousSnapshot, @NonNull Callback callback) {
         if (previousSnapshot != null && previousSnapshot != this) {
-            PagedList<T> storageSnapshot = (PagedList<T>) previousSnapshot;
-            //noinspection unchecked
-            dispatchUpdatesSinceSnapshot(storageSnapshot, callback);
+
+            if (previousSnapshot.isEmpty()) {
+                if (!mStorage.isEmpty()) {
+                    // If snapshot is empty, diff is trivial - just notify number new items.
+                    // Note: occurs in async init, when snapshot taken before init page arrives
+                    callback.onInserted(0, mStorage.size());
+                }
+            } else {
+                PagedList<T> storageSnapshot = (PagedList<T>) previousSnapshot;
+
+                //noinspection unchecked
+                dispatchUpdatesSinceSnapshot(storageSnapshot, callback);
+            }
         }
 
         // first, clean up any empty weak refs
@@ -608,7 +629,7 @@
     /**
      * Removes a previously added callback.
      *
-     * @param callback Callback, previously added.
+     * @param callback LoadCallback, previously added.
      * @see #addWeakCallback(List, Callback)
      */
     @SuppressWarnings("WeakerAccess")
@@ -645,6 +666,14 @@
         }
     }
 
+
+
+    /**
+     * Dispatch updates since the non-empty snapshot was taken.
+     *
+     * @param snapshot Non-empty snapshot.
+     * @param callback LoadCallback for updates that have occurred since snapshot.
+     */
     abstract void dispatchUpdatesSinceSnapshot(@NonNull PagedList<T> snapshot,
             @NonNull Callback callback);
 
@@ -826,13 +855,6 @@
              * This value is typically larger than page size, so on first load data there's a large
              * enough range of content loaded to cover small scrolls.
              * <p>
-             * If used with a {@link TiledDataSource}, this value is rounded to the nearest number
-             * of pages, with a minimum of two pages, and loaded with a single call to
-             * {@link TiledDataSource#loadRange(int, int)}.
-             * <p>
-             * If used with a {@link KeyedDataSource}, this value will be passed to
-             * {@link KeyedDataSource#loadInitial(int)}.
-             * <p>
              * If not set, defaults to three times page size.
              *
              * @param initialLoadSizeHint Number of items to load while initializing the PagedList.
@@ -873,13 +895,43 @@
     }
 
     /**
-     * WIP API for load-more-into-local-storage callbacks
+     * Signals when a PagedList has reached the end of available data.
+     * <p>
+     * This can be used to implement paging from the network into a local database - when the
+     * database has no more data to present, a BoundaryCallback can be used to fetch more data.
+     * <p>
+     * If an instance is shared across multiple PagedLists (e.g. when passed to
+     * {@link LivePagedListBuilder#setBoundaryCallback}), the callbacks may be issued multiple
+     * times. If for example {@link #onItemAtEndLoaded(Object)} triggers a network load, it should
+     * avoid triggering it again while the load is ongoing.
+     *
+     * @param <T> Type loaded by the PagedList.
      */
+    @MainThread
     public abstract static class BoundaryCallback<T> {
+        /**
+         * Called when zero items are returned from an initial load of the PagedList's data source.
+         */
         public abstract void onZeroItemsLoaded();
-        public abstract void onItemAtFrontLoaded(@NonNull List<T> pagedListSnapshot,
-                @NonNull T itemAtFront, int pagedListSize);
-        public abstract void onItemAtEndLoaded(@NonNull List<T> pagedListSnapshot,
-                @NonNull T itemAtEnd, int pagedListSize);
+
+        /**
+         * Called when the item at the front of the PagedList has been loaded, and access has
+         * occurred within {@link Config#prefetchDistance} of it.
+         * <p>
+         * No more data will be prepended to the PagedList before this item.
+         *
+         * @param itemAtFront The first item of PagedList
+         */
+        public abstract void onItemAtFrontLoaded(@NonNull T itemAtFront);
+
+        /**
+         * Called when the item at the end of the PagedList has been loaded, and access has
+         * occurred within {@link Config#prefetchDistance} of it.
+         * <p>
+         * No more data will be appended to the PagedList after this item.
+         *
+         * @param itemAtEnd The first item of PagedList
+         */
+        public abstract void onItemAtEndLoaded(@NonNull T itemAtEnd);
     }
 }
diff --git a/paging/common/src/main/java/android/arch/paging/PagedStorage.java b/paging/common/src/main/java/android/arch/paging/PagedStorage.java
index b857462..d4531d3 100644
--- a/paging/common/src/main/java/android/arch/paging/PagedStorage.java
+++ b/paging/common/src/main/java/android/arch/paging/PagedStorage.java
@@ -17,13 +17,21 @@
 package android.arch.paging;
 
 import android.support.annotation.NonNull;
+import android.support.annotation.Nullable;
 
 import java.util.AbstractList;
 import java.util.ArrayList;
-import java.util.Collections;
 import java.util.List;
 
-final class PagedStorage<K, V> extends AbstractList<V> {
+final class PagedStorage<T> extends AbstractList<T> {
+    /**
+     * Lists instances are compared (with instance equality) to PLACEHOLDER_LIST to check if an item
+     * in that position is already loading. We use a singleton placeholder list that is distinct
+     * from Collections.EMPTY_LIST for safety.
+     */
+    @SuppressWarnings("MismatchedQueryAndUpdateOfCollection")
+    private static final List PLACEHOLDER_LIST = new ArrayList();
+
     // Always set
     private int mLeadingNullCount;
     /**
@@ -37,7 +45,7 @@
      * Non-contiguous - mPages may have nulls or a placeholder page, isTiled() always returns true.
      *     mPages may have nulls, or placeholder (empty) pages while content is loading.
      */
-    private final ArrayList<Page<K, V>> mPages;
+    private final ArrayList<List<T>> mPages;
     private int mTrailingNullCount;
 
     private int mPositionOffset;
@@ -53,9 +61,6 @@
     private int mNumberPrepended;
     private int mNumberAppended;
 
-    // only used in tiling case
-    private Page<K, V> mPlaceholderPage;
-
     PagedStorage() {
         mLeadingNullCount = 0;
         mPages = new ArrayList<>();
@@ -67,12 +72,12 @@
         mNumberAppended = 0;
     }
 
-    PagedStorage(int leadingNulls, Page<K, V> page, int trailingNulls) {
+    PagedStorage(int leadingNulls, List<T> page, int trailingNulls) {
         this();
         init(leadingNulls, page, trailingNulls, 0);
     }
 
-    private PagedStorage(PagedStorage<K, V> other) {
+    private PagedStorage(PagedStorage<T> other) {
         mLeadingNullCount = other.mLeadingNullCount;
         mPages = new ArrayList<>(other.mPages);
         mTrailingNullCount = other.mTrailingNullCount;
@@ -81,40 +86,37 @@
         mPageSize = other.mPageSize;
         mNumberPrepended = other.mNumberPrepended;
         mNumberAppended = other.mNumberAppended;
-
-        // preserve placeholder page so we can locate placeholder pages if needed later
-        mPlaceholderPage = other.mPlaceholderPage;
     }
 
-    PagedStorage<K, V> snapshot() {
+    PagedStorage<T> snapshot() {
         return new PagedStorage<>(this);
     }
 
-    private void init(int leadingNulls, Page<K, V> page, int trailingNulls, int positionOffset) {
+    private void init(int leadingNulls, List<T> page, int trailingNulls, int positionOffset) {
         mLeadingNullCount = leadingNulls;
         mPages.clear();
         mPages.add(page);
         mTrailingNullCount = trailingNulls;
 
         mPositionOffset = positionOffset;
-        mStorageCount = page.items.size();
+        mStorageCount = page.size();
 
         // initialized as tiled. There may be 3 nulls, 2 items, but we still call this tiled
         // even if it will break if nulls convert.
-        mPageSize = page.items.size();
+        mPageSize = page.size();
 
         mNumberPrepended = 0;
         mNumberAppended = 0;
     }
 
-    void init(int leadingNulls, Page<K, V> page, int trailingNulls, int positionOffset,
+    void init(int leadingNulls, @NonNull List<T> page, int trailingNulls, int positionOffset,
             @NonNull Callback callback) {
         init(leadingNulls, page, trailingNulls, positionOffset);
         callback.onInitialized(size());
     }
 
     @Override
-    public V get(int i) {
+    public T get(int i) {
         if (i < 0 || i >= size()) {
             throw new IndexOutOfBoundsException("Index: " + i + ", Size: " + size());
         }
@@ -138,7 +140,7 @@
             pageInternalIndex = localIndex;
             final int localPageCount = mPages.size();
             for (localPageIndex = 0; localPageIndex < localPageCount; localPageIndex++) {
-                int pageSize = mPages.get(localPageIndex).items.size();
+                int pageSize = mPages.get(localPageIndex).size();
                 if (pageSize > pageInternalIndex) {
                     // stop, found the page
                     break;
@@ -147,12 +149,12 @@
             }
         }
 
-        Page<?, V> page = mPages.get(localPageIndex);
-        if (page == null || page.items.size() == 0) {
+        List<T> page = mPages.get(localPageIndex);
+        if (page == null || page.size() == 0) {
             // can only occur in tiled case, with untouched inner/placeholder pages
             return null;
         }
-        return page.items.get(pageInternalIndex);
+        return page.get(pageInternalIndex);
     }
 
     /**
@@ -207,8 +209,8 @@
         int total = mLeadingNullCount;
         final int pageCount = mPages.size();
         for (int i = 0; i < pageCount; i++) {
-            Page page = mPages.get(i);
-            if (page != null && page != mPlaceholderPage) {
+            List page = mPages.get(i);
+            if (page != null && page != PLACEHOLDER_LIST) {
                 break;
             }
             total += mPageSize;
@@ -219,8 +221,8 @@
     int computeTrailingNulls() {
         int total = mTrailingNullCount;
         for (int i = mPages.size() - 1; i >= 0; i--) {
-            Page page = mPages.get(i);
-            if (page != null && page != mPlaceholderPage) {
+            List page = mPages.get(i);
+            if (page != null && page != PLACEHOLDER_LIST) {
                 break;
             }
             total += mPageSize;
@@ -230,21 +232,21 @@
 
     // ---------------- Contiguous API -------------------
 
-    V getFirstLoadedItem() {
+    T getFirstLoadedItem() {
         // safe to access first page's first item here:
         // If contiguous, mPages can't be empty, can't hold null Pages, and items can't be empty
-        return mPages.get(0).items.get(0);
+        return mPages.get(0).get(0);
     }
 
-    V getLastLoadedItem() {
+    T getLastLoadedItem() {
         // safe to access last page's last item here:
         // If contiguous, mPages can't be empty, can't hold null Pages, and items can't be empty
-        Page<K, V> page = mPages.get(mPages.size() - 1);
-        return page.items.get(page.items.size() - 1);
+        List<T> page = mPages.get(mPages.size() - 1);
+        return page.get(page.size() - 1);
     }
 
-    public void prependPage(@NonNull Page<K, V> page, @NonNull Callback callback) {
-        final int count = page.items.size();
+    void prependPage(@NonNull List<T> page, @NonNull Callback callback) {
+        final int count = page.size();
         if (count == 0) {
             // Nothing returned from source, stop loading in this direction
             return;
@@ -274,8 +276,8 @@
         callback.onPagePrepended(mLeadingNullCount, changedCount, addedCount);
     }
 
-    public void appendPage(@NonNull Page<K, V> page, @NonNull Callback callback) {
-        final int count = page.items.size();
+    void appendPage(@NonNull List<T> page, @NonNull Callback callback) {
+        final int count = page.size();
         if (count == 0) {
             // Nothing returned from source, stop loading in this direction
             return;
@@ -284,7 +286,7 @@
         if (mPageSize > 0) {
             // if the previous page was smaller than mPageSize,
             // or if this page is larger than the previous, disable tiling
-            if (mPages.get(mPages.size() - 1).items.size() != mPageSize
+            if (mPages.get(mPages.size() - 1).size() != mPageSize
                     || count > mPageSize) {
                 mPageSize = -1;
             }
@@ -306,8 +308,30 @@
 
     // ------------------ Non-Contiguous API (tiling required) ----------------------
 
-    public void insertPage(int position, @NonNull Page<K, V> page, Callback callback) {
-        final int newPageSize = page.items.size();
+    void initAndSplit(int leadingNulls, @NonNull List<T> multiPageList,
+            int trailingNulls, int positionOffset, int pageSize, @NonNull Callback callback) {
+
+        int pageCount = (multiPageList.size() + (pageSize - 1)) / pageSize;
+        for (int i = 0; i < pageCount; i++) {
+            int beginInclusive = i * pageSize;
+            int endExclusive = Math.min(multiPageList.size(), (i + 1) * pageSize);
+
+            List<T> sublist = multiPageList.subList(beginInclusive, endExclusive);
+
+            if (i == 0) {
+                // Trailing nulls for first page includes other pages in multiPageList
+                int initialTrailingNulls = trailingNulls + multiPageList.size() - sublist.size();
+                init(leadingNulls, sublist, initialTrailingNulls, positionOffset);
+            } else {
+                int insertPosition = leadingNulls + beginInclusive;
+                insertPage(insertPosition, sublist, null);
+            }
+        }
+        callback.onInitialized(size());
+    }
+
+    public void insertPage(int position, @NonNull List<T> page, @Nullable Callback callback) {
+        final int newPageSize = page.size();
         if (newPageSize != mPageSize) {
             // differing page size is OK in 2 cases, when the page is being added:
             // 1) to the end (in which case, ignore new smaller size)
@@ -334,22 +358,15 @@
 
         int localPageIndex = pageIndex - mLeadingNullCount / mPageSize;
 
-        Page<K, V> oldPage = mPages.get(localPageIndex);
-        if (oldPage != null && oldPage != mPlaceholderPage) {
+        List<T> oldPage = mPages.get(localPageIndex);
+        if (oldPage != null && oldPage != PLACEHOLDER_LIST) {
             throw new IllegalArgumentException(
                     "Invalid position " + position + ": data already loaded");
         }
         mPages.set(localPageIndex, page);
-        callback.onPageInserted(position, page.items.size());
-    }
-
-    private Page<K, V> getPlaceholderPage() {
-        if (mPlaceholderPage == null) {
-            @SuppressWarnings("unchecked")
-            List<V> list = Collections.emptyList();
-            mPlaceholderPage = new Page<>(null, list, null);
+        if (callback != null) {
+            callback.onPageInserted(position, page.size());
         }
-        return mPlaceholderPage;
     }
 
     private void allocatePageRange(final int minimumPage, final int maximumPage) {
@@ -399,7 +416,8 @@
         for (int pageIndex = minimumPage; pageIndex <= maximumPage; pageIndex++) {
             int localPageIndex = pageIndex - leadingNullPages;
             if (mPages.get(localPageIndex) == null) {
-                mPages.set(localPageIndex, getPlaceholderPage());
+                //noinspection unchecked
+                mPages.set(localPageIndex, PLACEHOLDER_LIST);
                 callback.onPagePlaceholderInserted(pageIndex);
             }
         }
@@ -414,9 +432,9 @@
             return false;
         }
 
-        Page<K, V> page = mPages.get(index - leadingNullPages);
+        List<T> page = mPages.get(index - leadingNullPages);
 
-        return page != null && page != mPlaceholderPage;
+        return page != null && page != PLACEHOLDER_LIST;
     }
 
     @Override
diff --git a/paging/common/src/main/java/android/arch/paging/PositionalDataSource.java b/paging/common/src/main/java/android/arch/paging/PositionalDataSource.java
index fa2932a..5dd3a83 100644
--- a/paging/common/src/main/java/android/arch/paging/PositionalDataSource.java
+++ b/paging/common/src/main/java/android/arch/paging/PositionalDataSource.java
@@ -1,5 +1,5 @@
 /*
- * Copyright (C) 2017 The Android Open Source Project
+ * Copyright 2017 The Android Open Source Project
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -20,115 +20,139 @@
 import android.support.annotation.Nullable;
 import android.support.annotation.WorkerThread;
 
-import java.util.List;
+import java.util.Collections;
 
 /**
- * Incremental data loader for paging positional content, where content can be loaded based on its
- * integer position.
+ * Position-based data loader for a fixed-size, countable data set, supporting loads at arbitrary
+ * positions.
  * <p>
- * Use PositionalDataSource if you only need position as input for item loading - if for example,
- * you're asking the backend for items at positions 10 through 20, or using a limit/offset database
- * query to load items at query position 10 through 20.
+ * Extend PositionalDataSource if you can support counting your data set, and loading based on
+ * position information.
  * <p>
- * Implement a DataSource using PositionalDataSource if position is the only information you need to
- * load items.
+ * Note that unless {@link PagedList.Config#enablePlaceholders placeholders are disabled}
+ * PositionalDataSource requires counting the size of the dataset. This allows pages to be tiled in
+ * at arbitrary, non-contiguous locations based upon what the user observes in a {@link PagedList}.
  * <p>
- * Note that {@link BoundedDataSource} provides a simpler API for positional loading, if your
- * backend or data store doesn't require
- * <p>
- * @param <Value> Value type of items being loaded by the DataSource.
+ * Room can generate a Factory of PositionalDataSources for you:
+ * <pre>
+ * {@literal @}Dao
+ * interface UserDao {
+ *     {@literal @}Query("SELECT * FROM user ORDER BY mAge DESC")
+ *     public abstract DataSource.Factory&lt;Integer, User> loadUsersByAgeDesc();
+ * }</pre>
+ *
+ * @param <T> Type of items being loaded by the PositionalDataSource.
  */
-abstract class PositionalDataSource<Value> extends ContiguousDataSource<Integer, Value> {
+public abstract class PositionalDataSource<T> extends DataSource<Integer, T> {
 
     /**
-     * Number of items that this DataSource can provide in total, or COUNT_UNDEFINED.
-     *
-     * @return number of items that this DataSource can provide in total, or COUNT_UNDEFINED
-     * if difficult or undesired to compute.
-     */
-    public int countItems() {
-        return COUNT_UNDEFINED;
-    }
-
-    @Nullable
-    @Override
-    List<Value> loadAfterImpl(int currentEndIndex, @NonNull Value currentEndItem, int pageSize) {
-        return loadAfter(currentEndIndex + 1, pageSize);
-    }
-
-    @Nullable
-    @Override
-    List<Value> loadBeforeImpl(int currentBeginIndex, @NonNull Value currentBeginItem,
-            int pageSize) {
-        return loadBefore(currentBeginIndex - 1, pageSize);
-    }
-
-    @Override
-    void loadInitial(Integer position, int initialLoadSize, boolean enablePlaceholders,
-            @NonNull PageResult.Receiver<Integer, Value> receiver) {
-
-        final int convertPosition = position == null ? 0 : position;
-        final int loadPosition = Math.max(0, (convertPosition - initialLoadSize / 2));
-
-        int count = COUNT_UNDEFINED;
-        if (enablePlaceholders) {
-            count = countItems();
-        }
-        List<Value> data = loadAfter(loadPosition, initialLoadSize);
-
-        if (data == null) {
-            receiver.onPageResult(new PageResult<Integer, Value>(PageResult.INIT));
-            return;
-        }
-
-        final boolean uncounted = count == COUNT_UNDEFINED;
-        int leadingNullCount = uncounted ? 0 : loadPosition;
-        int trailingNullCount = uncounted ? 0 : count - leadingNullCount - data.size();
-        int positionOffset = uncounted ? loadPosition : 0;
-
-        receiver.onPageResult(new PageResult<>(
-                PageResult.INIT,
-                new Page<Integer, Value>(data),
-                leadingNullCount,
-                trailingNullCount,
-                positionOffset));
-    }
-
-    /**
-     * Load data after currently loaded content, starting at the provided index.
+     * Load initial list data.
      * <p>
-     * It's valid to return a different list size than the page size, if it's easier for this data
-     * source. It is generally safer to increase the number loaded than reduce.
+     * This method is called to load the initial page(s) from the DataSource.
+     * <p>
+     * Result list must be a multiple of pageSize to enable efficient tiling.
      *
-     * @param startIndex Load items starting at this index.
-     * @param pageSize Suggested number of items to load.
-     * @return List of items, starting at position currentEndIndex + 1. Null if the data source is
-     *         no longer valid, and should not be queried again.
+     * @param requestedStartPosition Initial load position requested. Note that this may not be
+     *                               within the bounds of your data set, it should be corrected
+     *                               before you make your query.
+     * @param requestedLoadSize Requested number of items to load. Note that this may be larger than
+     *                          available data.
+     * @param pageSize Defines page size acceptable for return values. List of items passed to the
+     *                 callback must be an integer multiple of page size.
+     * @param callback DataSource.InitialLoadCallback that receives initial load data, including
+     *                 position and total data set size.
      */
     @WorkerThread
-    @Nullable
-    public abstract List<Value> loadAfter(int startIndex, int pageSize);
+    public abstract void loadInitial(int requestedStartPosition, int requestedLoadSize,
+            int pageSize, @NonNull InitialLoadCallback<T> callback);
 
     /**
-     * Load data before the currently loaded content, starting at the provided index.
+     * Called to load a range of data from the DataSource.
      * <p>
-     * It's valid to return a different list size than the page size, if it's easier for this data
-     * source. It is generally safer to increase the number loaded than reduce.
+     * This method is called to load additional pages from the DataSource after the
+     * InitialLoadCallback passed to loadInitial has initialized a PagedList.
+     * <p>
+     * Unlike {@link #loadInitial(int, int, int, InitialLoadCallback)}, this method must return the
+     * number of items requested, at the position requested.
      *
-     * @param startIndex Load items, starting at this index.
-     * @param pageSize Suggested number of items to load.
-     * @return List of items, in descending order, starting at position currentBeginIndex - 1. Null
-     *         if the data source is no longer valid, and should not be queried again.
+     * @param startPosition Initial load position.
+     * @param count Number of items to load.
+     * @param callback DataSource.LoadCallback that receives loaded data.
      */
     @WorkerThread
-    @Nullable
-    public abstract List<Value> loadBefore(int startIndex, int pageSize);
+    public abstract void loadRange(int startPosition, int count,
+            @NonNull LoadCallback<T> callback);
 
     @Override
-    Integer getKey(int position, Value item) {
-        if (position < 0) {
-            return null;
+    boolean isContiguous() {
+        return false;
+    }
+
+    @NonNull
+    ContiguousDataSource<Integer, T> wrapAsContiguousWithoutPlaceholders() {
+        return new ContiguousWithoutPlaceholdersWrapper<>(this);
+    }
+
+    static int computeFirstLoadPosition(int position, int firstLoadSize, int pageSize, int size) {
+        int roundedPageStart = Math.round(position / pageSize) * pageSize;
+
+        // maximum start pos is that which will encompass end of list
+        int maximumLoadPage = ((size - firstLoadSize + pageSize - 1) / pageSize) * pageSize;
+        roundedPageStart = Math.min(maximumLoadPage, roundedPageStart);
+
+        // minimum start position is 0
+        roundedPageStart = Math.max(0, roundedPageStart);
+
+        return roundedPageStart;
+    }
+
+    @SuppressWarnings("deprecation")
+    static class ContiguousWithoutPlaceholdersWrapper<Value>
+            extends ContiguousDataSource<Integer, Value> {
+
+        @NonNull
+        final PositionalDataSource<Value> mPositionalDataSource;
+
+        ContiguousWithoutPlaceholdersWrapper(
+                @NonNull PositionalDataSource<Value> positionalDataSource) {
+            mPositionalDataSource = positionalDataSource;
         }
-        return position;
+
+        @Override
+        public void loadInitial(@Nullable Integer position, int initialLoadSize,
+                boolean enablePlaceholders, @NonNull InitialLoadCallback<Value> callback) {
+            final int convertPosition = position == null ? 0 : position;
+
+            // Note enablePlaceholders will be false here, but we don't have a way to communicate
+            // this to PositionalDataSource. This is fine, because only the list and its position
+            // offset will be consumed by the InitialLoadCallback.
+            mPositionalDataSource.loadInitial(
+                    convertPosition, initialLoadSize, initialLoadSize, callback);
+        }
+
+        @Override
+        void loadAfter(int currentEndIndex, @NonNull Value currentEndItem, int pageSize,
+                @NonNull LoadCallback<Value> callback) {
+            int startIndex = currentEndIndex + 1;
+            mPositionalDataSource.loadRange(startIndex, pageSize, callback);
+        }
+
+        @Override
+        void loadBefore(int currentBeginIndex, @NonNull Value currentBeginItem, int pageSize,
+                @NonNull LoadCallback<Value> callback) {
+            int startIndex = currentBeginIndex - 1;
+            if (startIndex < 0) {
+                callback.onResult(Collections.<Value>emptyList());
+            } else {
+                int loadSize = Math.min(pageSize, startIndex + 1);
+                startIndex = startIndex - loadSize + 1;
+                mPositionalDataSource.loadRange(startIndex, loadSize, callback);
+            }
+        }
+
+        @Override
+        Integer getKey(int position, Value item) {
+            return position;
+        }
     }
 }
diff --git a/paging/common/src/main/java/android/arch/paging/TiledDataSource.java b/paging/common/src/main/java/android/arch/paging/TiledDataSource.java
index 0ea9428..27aeda0 100644
--- a/paging/common/src/main/java/android/arch/paging/TiledDataSource.java
+++ b/paging/common/src/main/java/android/arch/paging/TiledDataSource.java
@@ -16,84 +16,24 @@
 
 package android.arch.paging;
 
-import android.support.annotation.Nullable;
+import android.support.annotation.NonNull;
+import android.support.annotation.RestrictTo;
 import android.support.annotation.WorkerThread;
 
 import java.util.Collections;
 import java.util.List;
 
 /**
- * Position-based data loader for fixed size, arbitrary positioned loading.
- * <p>
- * Extend TiledDataSource if you want to load arbitrary pages based solely on position information,
- * and can generate pages of a provided fixed size.
- * <p>
- * Room can generate a TiledDataSource for you:
- * <pre>
- * {@literal @}Dao
- * interface UserDao {
- *     {@literal @}Query("SELECT * FROM user ORDER BY mAge DESC")
- *     public abstract TiledDataSource&lt;User> loadUsersByAgeDesc();
- * }</pre>
+ * @param <T> Type loaded by the TiledDataSource.
  *
- * Under the hood, Room will generate code equivalent to the below, using a limit/offset SQL query:
- * <pre>
- * {@literal @}Dao
- * interface UserDao {
- *     {@literal @}Query("SELECT COUNT(*) from user")
- *     public abstract Integer getUserCount();
- *
- *     {@literal @}Query("SELECT * from user ORDER BY mName DESC LIMIT :limit OFFSET :offset")
- *     public abstract List&lt;User> userNameLimitOffset(int limit, int offset);
- * }
- *
- * public class OffsetUserQueryDataSource extends TiledDataSource&lt;User> {
- *     private MyDatabase mDb;
- *     private final UserDao mUserDao;
- *     {@literal @}SuppressWarnings("FieldCanBeLocal")
- *     private final InvalidationTracker.Observer mObserver;
- *
- *     public OffsetUserQueryDataSource(MyDatabase db) {
- *         mDb = db;
- *         mUserDao = db.getUserDao();
- *         mObserver = new InvalidationTracker.Observer("user") {
- *             {@literal @}Override
- *             public void onInvalidated({@literal @}NonNull Set&lt;String> tables) {
- *                 // the user table has been invalidated, invalidate the DataSource
- *                 invalidate();
- *             }
- *         };
- *         db.getInvalidationTracker().addWeakObserver(mObserver);
- *     }
- *
- *     {@literal @}Override
- *     public boolean isInvalid() {
- *         mDb.getInvalidationTracker().refreshVersionsSync();
- *         return super.isInvalid();
- *     }
- *
- *     {@literal @}Override
- *     public int countItems() {
- *         return mUserDao.getUserCount();
- *     }
- *
- *     {@literal @}Override
- *     public List&lt;User> loadRange(int startPosition, int loadCount) {
- *         return mUserDao.userNameLimitOffset(loadCount, startPosition);
- *     }
- * }</pre>
- *
- * @param <Type> Type of items being loaded by the TiledDataSource.
+ * @deprecated Use {@link PositionalDataSource}
+ * @hide
  */
-public abstract class TiledDataSource<Type> extends DataSource<Integer, Type> {
+@SuppressWarnings("DeprecatedIsStillUsed")
+@Deprecated
+@RestrictTo(RestrictTo.Scope.LIBRARY_GROUP)
+public abstract class TiledDataSource<T> extends PositionalDataSource<T> {
 
-    private int mItemCount;
-
-    /**
-     * Number of items that this DataSource can provide in total.
-     *
-     * @return Number of items this DataSource can provide. Must be <code>0</code> or greater.
-     */
     @WorkerThread
     public abstract int countItems();
 
@@ -102,111 +42,39 @@
         return false;
     }
 
-    /**
-     * Called to load items at from the specified position range.
-     * <p>
-     * This method must return a list of requested size, unless at the end of list. Fixed size pages
-     * enable TiledDataSource to navigate tiles efficiently, and quickly accesss any position in the
-     * data set.
-     * <p>
-     * If a list of a different size is returned, but it is not the last list in the data set based
-     * on the return value from {@link #countItems()}, an exception will be thrown.
-     *
-     * @param startPosition Index of first item to load.
-     * @param count         Number of items to load.
-     * @return List of loaded items, of the requested length unless at end of list. Null if the
-     *         DataSource is no longer valid, and should not be queried again.
-     */
     @WorkerThread
-    public abstract List<Type> loadRange(int startPosition, int count);
+    public abstract List<T> loadRange(int startPosition, int count);
 
-    /**
-     * blocking, and splits pages
-     */
-    void loadRangeInitial(int startPosition, int count, int pageSize, int itemCount,
-            PageResult.Receiver<Integer, Type> receiver) {
-        mItemCount = itemCount;
-
-        if (itemCount == 0) {
-            // no data to load, just immediately return empty
-            receiver.onPageResult(new PageResult<>(
-                    PageResult.INIT, new Page<Integer, Type>(Collections.<Type>emptyList()),
-                    0, 0, startPosition));
+    @Override
+    public void loadInitial(int requestedStartPosition, int requestedLoadSize, int pageSize,
+            @NonNull InitialLoadCallback<T> callback) {
+        int totalCount = countItems();
+        if (totalCount == 0) {
+            callback.onResult(Collections.<T>emptyList());
             return;
         }
 
-        List<Type> list = loadRangeWrapper(startPosition, count);
+        // bound the size requested, based on known count
+        final int firstLoadPosition = computeFirstLoadPosition(
+                requestedStartPosition, requestedLoadSize, pageSize, totalCount);
+        final int firstLoadSize = Math.min(totalCount - firstLoadPosition, requestedLoadSize);
 
-        count = Math.min(count, itemCount - startPosition);
-
-        if (list == null) {
-            // invalid data, pass to receiver
-            receiver.onPageResult(new PageResult<Integer, Type>(
-                    PageResult.INIT, null, 0, 0, startPosition));
-            return;
-        }
-
-        if (list.size() != count) {
-            throw new IllegalStateException("Invalid list, requested size: " + count
-                    + ", returned size: " + list.size());
-        }
-
-        // emit the results as multiple pages
-        int pageCount = (count + (pageSize - 1)) / pageSize;
-        for (int i = 0; i < pageCount; i++) {
-            int beginInclusive = i * pageSize;
-            int endExclusive = Math.min(count, (i + 1) * pageSize);
-
-            Page<Integer, Type> page = new Page<>(list.subList(beginInclusive, endExclusive));
-
-            int leadingNulls = startPosition + beginInclusive;
-            int trailingNulls = itemCount - leadingNulls - page.items.size();
-            receiver.onPageResult(new PageResult<>(
-                    PageResult.INIT, page, leadingNulls, trailingNulls, 0));
-        }
-    }
-
-    void loadRange(int startPosition, int count, PageResult.Receiver<Integer, Type> receiver) {
-        List<Type> list = loadRangeWrapper(startPosition, count);
-
-        Page<Integer, Type> page = null;
-        int trailingNulls = mItemCount - startPosition;
-
+        // convert from legacy behavior
+        List<T> list = loadRange(firstLoadPosition, firstLoadSize);
         if (list != null) {
-            page = new Page<Integer, Type>(list);
-            trailingNulls -= list.size();
+            callback.onResult(list, firstLoadPosition, totalCount);
+        } else {
+            invalidate();
         }
-        receiver.postOnPageResult(new PageResult<>(
-                PageResult.TILE, page, startPosition, trailingNulls, 0));
     }
 
-    private List<Type> loadRangeWrapper(int startPosition, int count) {
-        if (isInvalid()) {
-            return null;
-        }
-        List<Type> list = loadRange(startPosition, count);
-        if (isInvalid()) {
-            return null;
-        }
-        return list;
-    }
-
-    ContiguousDataSource<Integer, Type> getAsContiguous() {
-        return new TiledAsBoundedDataSource<>(this);
-    }
-
-    static class TiledAsBoundedDataSource<Value> extends BoundedDataSource<Value> {
-        final TiledDataSource<Value> mTiledDataSource;
-
-        TiledAsBoundedDataSource(TiledDataSource<Value> tiledDataSource) {
-            mTiledDataSource = tiledDataSource;
-        }
-
-        @WorkerThread
-        @Nullable
-        @Override
-        public List<Value> loadRange(int startPosition, int loadCount) {
-            return mTiledDataSource.loadRange(startPosition, loadCount);
+    @Override
+    public void loadRange(int startPosition, int count, @NonNull LoadCallback<T> callback) {
+        List<T> list = loadRange(startPosition, count);
+        if (list != null) {
+            callback.onResult(list);
+        } else {
+            invalidate();
         }
     }
 }
diff --git a/paging/common/src/main/java/android/arch/paging/TiledPagedList.java b/paging/common/src/main/java/android/arch/paging/TiledPagedList.java
index 76bb682..338d43c 100644
--- a/paging/common/src/main/java/android/arch/paging/TiledPagedList.java
+++ b/paging/common/src/main/java/android/arch/paging/TiledPagedList.java
@@ -25,32 +25,15 @@
 
 class TiledPagedList<T> extends PagedList<T>
         implements PagedStorage.Callback {
+    private final PositionalDataSource<T> mDataSource;
 
-    private final TiledDataSource<T> mDataSource;
-
-    @SuppressWarnings("unchecked")
-    private final PagedStorage<Integer, T> mKeyedStorage = (PagedStorage<Integer, T>) mStorage;
-
-    private final PageResult.Receiver<Integer, T> mReceiver =
-            new PageResult.Receiver<Integer, T>() {
-        @AnyThread
-        @Override
-        public void postOnPageResult(@NonNull final PageResult<Integer, T> pageResult) {
-            // NOTE: if we're already on main thread, this can delay page receive by a frame
-            mMainThreadExecutor.execute(new Runnable() {
-                @Override
-                public void run() {
-                    onPageResult(pageResult);
-                }
-            });
-        }
-
+    private PageResult.Receiver<T> mReceiver = new PageResult.Receiver<T>() {
         // Creation thread for initial synchronous load, otherwise main thread
         // Safe to access main thread only state - no other thread has reference during construction
         @AnyThread
         @Override
-        public void onPageResult(@NonNull PageResult<Integer, T> pageResult) {
-            if (pageResult.page == null) {
+        public void onPageResult(int type, @NonNull PageResult<T> pageResult) {
+            if (pageResult.isInvalid()) {
                 detach();
                 return;
             }
@@ -61,60 +44,62 @@
             }
 
             if (mStorage.getPageCount() == 0) {
-                mKeyedStorage.init(
+                mStorage.initAndSplit(
                         pageResult.leadingNulls, pageResult.page, pageResult.trailingNulls,
-                        pageResult.positionOffset, TiledPagedList.this);
+                        pageResult.positionOffset, mConfig.pageSize, TiledPagedList.this);
             } else {
-                mKeyedStorage.insertPage(pageResult.leadingNulls, pageResult.page,
+                mStorage.insertPage(pageResult.positionOffset, pageResult.page,
                         TiledPagedList.this);
             }
 
             if (mBoundaryCallback != null) {
                 boolean deferEmpty = mStorage.size() == 0;
-                boolean deferBegin = !deferEmpty && pageResult.leadingNulls == 0;
-                boolean deferEnd = !deferEmpty && pageResult.trailingNulls == 0;
+                boolean deferBegin = !deferEmpty
+                        && pageResult.leadingNulls == 0
+                        && pageResult.positionOffset == 0;
+                int size = size();
+                boolean deferEnd = !deferEmpty
+                        && ((type == PageResult.INIT && pageResult.trailingNulls == 0)
+                                || (type == PageResult.TILE
+                                        && pageResult.positionOffset
+                                                == (size - size % mConfig.pageSize)));
                 deferBoundaryCallbacks(deferEmpty, deferBegin, deferEnd);
             }
         }
     };
 
     @WorkerThread
-    TiledPagedList(@NonNull TiledDataSource<T> dataSource,
+    TiledPagedList(@NonNull PositionalDataSource<T> dataSource,
             @NonNull Executor mainThreadExecutor,
             @NonNull Executor backgroundThreadExecutor,
             @Nullable BoundaryCallback<T> boundaryCallback,
             @NonNull Config config,
             int position) {
-        super(new PagedStorage<Integer, T>(), mainThreadExecutor, backgroundThreadExecutor,
+        super(new PagedStorage<T>(), mainThreadExecutor, backgroundThreadExecutor,
                 boundaryCallback, config);
         mDataSource = dataSource;
 
         final int pageSize = mConfig.pageSize;
+        mLastLoad = position;
 
-        final int itemCount = mDataSource.countItems();
+        if (mDataSource.isInvalid()) {
+            detach();
+        } else {
+            final int firstLoadSize =
+                    (Math.max(Math.round(mConfig.initialLoadSizeHint / pageSize), 2)) * pageSize;
 
-        final int firstLoadSize = Math.min(itemCount,
-                (Math.max(mConfig.initialLoadSizeHint / pageSize, 2)) * pageSize);
-        final int firstLoadPosition = computeFirstLoadPosition(
-                position, firstLoadSize, pageSize, itemCount);
+            final int idealStart = position - firstLoadSize / 2;
+            final int roundedPageStart = Math.max(0, Math.round(idealStart / pageSize) * pageSize);
 
-        mDataSource.loadRangeInitial(firstLoadPosition, firstLoadSize, pageSize,
-                itemCount, mReceiver);
-    }
+            DataSource.InitialLoadCallback<T> callback =
+                    new DataSource.InitialLoadCallback<>(true, mDataSource, mReceiver);
+            mDataSource.loadInitial(roundedPageStart, firstLoadSize, pageSize, callback);
 
-    static int computeFirstLoadPosition(int position, int firstLoadSize, int pageSize, int size) {
-        int idealStart = position - firstLoadSize / 2;
-
-        int roundedPageStart = Math.round(idealStart / pageSize) * pageSize;
-
-        // minimum start position is 0
-        roundedPageStart = Math.max(0, roundedPageStart);
-
-        // maximum start pos is that which will encompass end of list
-        int maximumLoadPage = ((size - firstLoadSize + pageSize - 1) / pageSize) * pageSize;
-        roundedPageStart = Math.min(maximumLoadPage, roundedPageStart);
-
-        return roundedPageStart;
+            // If initialLoad's callback is not called within the body, we force any following calls
+            // to post to the UI thread. This constructor may be run on a background thread, but
+            // after constructor, mutation must happen on UI thread.
+            callback.setPostExecutor(mMainThreadExecutor);
+        }
     }
 
     @Override
@@ -132,7 +117,13 @@
     protected void dispatchUpdatesSinceSnapshot(@NonNull PagedList<T> pagedListSnapshot,
             @NonNull Callback callback) {
         //noinspection UnnecessaryLocalVariable
-        final PagedStorage<?, T> snapshot = pagedListSnapshot.mStorage;
+        final PagedStorage<T> snapshot = pagedListSnapshot.mStorage;
+
+        if (snapshot.isEmpty()
+                || mStorage.size() != snapshot.size()) {
+            throw new IllegalArgumentException("Invalid snapshot provided - doesn't appear"
+                    + " to be a snapshot of this PagedList");
+        }
 
         // loop through each page and signal the callback for any pages that are present now,
         // but not in the snapshot.
@@ -186,7 +177,17 @@
                     return;
                 }
                 final int pageSize = mConfig.pageSize;
-                mDataSource.loadRange(pageIndex * pageSize, pageSize, mReceiver);
+
+                if (mDataSource.isInvalid()) {
+                    detach();
+                } else {
+                    int startPosition = pageIndex * pageSize;
+                    int count = Math.min(pageSize, mStorage.size() - startPosition);
+                    DataSource.LoadCallback<T> callback = new DataSource.LoadCallback<>(
+                            PageResult.TILE, mMainThreadExecutor, mDataSource, mReceiver);
+                    callback.setPositionOffset(startPosition);
+                    mDataSource.loadRange(startPosition, count, callback);
+                }
             }
         });
     }
diff --git a/paging/common/src/test/java/android/arch/paging/AsyncListDataSource.kt b/paging/common/src/test/java/android/arch/paging/AsyncListDataSource.kt
new file mode 100644
index 0000000..48d6e29
--- /dev/null
+++ b/paging/common/src/test/java/android/arch/paging/AsyncListDataSource.kt
@@ -0,0 +1,41 @@
+/*
+ * Copyright 2017 The Android Open Source Project
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package android.arch.paging
+
+class AsyncListDataSource<T>(list: List<T>)
+    : PositionalDataSource<T>() {
+    val workItems: MutableList<() -> Unit> = ArrayList()
+    private val listDataSource = ListDataSource(list)
+
+    override fun loadInitial(requestedStartPosition: Int, requestedLoadSize: Int, pageSize: Int,
+            callback: InitialLoadCallback<T>) {
+        workItems.add {
+            listDataSource.loadInitial(requestedStartPosition, requestedLoadSize, pageSize, callback)
+        }
+    }
+
+    override fun loadRange(startPosition: Int, count: Int, callback: LoadCallback<T>) {
+        workItems.add {
+            listDataSource.loadRange(startPosition, count, callback)
+        }
+    }
+
+    fun flush() {
+        workItems.map { it() }
+        workItems.clear()
+    }
+}
diff --git a/paging/common/src/test/java/android/arch/paging/ContiguousPagedListTest.kt b/paging/common/src/test/java/android/arch/paging/ContiguousPagedListTest.kt
index 7331bfe..5ae0f9f 100644
--- a/paging/common/src/test/java/android/arch/paging/ContiguousPagedListTest.kt
+++ b/paging/common/src/test/java/android/arch/paging/ContiguousPagedListTest.kt
@@ -18,17 +18,16 @@
 
 import org.junit.Assert.assertArrayEquals
 import org.junit.Assert.assertEquals
+import org.junit.Assert.assertFalse
+import org.junit.Assert.assertTrue
 import org.junit.Test
 import org.junit.runner.RunWith
 import org.junit.runners.Parameterized
-import org.mockito.ArgumentCaptor
-import org.mockito.Mockito.any
-import org.mockito.Mockito.eq
 import org.mockito.Mockito.mock
 import org.mockito.Mockito.verify
 import org.mockito.Mockito.verifyNoMoreInteractions
 import org.mockito.Mockito.verifyZeroInteractions
-import java.util.Collections
+
 
 @RunWith(Parameterized::class)
 class ContiguousPagedListTest(private val mCounted: Boolean) {
@@ -44,33 +43,46 @@
     }
 
     private inner class TestSource(val listData: List<Item> = ITEMS)
-            : PositionalDataSource<Item>() {
-        override fun countItems(): Int {
-            return if (mCounted) {
-                listData.size
+            : ContiguousDataSource<Int, Item>() {
+        override fun loadInitial(key: Int?, initialLoadSize: Int,
+                enablePlaceholders: Boolean, callback: InitialLoadCallback<Item>) {
+
+            val convertPosition = key ?: 0
+            val loadPosition = Math.max(0, (convertPosition - initialLoadSize / 2))
+
+            val data = getClampedRange(loadPosition, loadPosition + initialLoadSize)
+
+
+            if (enablePlaceholders && mCounted) {
+                callback.onResult(data, loadPosition, listData.size)
             } else {
-                DataSource.COUNT_UNDEFINED
+                // still must pass offset, even if not counted
+                callback.onResult(data, loadPosition)
             }
         }
 
-        private fun getClampedRange(startInc: Int, endExc: Int, reverse: Boolean): List<Item> {
-            val list = listData.subList(Math.max(0, startInc), Math.min(listData.size, endExc))
-            if (reverse) {
-                Collections.reverse(list)
-            }
-            return list
+        override fun loadAfter(currentEndIndex: Int, currentEndItem: Item, pageSize: Int,
+                callback: LoadCallback<Item>) {
+            val startIndex = currentEndIndex + 1
+            callback.onResult(getClampedRange(startIndex, startIndex + pageSize))
         }
 
-        override fun loadAfter(startIndex: Int, pageSize: Int): List<Item>? {
-            return getClampedRange(startIndex, startIndex + pageSize, false)
+        override fun loadBefore(currentBeginIndex: Int, currentBeginItem: Item, pageSize: Int,
+                callback: LoadCallback<Item>) {
+            val startIndex = currentBeginIndex - 1
+            callback.onResult(getClampedRange(startIndex - pageSize + 1, startIndex + 1))
         }
 
-        override fun loadBefore(startIndex: Int, pageSize: Int): List<Item>? {
-            return getClampedRange(startIndex - pageSize + 1, startIndex + 1, true)
+        override fun getKey(position: Int, item: Item?): Int {
+            return 0
+        }
+
+        private fun getClampedRange(startInc: Int, endExc: Int): List<Item> {
+            return listData.subList(Math.max(0, startInc), Math.min(listData.size, endExc))
         }
     }
 
-    private fun verifyRange(start: Int, count: Int, actual: PagedStorage<*, Item>) {
+    private fun verifyRange(start: Int, count: Int, actual: PagedStorage<Item>) {
         if (mCounted) {
             // assert nulls + content
             val expected = arrayOfNulls<Item>(ITEMS.size)
@@ -98,41 +110,6 @@
         verifyRange(start, count, actual.mStorage)
     }
 
-    private fun verifyRange(start: Int, count: Int, actual: PageResult<Int, Item>) {
-        if (mCounted) {
-            assertEquals(start, actual.leadingNulls)
-            assertEquals(ITEMS.size - start - count, actual.trailingNulls)
-            assertEquals(0, actual.positionOffset)
-        } else {
-            assertEquals(0, actual.leadingNulls)
-            assertEquals(0, actual.trailingNulls)
-            assertEquals(start, actual.positionOffset)
-        }
-        assertEquals(ITEMS.subList(start, start + count), actual.page.items)
-    }
-
-    private fun verifyInitialLoad(start: Int, count : Int, initialPosition: Int, initialLoadSize: Int) {
-        @Suppress("UNCHECKED_CAST")
-        val receiver = mock(PageResult.Receiver::class.java) as PageResult.Receiver<Int, Item>
-
-        @Suppress("UNCHECKED_CAST")
-        val captor = ArgumentCaptor.forClass(PageResult::class.java)
-                as ArgumentCaptor<PageResult<Int, Item>>
-
-        TestSource().loadInitial(initialPosition, initialLoadSize, true, receiver)
-
-        verify(receiver).onPageResult(captor.capture())
-        verifyNoMoreInteractions(receiver)
-        verifyRange(start, count, captor.value)
-    }
-
-    @Test
-    fun initialLoad() {
-        verifyInitialLoad(30, 40, 50, 40)
-        verifyInitialLoad(0, 10, 5, 10)
-        verifyInitialLoad(90, 10, 95, 10)
-    }
-
     private fun createCountedPagedList(
             initialPosition: Int,
             pageSize: Int = 20,
@@ -182,7 +159,6 @@
         verifyNoMoreInteractions(callback)
     }
 
-
     @Test
     fun prepend() {
         val pagedList = createCountedPagedList(80)
@@ -311,6 +287,60 @@
     }
 
     @Test
+    fun initialLoadAsync() {
+        // Note: ignores Parameterized param
+        val asyncDataSource = AsyncListDataSource(ITEMS)
+        val dataSource = asyncDataSource.wrapAsContiguousWithoutPlaceholders()
+        val pagedList = ContiguousPagedList(
+                dataSource, mMainThread, mBackgroundThread, null,
+                PagedList.Config.Builder().setPageSize(10).build(), null)
+
+        assertTrue(pagedList.isEmpty())
+        drain()
+        assertTrue(pagedList.isEmpty())
+        asyncDataSource.flush()
+        assertTrue(pagedList.isEmpty())
+        mBackgroundThread.executeAll()
+        assertTrue(pagedList.isEmpty())
+
+        // Data source defers callbacks until flush, which posts result to main thread
+        mMainThread.executeAll()
+        assertFalse(pagedList.isEmpty())
+
+    }
+
+    @Test
+    fun addWeakCallbackEmpty() {
+        // Note: ignores Parameterized param
+        val asyncDataSource = AsyncListDataSource(ITEMS)
+        val dataSource = asyncDataSource.wrapAsContiguousWithoutPlaceholders()
+        val pagedList = ContiguousPagedList(
+                dataSource, mMainThread, mBackgroundThread, null,
+                PagedList.Config.Builder().setPageSize(10).build(), null)
+        val callback = mock(PagedList.Callback::class.java)
+
+        // capture empty snapshot
+        val emptySnapshot = pagedList.snapshot()
+        assertTrue(pagedList.isEmpty())
+        assertTrue(emptySnapshot.isEmpty())
+
+        // verify that adding callback notifies nothing going from empty -> empty
+        pagedList.addWeakCallback(emptySnapshot, callback)
+        verifyZeroInteractions(callback)
+        pagedList.removeWeakCallback(callback)
+
+        // data added in asynchronously
+        asyncDataSource.flush()
+        drain()
+        assertFalse(pagedList.isEmpty())
+
+        // verify that adding callback notifies insert going from empty -> content
+        pagedList.addWeakCallback(emptySnapshot, callback)
+        verify(callback).onInserted(0, pagedList.size)
+        verifyNoMoreInteractions(callback)
+    }
+
+    @Test
     fun boundaryCallback_empty() {
         @Suppress("UNCHECKED_CAST")
         val boundaryCallback =
@@ -347,8 +377,7 @@
         pagedList.loadAround(if (mCounted) 99 else 19)
         drain()
         verifyRange(80, 20, pagedList)
-        verify(boundaryCallback).onItemAtEndLoaded(
-                any(), eq(ITEMS.last()), eq(if (mCounted) 100 else 20))
+        verify(boundaryCallback).onItemAtEndLoaded(ITEMS.last())
         verifyNoMoreInteractions(boundaryCallback)
 
 
@@ -371,7 +400,7 @@
         // ... finally try prepend, see 0 items, which will dispatch front callback
         pagedList.loadAround(0)
         drain()
-        verify(boundaryCallback).onItemAtFrontLoaded(any(), eq(ITEMS.first()), eq(100))
+        verify(boundaryCallback).onItemAtFrontLoaded(ITEMS.first())
         verifyNoMoreInteractions(boundaryCallback)
     }
 
diff --git a/paging/common/src/test/java/android/arch/paging/KeyedDataSourceTest.kt b/paging/common/src/test/java/android/arch/paging/KeyedDataSourceTest.kt
index 115fea2..e6c95e5 100644
--- a/paging/common/src/test/java/android/arch/paging/KeyedDataSourceTest.kt
+++ b/paging/common/src/test/java/android/arch/paging/KeyedDataSourceTest.kt
@@ -16,7 +16,6 @@
 
 package android.arch.paging
 
-import org.junit.Assert.assertArrayEquals
 import org.junit.Assert.assertEquals
 import org.junit.Assert.assertNotNull
 import org.junit.Assert.assertTrue
@@ -24,6 +23,7 @@
 import org.junit.runner.RunWith
 import org.junit.runners.JUnit4
 import org.mockito.ArgumentCaptor
+import org.mockito.ArgumentMatchers.anyInt
 import org.mockito.Mockito.mock
 import org.mockito.Mockito.verify
 import org.mockito.Mockito.verifyNoMoreInteractions
@@ -33,31 +33,19 @@
 
     // ----- STANDARD -----
 
-    @Test
-    fun loadInitial_validateTestDataSource() {
-        val dataSource = ItemDataSource()
-
-        // all
-        assertEquals(ITEMS_BY_NAME_ID, dataSource.loadInitial(ITEMS_BY_NAME_ID.size))
-
-        // 10
-        assertEquals(ITEMS_BY_NAME_ID.subList(0, 10), dataSource.loadInitial(10))
-
-        // too many
-        assertEquals(ITEMS_BY_NAME_ID, dataSource.loadInitial(ITEMS_BY_NAME_ID.size + 10))
-    }
-
     private fun loadInitial(dataSource: ItemDataSource, key: Key?, initialLoadSize: Int,
-            enablePlaceholders: Boolean): PageResult<Key, Item> {
+            enablePlaceholders: Boolean): PageResult<Item> {
         @Suppress("UNCHECKED_CAST")
-        val receiver = mock(PageResult.Receiver::class.java) as PageResult.Receiver<Key, Item>
+        val receiver = mock(PageResult.Receiver::class.java) as PageResult.Receiver<Item>
         @Suppress("UNCHECKED_CAST")
         val captor = ArgumentCaptor.forClass(PageResult::class.java)
-                as ArgumentCaptor<PageResult<Key, Item>>
+                as ArgumentCaptor<PageResult<Item>>
 
-        dataSource.loadInitial(key, initialLoadSize, enablePlaceholders, receiver)
+        val callback = DataSource.InitialLoadCallback(true, dataSource, receiver)
 
-        verify(receiver).onPageResult(captor.capture())
+        dataSource.loadInitial(key, initialLoadSize, enablePlaceholders, callback)
+
+        verify(receiver).onPageResult(anyInt(), captor.capture())
         verifyNoMoreInteractions(receiver)
         assertNotNull(captor.value)
         return captor.value
@@ -69,7 +57,7 @@
         val result = loadInitial(dataSource, dataSource.getKey(ITEMS_BY_NAME_ID[49]), 10, true)
 
         assertEquals(45, result.leadingNulls)
-        assertEquals(ITEMS_BY_NAME_ID.subList(45, 55), result.page.items)
+        assertEquals(ITEMS_BY_NAME_ID.subList(45, 55), result.page)
         assertEquals(45, result.trailingNulls)
     }
 
@@ -81,7 +69,7 @@
         val result = loadInitial(dataSource, dataSource.getKey(ITEMS_BY_NAME_ID[0]), 20, true)
 
         assertEquals(0, result.leadingNulls)
-        assertEquals(ITEMS_BY_NAME_ID.subList(0, 1), result.page.items)
+        assertEquals(ITEMS_BY_NAME_ID.subList(0, 1), result.page)
         assertEquals(0, result.trailingNulls)
     }
 
@@ -93,8 +81,8 @@
         val key = dataSource.getKey(ITEMS_BY_NAME_ID.last())
         val result = loadInitial(dataSource, key, 20, true)
 
-        assertEquals(89, result.leadingNulls)
-        assertEquals(ITEMS_BY_NAME_ID.subList(89, 100), result.page.items)
+        assertEquals(90, result.leadingNulls)
+        assertEquals(ITEMS_BY_NAME_ID.subList(90, 100), result.page)
         assertEquals(0, result.trailingNulls)
     }
 
@@ -106,7 +94,7 @@
         val result = loadInitial(dataSource, null, 10, true)
 
         assertEquals(0, result.leadingNulls)
-        assertEquals(ITEMS_BY_NAME_ID.subList(0, 10), result.page.items)
+        assertEquals(ITEMS_BY_NAME_ID.subList(0, 10), result.page)
         assertEquals(90, result.trailingNulls)
     }
 
@@ -122,7 +110,7 @@
         // do: load after was empty, so pass full size to load before, since this can incur larger
         // loads than requested (see keyMatchesLastItem test)
         assertEquals(95, result.leadingNulls)
-        assertEquals(ITEMS_BY_NAME_ID.subList(95, 100), result.page.items)
+        assertEquals(ITEMS_BY_NAME_ID.subList(95, 100), result.page)
         assertEquals(0, result.trailingNulls)
     }
 
@@ -137,7 +125,7 @@
         val result = loadInitial(dataSource, key, 10, false)
 
         assertEquals(0, result.leadingNulls)
-        assertEquals(ITEMS_BY_NAME_ID.subList(45, 55), result.page.items)
+        assertEquals(ITEMS_BY_NAME_ID.subList(45, 55), result.page)
         assertEquals(0, result.trailingNulls)
     }
 
@@ -150,7 +138,7 @@
         val result = loadInitial(dataSource, key, 10, true)
 
         assertEquals(0, result.leadingNulls)
-        assertEquals(ITEMS_BY_NAME_ID.subList(45, 55), result.page.items)
+        assertEquals(ITEMS_BY_NAME_ID.subList(45, 55), result.page)
         assertEquals(0, result.trailingNulls)
     }
 
@@ -162,7 +150,7 @@
         val result = loadInitial(dataSource, null, 10, true)
 
         assertEquals(0, result.leadingNulls)
-        assertEquals(ITEMS_BY_NAME_ID.subList(0, 10), result.page.items)
+        assertEquals(ITEMS_BY_NAME_ID.subList(0, 10), result.page)
         assertEquals(0, result.trailingNulls)
     }
 
@@ -177,7 +165,7 @@
         val result = loadInitial(dataSource, key, 10, true)
 
         assertEquals(0, result.leadingNulls)
-        assertTrue(result.page.items.isEmpty())
+        assertTrue(result.page.isEmpty())
         assertEquals(0, result.trailingNulls)
     }
 
@@ -187,7 +175,7 @@
         val result = loadInitial(dataSource, null, 10, true)
 
         assertEquals(0, result.leadingNulls)
-        assertTrue(result.page.items.isEmpty())
+        assertTrue(result.page.isEmpty())
         assertEquals(0, result.trailingNulls)
     }
 
@@ -197,21 +185,18 @@
     fun loadBefore() {
         val dataSource = ItemDataSource()
         @Suppress("UNCHECKED_CAST")
-        val receiver = mock(PageResult.Receiver::class.java)
-                as PageResult.Receiver<Key, Item>
+        val callback = mock(DataSource.LoadCallback::class.java) as DataSource.LoadCallback<Item>
 
-        dataSource.loadBefore(5, ITEMS_BY_NAME_ID[5], 5, receiver)
+        dataSource.loadBefore(5, ITEMS_BY_NAME_ID[5], 5, callback)
 
         @Suppress("UNCHECKED_CAST")
-        val argument = ArgumentCaptor.forClass(PageResult::class.java)
-                as ArgumentCaptor<PageResult<Key, Item>>
-        verify(receiver).postOnPageResult(argument.capture())
-        verifyNoMoreInteractions(receiver)
+        val argument = ArgumentCaptor.forClass(List::class.java) as ArgumentCaptor<List<Item>>
+        verify(callback).onResult(argument.capture())
+        verifyNoMoreInteractions(callback)
 
         val observed = argument.value
 
-        assertEquals(PageResult.PREPEND, observed.type)
-        assertEquals(ITEMS_BY_NAME_ID.subList(0, 5), observed.page.items)
+        assertEquals(ITEMS_BY_NAME_ID.subList(0, 5), observed)
     }
 
     internal data class Key(val name: String, val id: Int)
@@ -219,62 +204,53 @@
     internal data class Item(
             val name: String, val id: Int, val balance: Double, val address: String)
 
-    internal class ItemDataSource(val counted: Boolean = true,
-                                  val items: List<Item> = ITEMS_BY_NAME_ID)
+    internal class ItemDataSource(private val counted: Boolean = true,
+                                  private val items: List<Item> = ITEMS_BY_NAME_ID)
             : KeyedDataSource<Key, Item>() {
+        override fun loadInitial(initialLoadKey: Key?, initialLoadSize: Int,
+                enablePlaceholders: Boolean, callback: InitialLoadCallback<Item>) {
+            val key = initialLoadKey ?: Key("", Integer.MAX_VALUE)
+            val start = Math.max(0, findFirstIndexAfter(key) - initialLoadSize / 2)
+            val endExclusive = Math.min(start + initialLoadSize, items.size)
+
+
+            if (enablePlaceholders && counted) {
+                callback.onResult(items.subList(start, endExclusive), start, items.size)
+            } else {
+                callback.onResult(items.subList(start, endExclusive))
+            }
+        }
+
+        override fun loadAfter(currentEndKey: Key, pageSize: Int, callback: LoadCallback<Item>) {
+            val start = findFirstIndexAfter(currentEndKey)
+            val endExclusive = Math.min(start + pageSize, items.size)
+
+            callback.onResult(items.subList(start, endExclusive))
+        }
+
+        override fun loadBefore(currentBeginKey: Key, pageSize: Int, callback: LoadCallback<Item>) {
+            val firstIndexBefore = findFirstIndexBefore(currentBeginKey)
+            val endExclusive = Math.max(0, firstIndexBefore + 1)
+            val start = Math.max(0, firstIndexBefore - pageSize + 1)
+
+            callback.onResult(items.subList(start, endExclusive))
+        }
 
         override fun getKey(item: Item): Key {
             return Key(item.name, item.id)
         }
 
-        override fun loadInitial(pageSize: Int): List<Item>? {
-            // call loadAfter with a default key
-            return loadAfter(Key("", Integer.MAX_VALUE), pageSize)
-        }
-
-        fun findFirstIndexAfter(key: Key): Int {
+        private fun findFirstIndexAfter(key: Key): Int {
             return items.indices.firstOrNull {
                 KEY_COMPARATOR.compare(key, getKey(items[it])) < 0
             } ?: items.size
         }
 
-        fun findFirstIndexBefore(key: Key): Int {
+        private fun findFirstIndexBefore(key: Key): Int {
             return items.indices.reversed().firstOrNull {
                 KEY_COMPARATOR.compare(key, getKey(items[it])) > 0
             } ?: -1
         }
-
-        override fun countItemsBefore(key: Key): Int {
-            if (!counted) {
-                return DataSource.COUNT_UNDEFINED
-            }
-
-            return findFirstIndexBefore(key) + 1
-        }
-
-        override fun countItemsAfter(key: Key): Int {
-            if (!counted) {
-                return DataSource.COUNT_UNDEFINED
-            }
-
-            return items.size - findFirstIndexAfter(key)
-        }
-
-        override fun loadAfter(key: Key, pageSize: Int): List<Item>? {
-            val start = findFirstIndexAfter(key)
-            val endExclusive = Math.min(start + pageSize, items.size)
-
-            return items.subList(start, endExclusive)
-        }
-
-        override fun loadBefore(key: Key, pageSize: Int): List<Item>? {
-            val firstIndexBefore = findFirstIndexBefore(key)
-            val endExclusive = Math.max(0, firstIndexBefore + 1)
-            val start = Math.max(0, firstIndexBefore - pageSize + 1)
-
-            val list = items.subList(start, endExclusive)
-            return list.reversed()
-        }
     }
 
     companion object {
diff --git a/paging/common/src/test/java/android/arch/paging/PagedStorageTest.kt b/paging/common/src/test/java/android/arch/paging/PagedStorageTest.kt
index 92b6c87..8fb54e0 100644
--- a/paging/common/src/test/java/android/arch/paging/PagedStorageTest.kt
+++ b/paging/common/src/test/java/android/arch/paging/PagedStorageTest.kt
@@ -29,8 +29,8 @@
 
 @RunWith(JUnit4::class)
 class PagedStorageTest {
-    private fun createPage(vararg strings: String): Page<Int, String> {
-        return Page(strings.asList())
+    private fun createPage(vararg strings: String): List<String> {
+        return strings.asList()
     }
 
     @Test
@@ -216,7 +216,7 @@
     @Test
     fun insertOne() {
         val callback = mock(PagedStorage.Callback::class.java)
-        val storage = PagedStorage<Int, String>()
+        val storage = PagedStorage<String>()
 
         storage.init(2, createPage("c", "d"), 3, 0, callback)
 
@@ -236,7 +236,7 @@
     @Test
     fun insertThree() {
         val callback = mock(PagedStorage.Callback::class.java)
-        val storage = PagedStorage<Int, String>()
+        val storage = PagedStorage<String>()
 
         storage.init(2, createPage("c", "d"), 3, 0, callback)
 
@@ -273,7 +273,7 @@
     @Test
     fun insertLastFirst() {
         val callback = mock(PagedStorage.Callback::class.java)
-        val storage = PagedStorage<Int, String>()
+        val storage = PagedStorage<String>()
 
         storage.init(6, createPage("g"), 0, 0, callback)
 
@@ -294,7 +294,7 @@
     @Test(expected = IllegalArgumentException::class)
     fun insertFailure_decreaseLast() {
         val callback = mock(PagedStorage.Callback::class.java)
-        val storage = PagedStorage<Int, String>()
+        val storage = PagedStorage<String>()
 
         storage.init(2, createPage("c", "d"), 0, 0, callback)
 
@@ -305,7 +305,7 @@
     @Test(expected = IllegalArgumentException::class)
     fun insertFailure_increase() {
         val callback = mock(PagedStorage.Callback::class.java)
-        val storage = PagedStorage<Int, String>()
+        val storage = PagedStorage<String>()
 
         storage.init(0, createPage("a", "b"), 3, 0, callback)
 
@@ -316,7 +316,7 @@
     @Test
     fun allocatePlaceholders_simple() {
         val callback = mock(PagedStorage.Callback::class.java)
-        val storage = PagedStorage<Int, String>()
+        val storage = PagedStorage<String>()
 
         storage.init(2, createPage("c"), 2, 0, callback)
 
@@ -334,7 +334,7 @@
     @Test
     fun allocatePlaceholders_adoptPageSize() {
         val callback = mock(PagedStorage.Callback::class.java)
-        val storage = PagedStorage<Int, String>()
+        val storage = PagedStorage<String>()
 
         storage.init(4, createPage("e"), 0, 0, callback)
 
@@ -352,7 +352,7 @@
     @Test(expected = IllegalArgumentException::class)
     fun allocatePlaceholders_cannotShrinkPageSize() {
         val callback = mock(PagedStorage.Callback::class.java)
-        val storage = PagedStorage<Int, String>()
+        val storage = PagedStorage<String>()
 
         storage.init(4, createPage("e", "f"), 0, 0, callback)
 
@@ -365,7 +365,7 @@
     @Test(expected = IllegalArgumentException::class)
     fun allocatePlaceholders_cannotAdoptPageSize() {
         val callback = mock(PagedStorage.Callback::class.java)
-        val storage = PagedStorage<Int, String>()
+        val storage = PagedStorage<String>()
 
         storage.init(2, createPage("c", "d"), 2, 0, callback)
 
@@ -377,7 +377,7 @@
     @Test
     fun get_placeholdersMulti() {
         val callback = mock(PagedStorage.Callback::class.java)
-        val storage = PagedStorage<Int, String>()
+        val storage = PagedStorage<String>()
 
         storage.init(2, createPage("c", "d"), 3, 0, callback)
 
@@ -392,7 +392,7 @@
     @Test
     fun hasPage() {
         val callback = mock(PagedStorage.Callback::class.java)
-        val storage = PagedStorage<Int, String>()
+        val storage = PagedStorage<String>()
 
         storage.init(4, createPage("e"), 0, 0, callback)
 
diff --git a/paging/common/src/test/java/android/arch/paging/PositionalDataSourceTest.kt b/paging/common/src/test/java/android/arch/paging/PositionalDataSourceTest.kt
new file mode 100644
index 0000000..34e0a57
--- /dev/null
+++ b/paging/common/src/test/java/android/arch/paging/PositionalDataSourceTest.kt
@@ -0,0 +1,50 @@
+/*
+ * Copyright 2017 The Android Open Source Project
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package android.arch.paging
+
+import org.junit.Assert.assertEquals
+import org.junit.Test
+import org.junit.runner.RunWith
+import org.junit.runners.JUnit4
+
+@RunWith(JUnit4::class)
+class PositionalDataSourceTest {
+    @Test
+    fun computeFirstLoadPositionZero() {
+        assertEquals(0, PositionalDataSource.computeFirstLoadPosition(0, 30, 10, 100))
+    }
+
+    @Test
+    fun computeFirstLoadPositionRequestedPositionIncluded() {
+        assertEquals(10, PositionalDataSource.computeFirstLoadPosition(10, 10, 10, 100))
+    }
+
+    @Test
+    fun computeFirstLoadPositionRound() {
+        assertEquals(10, PositionalDataSource.computeFirstLoadPosition(13, 30, 10, 100))
+    }
+
+    @Test
+    fun computeFirstLoadPositionEndAdjusted() {
+        assertEquals(70, PositionalDataSource.computeFirstLoadPosition(99, 30, 10, 100))
+    }
+
+    @Test
+    fun computeFirstLoadPositionEndAdjustedAndAligned() {
+        assertEquals(70, PositionalDataSource.computeFirstLoadPosition(99, 35, 10, 100))
+    }
+}
diff --git a/paging/common/src/test/java/android/arch/paging/TiledDataSourceTest.kt b/paging/common/src/test/java/android/arch/paging/TiledDataSourceTest.kt
index 2b16fb2..b3ffdfb 100644
--- a/paging/common/src/test/java/android/arch/paging/TiledDataSourceTest.kt
+++ b/paging/common/src/test/java/android/arch/paging/TiledDataSourceTest.kt
@@ -1,45 +1,86 @@
+/*
+ * Copyright 2017 The Android Open Source Project
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
 package android.arch.paging
 
-import org.junit.Assert.assertEquals
+import junit.framework.Assert.assertEquals
 import org.junit.Test
 import org.junit.runner.RunWith
 import org.junit.runners.JUnit4
 import org.mockito.ArgumentCaptor
+import org.mockito.ArgumentMatchers.eq
 import org.mockito.Mockito.mock
 import org.mockito.Mockito.verify
 import org.mockito.Mockito.verifyNoMoreInteractions
 import java.util.Collections
 
-
+@Suppress("DEPRECATION")
 @RunWith(JUnit4::class)
 class TiledDataSourceTest {
-    @Test
-    fun loadInitialEmpty() {
+
+    fun TiledDataSource<String>.loadInitial(startPosition: Int, count: Int, pageSize: Int)
+            : List<String> {
         @Suppress("UNCHECKED_CAST")
-        val receiver = mock(PageResult.Receiver::class.java) as PageResult.Receiver<Int, String>
-        val dataSource = EmptyDataSource()
-        dataSource.loadRangeInitial(0, 0, 1, 0, receiver)
+        val receiver = mock(PageResult.Receiver::class.java) as PageResult.Receiver<String>
+
+        val callback = DataSource.InitialLoadCallback(true, this, receiver)
+
+        this.loadInitial(startPosition, count, pageSize, callback)
 
         @Suppress("UNCHECKED_CAST")
         val argument = ArgumentCaptor.forClass(PageResult::class.java)
-                as ArgumentCaptor<PageResult<Int, String>>
-        verify(receiver).onPageResult(argument.capture())
+                as ArgumentCaptor<PageResult<String>>
+        verify(receiver).onPageResult(eq(PageResult.INIT), argument.capture())
         verifyNoMoreInteractions(receiver)
 
         val observed = argument.value
 
-        assertEquals(PageResult.INIT, observed.type)
-        assertEquals(Collections.EMPTY_LIST, observed.page.items)
+        return observed.page
     }
 
-    class EmptyDataSource : TiledDataSource<String>() {
-        override fun countItems(): Int {
-            return 0
+    @Test
+    fun loadInitialEmpty() {
+        class EmptyDataSource : TiledDataSource<String>() {
+            override fun countItems(): Int {
+                return 0
+            }
+
+            override fun loadRange(startPosition: Int, count: Int): List<String> {
+                return emptyList()
+            }
         }
 
-        override fun loadRange(startPosition: Int, count: Int): List<String> {
-            @Suppress("UNCHECKED_CAST")
-            return Collections.EMPTY_LIST as List<String>
-        }
+        assertEquals(Collections.EMPTY_LIST, EmptyDataSource().loadInitial(0, 1, 5))
     }
-}
\ No newline at end of file
+
+    @Test
+    fun loadInitialTooLong() {
+        val list = List(26) { "" + 'a' + it}
+        class AlphabetDataSource : TiledDataSource<String>() {
+            override fun countItems(): Int {
+                return list.size
+            }
+
+            override fun loadRange(startPosition: Int, count: Int): List<String> {
+                return list.subList(startPosition, startPosition + count)
+            }
+        }
+        // baseline behavior
+        assertEquals(list, AlphabetDataSource().loadInitial(0, 26, 10))
+        assertEquals(emptyList<String>(), AlphabetDataSource().loadInitial(0, 0, 10))
+        assertEquals(list, AlphabetDataSource().loadInitial(50, 26, 10))
+    }
+}
diff --git a/paging/common/src/test/java/android/arch/paging/TiledPagedListTest.kt b/paging/common/src/test/java/android/arch/paging/TiledPagedListTest.kt
index 6524008..10996a5 100644
--- a/paging/common/src/test/java/android/arch/paging/TiledPagedListTest.kt
+++ b/paging/common/src/test/java/android/arch/paging/TiledPagedListTest.kt
@@ -17,14 +17,13 @@
 package android.arch.paging
 
 import org.junit.Assert.assertEquals
+import org.junit.Assert.assertFalse
 import org.junit.Assert.assertNull
 import org.junit.Assert.assertSame
 import org.junit.Assert.assertTrue
 import org.junit.Test
 import org.junit.runner.RunWith
 import org.junit.runners.JUnit4
-import org.mockito.Mockito.any
-import org.mockito.Mockito.eq
 import org.mockito.Mockito.mock
 import org.mockito.Mockito.verify
 import org.mockito.Mockito.verifyNoMoreInteractions
@@ -43,12 +42,12 @@
         }
     }
 
-    private fun verifyLoadedPages(list: List<Item>, vararg loadedPages: Int, expected: List<Item> = ITEMS) {
+    private fun verifyLoadedPages(list: List<Item>, vararg loadedPages: Int) {
         val loadedPageList = loadedPages.asList()
-        assertEquals(expected.size, list.size)
+        assertEquals(ITEMS.size, list.size)
         for (i in list.indices) {
             if (loadedPageList.contains(i / PAGE_SIZE)) {
-                assertSame("Index $i", expected[i], list[i])
+                assertSame("Index $i", ITEMS[i], list[i])
             } else {
                 assertNull("Index $i", list[i])
             }
@@ -70,21 +69,6 @@
     }
 
     @Test
-    fun computeFirstLoadPosition_zero() {
-        assertEquals(0, TiledPagedList.computeFirstLoadPosition(0, 30, 10, 100))
-    }
-
-    @Test
-    fun computeFirstLoadPosition_requestedPositionIncluded() {
-        assertEquals(0, TiledPagedList.computeFirstLoadPosition(10, 10, 10, 100))
-    }
-
-    @Test
-    fun computeFirstLoadPosition_endAdjusted() {
-        assertEquals(70, TiledPagedList.computeFirstLoadPosition(99, 30, 10, 100))
-    }
-
-    @Test
     fun initialLoad_onePage() {
         val pagedList = createTiledPagedList(loadPosition = 0, initPageCount = 1)
         verifyLoadedPages(pagedList, 0, 1)
@@ -121,6 +105,56 @@
     }
 
     @Test
+    fun initialLoad_initializesLastKey() {
+        val pagedList = createTiledPagedList(loadPosition = 44, initPageCount = 2)
+        assertEquals(44, pagedList.lastKey)
+    }
+
+    @Test
+    fun initialLoadAsync() {
+        val dataSource = AsyncListDataSource(ITEMS)
+        val pagedList = TiledPagedList(
+                dataSource, mMainThread, mBackgroundThread, null,
+                PagedList.Config.Builder().setPageSize(10).build(), 0)
+
+        assertTrue(pagedList.isEmpty())
+        drain()
+        assertTrue(pagedList.isEmpty())
+        dataSource.flush()
+        assertTrue(pagedList.isEmpty())
+        mBackgroundThread.executeAll()
+        assertTrue(pagedList.isEmpty())
+
+        // Data source defers callbacks until flush, which posts result to main thread
+        mMainThread.executeAll()
+        assertFalse(pagedList.isEmpty())
+    }
+
+    @Test
+    fun addWeakCallbackEmpty() {
+        val dataSource = AsyncListDataSource(ITEMS)
+        val pagedList = TiledPagedList(
+                dataSource, mMainThread, mBackgroundThread, null,
+                PagedList.Config.Builder().setPageSize(10).build(), 0)
+
+        // capture empty snapshot
+        val emptySnapshot = pagedList.snapshot()
+        assertTrue(pagedList.isEmpty())
+        assertTrue(emptySnapshot.isEmpty())
+
+        // data added in asynchronously
+        dataSource.flush()
+        drain()
+        assertFalse(pagedList.isEmpty())
+
+        // verify that adding callback works with empty start point
+        val callback = mock(PagedList.Callback::class.java)
+        pagedList.addWeakCallback(emptySnapshot, callback)
+        verify(callback).onInserted(0, pagedList.size)
+        verifyNoMoreInteractions(callback)
+    }
+
+    @Test
     fun append() {
         val pagedList = createTiledPagedList(loadPosition = 0, initPageCount = 1)
         val callback = mock(PagedList.Callback::class.java)
@@ -305,8 +339,8 @@
 
         // callbacks posted, since creation often happens on BG thread
         drain()
-        verify(boundaryCallback).onItemAtFrontLoaded(any(), eq(ITEMS[0]), eq(2))
-        verify(boundaryCallback).onItemAtEndLoaded(any(), eq(ITEMS[1]), eq(2))
+        verify(boundaryCallback).onItemAtFrontLoaded(ITEMS[0])
+        verify(boundaryCallback).onItemAtEndLoaded(ITEMS[1])
         verifyNoMoreInteractions(boundaryCallback)
     }
 
@@ -332,8 +366,8 @@
 
         drain()
         // first/last items loaded now, so callbacks dispatched
-        verify(boundaryCallback).onItemAtFrontLoaded(any(), eq(ITEMS.first()), eq(45))
-        verify(boundaryCallback).onItemAtEndLoaded(any(), eq(ITEMS.last()), eq(45))
+        verify(boundaryCallback).onItemAtFrontLoaded(ITEMS.first())
+        verify(boundaryCallback).onItemAtEndLoaded(ITEMS.last())
         verifyNoMoreInteractions(boundaryCallback)
     }
 
@@ -360,8 +394,8 @@
         drain()
 
         // items accessed, so now posted callbacks are run
-        verify(boundaryCallback).onItemAtFrontLoaded(any(), eq(ITEMS.first()), eq(45))
-        verify(boundaryCallback).onItemAtEndLoaded(any(), eq(ITEMS.last()), eq(45))
+        verify(boundaryCallback).onItemAtFrontLoaded(ITEMS.first())
+        verify(boundaryCallback).onItemAtEndLoaded(ITEMS.last())
         verifyNoMoreInteractions(boundaryCallback)
     }
 
diff --git a/paging/integration-tests/testapp/src/main/java/android/arch/paging/integration/testapp/ItemDataSource.java b/paging/integration-tests/testapp/src/main/java/android/arch/paging/integration/testapp/ItemDataSource.java
index 4690533..bbbfabb 100644
--- a/paging/integration-tests/testapp/src/main/java/android/arch/paging/integration/testapp/ItemDataSource.java
+++ b/paging/integration-tests/testapp/src/main/java/android/arch/paging/integration/testapp/ItemDataSource.java
@@ -16,9 +16,10 @@
 
 package android.arch.paging.integration.testapp;
 
-import android.arch.paging.BoundedDataSource;
+import android.arch.paging.PositionalDataSource;
 import android.graphics.Color;
 import android.support.annotation.ColorInt;
+import android.support.annotation.NonNull;
 
 import java.util.ArrayList;
 import java.util.List;
@@ -26,7 +27,7 @@
 /**
  * Sample data source with artificial data.
  */
-class ItemDataSource extends BoundedDataSource<Item> {
+class ItemDataSource extends PositionalDataSource<Item> {
     private static final int COUNT = 500;
 
     @ColorInt
@@ -39,18 +40,7 @@
     private static int sGenerationId;
     private final int mGenerationId = sGenerationId++;
 
-    @Override
-    public int countItems() {
-        return COUNT;
-    }
-
-    @Override
-    public List<Item> loadRange(int startPosition, int loadCount) {
-        if (isInvalid()) {
-            // abort!
-            return null;
-        }
-
+    private List<Item> loadRangeInternal(int startPosition, int loadCount) {
         List<Item> items = new ArrayList<>();
         int end = Math.min(COUNT, startPosition + loadCount);
         int bgColor = COLORS[mGenerationId % COLORS.length];
@@ -63,11 +53,38 @@
         for (int i = startPosition; i != end; i++) {
             items.add(new Item(i, "item " + i, bgColor));
         }
-
-        if (isInvalid()) {
-            // abort!
-            return null;
-        }
         return items;
     }
+
+    // TODO: open up this API in PositionalDataSource?
+    private static int computeFirstLoadPosition(int position, int firstLoadSize,
+            int pageSize, int size) {
+        int roundedPageStart = Math.round(position / pageSize) * pageSize;
+
+        // minimum start position is 0
+        roundedPageStart = Math.max(0, roundedPageStart);
+
+        // maximum start pos is that which will encompass end of list
+        int maximumLoadPage = ((size - firstLoadSize + pageSize - 1) / pageSize) * pageSize;
+        roundedPageStart = Math.min(maximumLoadPage, roundedPageStart);
+
+        return roundedPageStart;
+    }
+
+    @Override
+    public void loadInitial(int requestedStartPosition, int requestedLoadSize,
+            int pageSize, @NonNull InitialLoadCallback<Item> callback) {
+        requestedStartPosition = computeFirstLoadPosition(
+                requestedStartPosition, requestedLoadSize, pageSize, COUNT);
+
+        requestedLoadSize = Math.min(COUNT - requestedStartPosition, requestedLoadSize);
+        List<Item> data = loadRangeInternal(requestedStartPosition, requestedLoadSize);
+        callback.onResult(data, requestedStartPosition, COUNT);
+    }
+
+    @Override
+    public void loadRange(int startPosition, int count, @NonNull LoadCallback<Item> callback) {
+        List<Item> data = loadRangeInternal(startPosition, count);
+        callback.onResult(data);
+    }
 }
diff --git a/paging/integration-tests/testapp/src/main/java/android/arch/paging/integration/testapp/PagedListItemViewModel.java b/paging/integration-tests/testapp/src/main/java/android/arch/paging/integration/testapp/PagedListItemViewModel.java
index 974eab9..be14bc1 100644
--- a/paging/integration-tests/testapp/src/main/java/android/arch/paging/integration/testapp/PagedListItemViewModel.java
+++ b/paging/integration-tests/testapp/src/main/java/android/arch/paging/integration/testapp/PagedListItemViewModel.java
@@ -27,10 +27,24 @@
  */
 @SuppressWarnings("WeakerAccess")
 public class PagedListItemViewModel extends ViewModel {
-    private LiveData<PagedList<Item>> mLivePagedList;
     private ItemDataSource mDataSource;
     private final Object mDataSourceLock = new Object();
 
+    private final DataSource.Factory<Integer, Item> mFactory =
+            new DataSource.Factory<Integer, Item>() {
+        @Override
+        public DataSource<Integer, Item> create() {
+            ItemDataSource newDataSource = new ItemDataSource();
+            synchronized (mDataSourceLock) {
+                mDataSource = newDataSource;
+                return mDataSource;
+            }
+        }
+    };
+
+    private LiveData<PagedList<Item>> mLivePagedList =
+            new LivePagedListBuilder<>(mFactory, 20).build();
+
     void invalidateList() {
         synchronized (mDataSourceLock) {
             if (mDataSource != null) {
@@ -40,22 +54,6 @@
     }
 
     LiveData<PagedList<Item>> getLivePagedList() {
-        if (mLivePagedList == null) {
-            mLivePagedList = new LivePagedListBuilder<Integer, Item>()
-                    .setPagingConfig(20)
-                    .setDataSourceFactory(new DataSource.Factory<Integer, Item>() {
-                        @Override
-                        public DataSource<Integer, Item> create() {
-                            ItemDataSource newDataSource = new ItemDataSource();
-                            synchronized (mDataSourceLock) {
-                                mDataSource = newDataSource;
-                                return mDataSource;
-                            }
-                        }
-                    })
-                    .build();
-        }
-
         return mLivePagedList;
     }
 }
diff --git a/paging/runtime/src/androidTest/java/android/arch/paging/PagedStorageDiffHelperTest.kt b/paging/runtime/src/androidTest/java/android/arch/paging/PagedStorageDiffHelperTest.kt
index 64501f7..b64b27b 100644
--- a/paging/runtime/src/androidTest/java/android/arch/paging/PagedStorageDiffHelperTest.kt
+++ b/paging/runtime/src/androidTest/java/android/arch/paging/PagedStorageDiffHelperTest.kt
@@ -35,17 +35,17 @@
     @Test
     fun sameListNoUpdates() {
         validateTwoListDiff(
-                PagedStorage(5, createPage("a", "b", "c"), 5),
-                PagedStorage(5, createPage("a", "b", "c"), 5)) {
+                PagedStorage(5, listOf("a", "b", "c"), 5),
+                PagedStorage(5, listOf("a", "b", "c"), 5)) {
             verifyZeroInteractions(it)
         }
     }
 
     @Test
     fun sameListNoUpdatesPlaceholder() {
-        val storageNoPlaceholder = PagedStorage(0, createPage("a", "b", "c"), 10)
+        val storageNoPlaceholder = PagedStorage(0, listOf("a", "b", "c"), 10)
 
-        val storageWithPlaceholder = PagedStorage(0, createPage("a", "b", "c"), 10)
+        val storageWithPlaceholder = PagedStorage(0, listOf("a", "b", "c"), 10)
         storageWithPlaceholder.allocatePlaceholders(3, 0, 3,
                 /* ignored */ mock(PagedStorage.Callback::class.java))
 
@@ -64,8 +64,8 @@
     @Test
     fun appendFill() {
         validateTwoListDiff(
-                PagedStorage(5, createPage("a", "b"), 5),
-                PagedStorage(5, createPage("a", "b", "c"), 4)) {
+                PagedStorage(5, listOf("a", "b"), 5),
+                PagedStorage(5, listOf("a", "b", "c"), 4)) {
             verify(it).onRemoved(11, 1)
             verify(it).onInserted(7, 1)
             // NOTE: ideally would be onChanged(7, 1, null)
@@ -76,8 +76,8 @@
     @Test
     fun prependFill() {
         validateTwoListDiff(
-                PagedStorage(5, createPage("b", "c"), 5),
-                PagedStorage(4, createPage("a", "b", "c"), 5)) {
+                PagedStorage(5, listOf("b", "c"), 5),
+                PagedStorage(4, listOf("a", "b", "c"), 5)) {
             verify(it).onRemoved(0, 1)
             verify(it).onInserted(4, 1)
             //NOTE: ideally would be onChanged(4, 1, null);
@@ -88,8 +88,8 @@
     @Test
     fun change() {
         validateTwoListDiff(
-                PagedStorage(5, createPage("a1", "b1", "c1"), 5),
-                PagedStorage(5, createPage("a2", "b1", "c2"), 5)) {
+                PagedStorage(5, listOf("a1", "b1", "c1"), 5),
+                PagedStorage(5, listOf("a2", "b1", "c2"), 5)) {
             verify(it).onChanged(5, 1, null)
             verify(it).onChanged(7, 1, null)
             verifyNoMoreInteractions(it)
@@ -108,12 +108,8 @@
             }
         }
 
-        private fun createPage(vararg items: String): Page<Int, String> {
-            return Page(items.toList())
-        }
-
-        private fun validateTwoListDiff(oldList: PagedStorage<*, String>,
-                                        newList: PagedStorage<*, String>,
+        private fun validateTwoListDiff(oldList: PagedStorage<String>,
+                                        newList: PagedStorage<String>,
                                         validator: (callback: ListUpdateCallback) -> Unit) {
             val diffResult = PagedStorageDiffHelper.computeDiff(
                     oldList, newList, DIFF_CALLBACK)
diff --git a/paging/runtime/src/androidTest/java/android/arch/paging/StringPagedList.kt b/paging/runtime/src/androidTest/java/android/arch/paging/StringPagedList.kt
index c2e5ec7..48d51bc 100644
--- a/paging/runtime/src/androidTest/java/android/arch/paging/StringPagedList.kt
+++ b/paging/runtime/src/androidTest/java/android/arch/paging/StringPagedList.kt
@@ -17,13 +17,13 @@
 package android.arch.paging
 
 class StringPagedList constructor(leadingNulls: Int, trailingNulls: Int, vararg items: String)
-        : PagedList<String>(PagedStorage<Int, String>(), TestExecutor(), TestExecutor(), null,
+        : PagedList<String>(PagedStorage<String>(), TestExecutor(), TestExecutor(), null,
                 PagedList.Config.Builder().setPageSize(1).build()), PagedStorage.Callback {
     init {
         @Suppress("UNCHECKED_CAST")
-        val keyedStorage = mStorage as PagedStorage<Int, String>
+        val keyedStorage = mStorage as PagedStorage<String>
         keyedStorage.init(leadingNulls,
-                Page<Int, String>(null, items.toList(), null),
+                items.toList(),
                 trailingNulls,
                 0,
                 this)
diff --git a/paging/runtime/src/main/java/android/arch/paging/LivePagedListBuilder.java b/paging/runtime/src/main/java/android/arch/paging/LivePagedListBuilder.java
index ee1810b..2edbcdf 100644
--- a/paging/runtime/src/main/java/android/arch/paging/LivePagedListBuilder.java
+++ b/paging/runtime/src/main/java/android/arch/paging/LivePagedListBuilder.java
@@ -25,42 +25,76 @@
 
 import java.util.concurrent.Executor;
 
+/**
+ * Builder for {@code LiveData<PagedList>}, given a {@link DataSource.Factory} and a
+ * {@link PagedList.Config}.
+ * <p>
+ * The required parameters are in the constructor, so you can simply construct and build, or
+ * optionally enable extra features (such as initial load key, or BoundaryCallback.
+ *
+ * @param <Key> Type of input valued used to load data from the DataSource. Must be integer if
+ *             you're using PositionalDataSource.
+ * @param <Value> Item type being presented.
+ */
 public class LivePagedListBuilder<Key, Value> {
     private Key mInitialLoadKey;
     private PagedList.Config mConfig;
     private DataSource.Factory<Key, Value> mDataSourceFactory;
     private PagedList.BoundaryCallback mBoundaryCallback;
-    private Executor mMainThreadExecutor;
     private Executor mBackgroundThreadExecutor;
 
-    @SuppressWarnings("WeakerAccess")
+    /**
+     * Creates a LivePagedListBuilder with required parameters.
+     *
+     * @param dataSourceFactory DataSource factory providing DataSource generations.
+     * @param config Paging configuration.
+     */
+    public LivePagedListBuilder(@NonNull DataSource.Factory<Key, Value> dataSourceFactory,
+            @NonNull PagedList.Config config) {
+        mDataSourceFactory = dataSourceFactory;
+        mConfig = config;
+    }
+
+    /**
+     * Creates a LivePagedListBuilder with required parameters.
+     * <p>
+     * This method is a convenience for
+     * <pre>
+     * LivePagedListBuilder(dataSourceFactory,
+     *         PagedList.Config.Builder().setPageSize(pageSize)).build()}
+     * </pre>
+     *
+     * @param dataSourceFactory
+     * @param pageSize
+     */
+    public LivePagedListBuilder(@NonNull DataSource.Factory<Key, Value> dataSourceFactory,
+            int pageSize) {
+        this(dataSourceFactory, new PagedList.Config.Builder().setPageSize(pageSize).build());
+    }
+
+    /**
+     * First loading key passed to the first PagedList/DataSource.
+     * <p>
+     * When a new PagedList/DataSource pair is created after the first, it acquires a load key from
+     * the previous generation so that data is loaded around the position already being observed.
+     *
+     * @param key Initial load key passed to the first PagedList/DataSource.
+     * @return this
+     */
     @NonNull
     public LivePagedListBuilder<Key, Value> setInitialLoadKey(@Nullable Key key) {
         mInitialLoadKey = key;
         return this;
     }
 
-    @SuppressWarnings("WeakerAccess")
-    @NonNull
-    public LivePagedListBuilder<Key, Value> setPagingConfig(@NonNull PagedList.Config config) {
-        mConfig = config;
-        return this;
-    }
-
-    @SuppressWarnings("WeakerAccess")
-    @NonNull
-    public LivePagedListBuilder<Key, Value> setPagingConfig(int pageSize) {
-        mConfig = new PagedList.Config.Builder().setPageSize(pageSize).build();
-        return this;
-    }
-
-    @NonNull
-    public LivePagedListBuilder<Key, Value> setDataSourceFactory(
-            @NonNull DataSource.Factory<Key, Value> dataSourceFactory) {
-        mDataSourceFactory = dataSourceFactory;
-        return this;
-    }
-
+    /**
+     * Sets a {@link PagedList.BoundaryCallback} on each PagedList created.
+     * <p>
+     * This can be used to
+     *
+     * @param boundaryCallback The boundary callback for listening to PagedList load state.
+     * @return this
+     */
     @SuppressWarnings("unused")
     @NonNull
     public LivePagedListBuilder<Key, Value> setBoundaryCallback(
@@ -69,14 +103,15 @@
         return this;
     }
 
-    @SuppressWarnings("unused")
-    @NonNull
-    public LivePagedListBuilder<Key, Value> setMainThreadExecutor(
-            @NonNull Executor mainThreadExecutor) {
-        mMainThreadExecutor = mainThreadExecutor;
-        return this;
-    }
-
+    /**
+     * Sets executor which will be used for background loading of pages.
+     * <p>
+     * Does not affect initial load, which will be always be done on done on the Arch components
+     * I/O thread.
+     *
+     * @param backgroundThreadExecutor Executor for background DataSource loading.
+     * @return this
+     */
     @SuppressWarnings("unused")
     @NonNull
     public LivePagedListBuilder<Key, Value> setBackgroundThreadExecutor(
@@ -85,6 +120,14 @@
         return this;
     }
 
+    /**
+     * Constructs the {@code LiveData<PagedList>}.
+     * <p>
+     * No work (such as loading) is done immediately, the creation of the first PagedList is is
+     * deferred until the LiveData is observed.
+     *
+     * @return The LiveData of PagedLists
+     */
     @NonNull
     public LiveData<PagedList<Value>> build() {
         if (mConfig == null) {
@@ -93,20 +136,17 @@
         if (mDataSourceFactory == null) {
             throw new IllegalArgumentException("DataSource.Factory must be provided");
         }
-        if (mMainThreadExecutor == null) {
-            mMainThreadExecutor = ArchTaskExecutor.getMainThreadExecutor();
-        }
         if (mBackgroundThreadExecutor == null) {
             mBackgroundThreadExecutor = ArchTaskExecutor.getIOThreadExecutor();
         }
 
         return create(mInitialLoadKey, mConfig, mBoundaryCallback, mDataSourceFactory,
-                mMainThreadExecutor, mBackgroundThreadExecutor);
+                ArchTaskExecutor.getMainThreadExecutor(), mBackgroundThreadExecutor);
     }
 
     @AnyThread
     @NonNull
-    public static <Key, Value> LiveData<PagedList<Value>> create(
+    private static <Key, Value> LiveData<PagedList<Value>> create(
             @Nullable final Key initialLoadKey,
             @NonNull final PagedList.Config config,
             @Nullable final PagedList.BoundaryCallback boundaryCallback,
diff --git a/paging/runtime/src/main/java/android/arch/paging/LivePagedListProvider.java b/paging/runtime/src/main/java/android/arch/paging/LivePagedListProvider.java
index e0a03cb..44b71a8 100644
--- a/paging/runtime/src/main/java/android/arch/paging/LivePagedListProvider.java
+++ b/paging/runtime/src/main/java/android/arch/paging/LivePagedListProvider.java
@@ -22,45 +22,30 @@
 import android.support.annotation.Nullable;
 import android.support.annotation.WorkerThread;
 
+// NOTE: Room 1.0 depends on this class, so it should not be removed
+// until Room switches to using DataSource.Factory directly
 /**
  * Provides a {@code LiveData<PagedList>}, given a means to construct a DataSource.
  * <p>
  * Return type for data-loading system of an application or library to produce a
  * {@code LiveData<PagedList>}, while leaving the details of the paging mechanism up to the
  * consumer.
- * <p>
- * If you're using Room, it can generate a LivePagedListProvider from a query:
- * <pre>
- * {@literal @}Dao
- * interface UserDao {
- *     {@literal @}Query("SELECT * FROM user ORDER BY lastName ASC")
- *     public abstract LivePagedListProvider&lt;Integer, User> usersByLastName();
- * }</pre>
- * In the above sample, {@code Integer} is used because it is the {@code Key} type of
- * {@link TiledDataSource}. Currently, Room can only generate a {@code LIMIT}/{@code OFFSET},
- * position based loader that uses TiledDataSource under the hood, and specifying {@code Integer}
- * here lets you pass an initial loading position as an integer.
- * <p>
- * In the future, Room plans to offer other key types to support paging content with a
- * {@link KeyedDataSource}.
  *
  * @param <Key> Type of input valued used to load data from the DataSource. Must be integer if
- *             you're using TiledDataSource.
+ *             you're using PositionalDataSource.
  * @param <Value> Data type produced by the DataSource, and held by the PagedLists.
  *
  * @see PagedListAdapter
  * @see DataSource
  * @see PagedList
  *
- * @deprecated To construct a {@code LiveData<PagedList>}, use {@link LivePagedListBuilder}, which
- * provides the same construction capability with more customization, and better defaults. The role
+ * @deprecated use {@link LivePagedListBuilder} to construct a {@code LiveData<PagedList>}. It
+ * provides the same construction capability with more customization, and simpler defaults. The role
  * of DataSource construction has been separated out to {@link DataSource.Factory} to access or
  * provide a self-invalidating sequence of DataSources. If you were acquiring this from Room, you
- * can switch to having your Dao return a {@link DataSource.Factory} instead, and create a LiveData
- * of PagedList with a {@link LivePagedListBuilder}.
+ * can switch to having your Dao return a {@link DataSource.Factory} instead, and create a
+ * {@code LiveData<PagedList>} with a {@link LivePagedListBuilder}.
  */
-// NOTE: Room 1.0 depends on this class, so it should not be removed
-// until Room switches to using DataSource.Factory directly
 @Deprecated
 public abstract class LivePagedListProvider<Key, Value> implements DataSource.Factory<Key, Value> {
 
@@ -93,10 +78,8 @@
     @AnyThread
     @NonNull
     public LiveData<PagedList<Value>> create(@Nullable Key initialLoadKey, int pageSize) {
-        return new LivePagedListBuilder<Key, Value>()
+        return new LivePagedListBuilder<>(this, pageSize)
                 .setInitialLoadKey(initialLoadKey)
-                .setPagingConfig(pageSize)
-                .setDataSourceFactory(this)
                 .build();
     }
 
@@ -116,10 +99,8 @@
     @NonNull
     public LiveData<PagedList<Value>> create(@Nullable Key initialLoadKey,
             @NonNull PagedList.Config config) {
-        return new LivePagedListBuilder<Key, Value>()
+        return new LivePagedListBuilder<>(this, config)
                 .setInitialLoadKey(initialLoadKey)
-                .setPagingConfig(config)
-                .setDataSourceFactory(this)
                 .build();
     }
 }
diff --git a/paging/runtime/src/main/java/android/arch/paging/PagedListAdapter.java b/paging/runtime/src/main/java/android/arch/paging/PagedListAdapter.java
index 89b9c2e..a8158c2 100644
--- a/paging/runtime/src/main/java/android/arch/paging/PagedListAdapter.java
+++ b/paging/runtime/src/main/java/android/arch/paging/PagedListAdapter.java
@@ -44,18 +44,14 @@
  * {@literal @}Dao
  * interface UserDao {
  *     {@literal @}Query("SELECT * FROM user ORDER BY lastName ASC")
- *     public abstract LivePagedListProvider&lt;Integer, User> usersByLastName();
+ *     public abstract DataSource.Factory&lt;Integer, User> usersByLastName();
  * }
  *
  * class MyViewModel extends ViewModel {
  *     public final LiveData&lt;PagedList&lt;User>> usersList;
  *     public MyViewModel(UserDao userDao) {
- *         usersList = userDao.usersByLastName().create(
- *                 /* initial load position {@literal *}/ 0,
- *                 new PagedList.Config.Builder()
- *                         .setPageSize(50)
- *                         .setPrefetchDistance(50)
- *                         .build());
+ *         usersList = LivePagedListBuilder&lt;>(
+ *                 userDao.usersByLastName(), /* page size {@literal *}/ 20).build();
  *     }
  * }
  *
diff --git a/paging/runtime/src/main/java/android/arch/paging/PagedListAdapterHelper.java b/paging/runtime/src/main/java/android/arch/paging/PagedListAdapterHelper.java
index 51a6e37..7a0b81a 100644
--- a/paging/runtime/src/main/java/android/arch/paging/PagedListAdapterHelper.java
+++ b/paging/runtime/src/main/java/android/arch/paging/PagedListAdapterHelper.java
@@ -48,18 +48,14 @@
  * {@literal @}Dao
  * interface UserDao {
  *     {@literal @}Query("SELECT * FROM user ORDER BY lastName ASC")
- *     public abstract LivePagedListProvider&lt;Integer, User> usersByLastName();
+ *     public abstract DataSource.Factory&lt;Integer, User> usersByLastName();
  * }
  *
  * class MyViewModel extends ViewModel {
  *     public final LiveData&lt;PagedList&lt;User>> usersList;
  *     public MyViewModel(UserDao userDao) {
- *         usersList = userDao.usersByLastName().create(
- *                 /* initial load position {@literal *}/ 0,
- *                 new PagedList.Config.Builder()
- *                         .setPageSize(50)
- *                         .setPrefetchDistance(50)
- *                         .build());
+ *         usersList = LivePagedListBuilder&lt;>(
+ *                 userDao.usersByLastName(), /* page size {@literal *}/ 20).build();
  *     }
  * }
  *
diff --git a/paging/runtime/src/main/java/android/arch/paging/PagedStorageDiffHelper.java b/paging/runtime/src/main/java/android/arch/paging/PagedStorageDiffHelper.java
index 6fc7039..d991b72 100644
--- a/paging/runtime/src/main/java/android/arch/paging/PagedStorageDiffHelper.java
+++ b/paging/runtime/src/main/java/android/arch/paging/PagedStorageDiffHelper.java
@@ -26,8 +26,8 @@
     }
 
     static <T> DiffUtil.DiffResult computeDiff(
-            final PagedStorage<?, T> oldList,
-            final PagedStorage<?, T> newList,
+            final PagedStorage<T> oldList,
+            final PagedStorage<T> newList,
             final DiffCallback<T> diffCallback) {
         final int oldOffset = oldList.computeLeadingNulls();
         final int newOffset = newList.computeLeadingNulls();
@@ -131,8 +131,8 @@
      * immediately after dispatching this diff.
      */
     static <T> void dispatchDiff(ListUpdateCallback callback,
-            final PagedStorage<?, T> oldList,
-            final PagedStorage<?, T> newList,
+            final PagedStorage<T> oldList,
+            final PagedStorage<T> newList,
             final DiffUtil.DiffResult diffResult) {
 
         final int trailingOld = oldList.computeTrailingNulls();
diff --git a/room/integration-tests/testapp/src/androidTest/java/android/arch/persistence/room/integration/testapp/dao/UserDao.java b/room/integration-tests/testapp/src/androidTest/java/android/arch/persistence/room/integration/testapp/dao/UserDao.java
index cb2bb03..0b184a9 100644
--- a/room/integration-tests/testapp/src/androidTest/java/android/arch/persistence/room/integration/testapp/dao/UserDao.java
+++ b/room/integration-tests/testapp/src/androidTest/java/android/arch/persistence/room/integration/testapp/dao/UserDao.java
@@ -190,8 +190,12 @@
     @Query("SELECT * FROM user where mAge > :age")
     public abstract LivePagedListProvider<Integer, User> loadPagedByAge_legacy(int age);
 
+    // TODO: switch to PositionalDataSource once Room supports it
     @Query("SELECT * FROM user ORDER BY mAge DESC")
-    public abstract TiledDataSource<User> loadUsersByAgeDesc();
+    public abstract DataSource.Factory<Integer, User> loadUsersByAgeDesc();
+
+    @Query("SELECT * FROM user ORDER BY mAge DESC")
+    public abstract TiledDataSource<User> loadUsersByAgeDesc_legacy();
 
     @Query("DELETE FROM User WHERE mId IN (:ids) AND mAge == :age")
     public abstract int deleteByAgeAndIds(int age, List<Integer> ids);
diff --git a/room/integration-tests/testapp/src/androidTest/java/android/arch/persistence/room/integration/testapp/paging/DataSourceFactoryTest.java b/room/integration-tests/testapp/src/androidTest/java/android/arch/persistence/room/integration/testapp/paging/DataSourceFactoryTest.java
index c546531..b54abe8 100644
--- a/room/integration-tests/testapp/src/androidTest/java/android/arch/persistence/room/integration/testapp/paging/DataSourceFactoryTest.java
+++ b/room/integration-tests/testapp/src/androidTest/java/android/arch/persistence/room/integration/testapp/paging/DataSourceFactoryTest.java
@@ -63,12 +63,12 @@
         validateUsersAsPagedList(new LivePagedListFactory() {
             @Override
             public LiveData<PagedList<User>> create() {
-                return new LivePagedListBuilder<Integer, User>()
-                        .setPagingConfig(new PagedList.Config.Builder()
+                return new LivePagedListBuilder<>(
+                        mUserDao.loadPagedByAge(3),
+                        new PagedList.Config.Builder()
                                 .setPageSize(10)
                                 .setPrefetchDistance(1)
                                 .setInitialLoadSizeHint(10).build())
-                        .setDataSourceFactory(mUserDao.loadPagedByAge(3))
                         .build();
             }
         });
diff --git a/room/integration-tests/testapp/src/androidTest/java/android/arch/persistence/room/integration/testapp/paging/LimitOffsetDataSourceTest.java b/room/integration-tests/testapp/src/androidTest/java/android/arch/persistence/room/integration/testapp/paging/LimitOffsetDataSourceTest.java
index 8226759..f0285a0 100644
--- a/room/integration-tests/testapp/src/androidTest/java/android/arch/persistence/room/integration/testapp/paging/LimitOffsetDataSourceTest.java
+++ b/room/integration-tests/testapp/src/androidTest/java/android/arch/persistence/room/integration/testapp/paging/LimitOffsetDataSourceTest.java
@@ -45,8 +45,17 @@
         mUserDao.deleteEverything();
     }
 
+    // TODO: delete this and factory abstraction when LivePagedListProvider is removed
+    @Test
+    public void limitOffsetDataSource_legacyTiledDataSource() {
+        // Simple verification that loading a TiledDataSource still works.
+        LimitOffsetDataSource<User> dataSource =
+                (LimitOffsetDataSource<User>) mUserDao.loadUsersByAgeDesc_legacy();
+        assertThat(dataSource.countItems(), is(0));
+    }
+
     private LimitOffsetDataSource<User> loadUsersByAgeDesc() {
-        return (LimitOffsetDataSource<User>) mUserDao.loadUsersByAgeDesc();
+        return (LimitOffsetDataSource<User>) mUserDao.loadUsersByAgeDesc().create();
     }
 
     @Test
diff --git a/room/integration-tests/testapp/src/androidTest/java/android/arch/persistence/room/integration/testapp/test/QueryTransactionTest.java b/room/integration-tests/testapp/src/androidTest/java/android/arch/persistence/room/integration/testapp/test/QueryTransactionTest.java
index 291cfd6..f076cf1 100644
--- a/room/integration-tests/testapp/src/androidTest/java/android/arch/persistence/room/integration/testapp/test/QueryTransactionTest.java
+++ b/room/integration-tests/testapp/src/androidTest/java/android/arch/persistence/room/integration/testapp/test/QueryTransactionTest.java
@@ -203,10 +203,8 @@
 
     @Test
     public void pagedList() {
-        LiveData<PagedList<Entity1>> pagedList = new LivePagedListBuilder<Integer, Entity1>()
-                .setDataSourceFactory(mDao.pagedList())
-                .setPagingConfig(10)
-                .build();
+        LiveData<PagedList<Entity1>> pagedList =
+                new LivePagedListBuilder<>(mDao.pagedList(), 10).build();
         observeForever(pagedList);
         drain();
         assertThat(sStartedTransactionCount.get(), is(mUseTransactionDao ? 0 : 0));
@@ -228,6 +226,7 @@
         mDao.insert(new Entity1(2, "bar"));
         drain();
         resetTransactionCount();
+        @SuppressWarnings("deprecation")
         TiledDataSource<Entity1> dataSource = mDao.dataSource();
         dataSource.loadRange(0, 10);
         assertThat(sStartedTransactionCount.get(), is(mUseTransactionDao ? 1 : 0));
diff --git a/room/integration-tests/testapp/src/main/java/android/arch/persistence/room/integration/testapp/CustomerViewModel.java b/room/integration-tests/testapp/src/main/java/android/arch/persistence/room/integration/testapp/CustomerViewModel.java
index 89d16b7..d8cd8d4 100644
--- a/room/integration-tests/testapp/src/main/java/android/arch/persistence/room/integration/testapp/CustomerViewModel.java
+++ b/room/integration-tests/testapp/src/main/java/android/arch/persistence/room/integration/testapp/CustomerViewModel.java
@@ -83,13 +83,12 @@
 
     private static <K> LiveData<PagedList<Customer>> getLivePagedList(
             K initialLoadKey, DataSource.Factory<K, Customer> dataSourceFactory) {
-        return new LivePagedListBuilder<K, Customer>()
+        PagedList.Config config = new PagedList.Config.Builder()
+                .setPageSize(10)
+                .setEnablePlaceholders(false)
+                .build();
+        return new LivePagedListBuilder<>(dataSourceFactory, config)
                 .setInitialLoadKey(initialLoadKey)
-                .setPagingConfig(new PagedList.Config.Builder()
-                        .setPageSize(10)
-                        .setEnablePlaceholders(false)
-                        .build())
-                .setDataSourceFactory(dataSourceFactory)
                 .build();
     }
 
diff --git a/room/integration-tests/testapp/src/main/java/android/arch/persistence/room/integration/testapp/RoomPagedListActivity.java b/room/integration-tests/testapp/src/main/java/android/arch/persistence/room/integration/testapp/RoomPagedListActivity.java
index cdd464e..0c79aee 100644
--- a/room/integration-tests/testapp/src/main/java/android/arch/persistence/room/integration/testapp/RoomPagedListActivity.java
+++ b/room/integration-tests/testapp/src/main/java/android/arch/persistence/room/integration/testapp/RoomPagedListActivity.java
@@ -16,7 +16,6 @@
 
 package android.arch.persistence.room.integration.testapp;
 
-import android.arch.lifecycle.LifecycleRegistry;
 import android.arch.lifecycle.LiveData;
 import android.arch.lifecycle.Observer;
 import android.arch.lifecycle.ViewModelProviders;
@@ -75,7 +74,7 @@
                 mAdapter.setList(items);
             }
         });
-        final Button button = findViewById(R.id.button);
+        final Button button = findViewById(R.id.addButton);
         button.setOnClickListener(new View.OnClickListener() {
             @Override
             public void onClick(View v) {
@@ -114,11 +113,4 @@
     protected boolean useKeyedQuery() {
         return false;
     }
-
-    private LifecycleRegistry mLifecycleRegistry = new LifecycleRegistry(this);
-
-    @Override
-    public LifecycleRegistry getLifecycle() {
-        return mLifecycleRegistry;
-    }
 }
diff --git a/room/integration-tests/testapp/src/main/java/android/arch/persistence/room/integration/testapp/database/LastNameAscCustomerDataSource.java b/room/integration-tests/testapp/src/main/java/android/arch/persistence/room/integration/testapp/database/LastNameAscCustomerDataSource.java
index a38d6ae..2db543b 100644
--- a/room/integration-tests/testapp/src/main/java/android/arch/persistence/room/integration/testapp/database/LastNameAscCustomerDataSource.java
+++ b/room/integration-tests/testapp/src/main/java/android/arch/persistence/room/integration/testapp/database/LastNameAscCustomerDataSource.java
@@ -21,6 +21,7 @@
 import android.support.annotation.NonNull;
 import android.support.annotation.Nullable;
 
+import java.util.Collections;
 import java.util.List;
 import java.util.Set;
 
@@ -60,7 +61,6 @@
     @Override
     public boolean isInvalid() {
         mDb.getInvalidationTracker().refreshVersionsSync();
-
         return super.isInvalid();
     }
 
@@ -76,30 +76,48 @@
     }
 
     @Override
-    public int countItemsBefore(@NonNull String customerName) {
-        return mCustomerDao.customerNameCountBefore(customerName);
+    public void loadInitial(@Nullable String customerName, int initialLoadSize,
+            boolean enablePlaceholders, @NonNull InitialLoadCallback<Customer> callback) {
+        List<Customer> list;
+        if (customerName != null) {
+            // initial keyed load - load before 'customerName',
+            // and load after last item in before list
+            int pageSize = initialLoadSize / 2;
+            String key = customerName;
+            list = mCustomerDao.customerNameLoadBefore(key, pageSize);
+            Collections.reverse(list);
+            if (!list.isEmpty()) {
+                key = getKey(list.get(list.size() - 1));
+            }
+            list.addAll(mCustomerDao.customerNameLoadAfter(key, pageSize));
+        } else {
+            list = mCustomerDao.customerNameInitial(initialLoadSize);
+        }
+
+        if (enablePlaceholders && !list.isEmpty()) {
+            String firstKey = getKey(list.get(0));
+            String lastKey = getKey(list.get(list.size() - 1));
+
+            // only bother counting if placeholders are desired
+            final int position = mCustomerDao.customerNameCountBefore(firstKey);
+            final int count = position + list.size() + mCustomerDao.customerNameCountAfter(lastKey);
+            callback.onResult(list, position, count);
+        } else {
+            callback.onResult(list);
+        }
     }
 
     @Override
-    public int countItemsAfter(@NonNull String customerName) {
-        return mCustomerDao.customerNameCountAfter(customerName);
+    public void loadAfter(@NonNull String currentEndKey, int pageSize,
+            @NonNull LoadCallback<Customer> callback) {
+        callback.onResult(mCustomerDao.customerNameLoadAfter(currentEndKey, pageSize));
     }
 
-    @Nullable
     @Override
-    public List<Customer> loadInitial(int pageSize) {
-        return mCustomerDao.customerNameInitial(pageSize);
-    }
-
-    @Nullable
-    @Override
-    public List<Customer> loadBefore(@NonNull String customerName, int pageSize) {
-        return mCustomerDao.customerNameLoadBefore(customerName, pageSize);
-    }
-
-    @Nullable
-    @Override
-    public List<Customer> loadAfter(@Nullable String customerName, int pageSize) {
-        return mCustomerDao.customerNameLoadAfter(customerName, pageSize);
+    public void loadBefore(@NonNull String currentBeginKey, int pageSize,
+            @NonNull LoadCallback<Customer> callback) {
+        List<Customer> list = mCustomerDao.customerNameLoadBefore(currentBeginKey, pageSize);
+        Collections.reverse(list);
+        callback.onResult(list);
     }
 }
diff --git a/room/integration-tests/testapp/src/main/res/layout/activity_recycler_view.xml b/room/integration-tests/testapp/src/main/res/layout/activity_recycler_view.xml
index 71dae1b..34e4491 100644
--- a/room/integration-tests/testapp/src/main/res/layout/activity_recycler_view.xml
+++ b/room/integration-tests/testapp/src/main/res/layout/activity_recycler_view.xml
@@ -35,7 +35,7 @@
         android:paddingTop="@dimen/activity_vertical_margin"
     />
     <Button
-        android:id="@+id/button"
+        android:id="@+id/addButton"
         android:layout_width="wrap_content"
         android:layout_height="wrap_content"
         android:layout_alignParentBottom="true"