core/java/android/app/LoaderManager.java

/*
 * Copyright (C) 2010 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.app;

import android.content.Loader;
import android.os.Bundle;
import android.util.Log;
import android.util.SparseArray;

/**
 * Interface associated with an {@link Activity} or {@link Fragment} for managing
 * one or more {@link android.content.Loader} instances associated with it.
 */
public interface LoaderManager {
    /**
     * Callback interface for a client to interact with the manager.
     */
    public interface LoaderCallbacks<D> {
        /**
         * Instantiate and return a new Loader for the given ID.
         *
         * @param id The ID whose loader is to be created.
         * @param args Any arguments supplied by the caller.
         * @return Return a new Loader instance that is ready to start loading.
         */
        public Loader<D> onCreateLoader(int id, Bundle args);

        /**
         * Called when a previously created loader has finished its load.
         * @param loader The Loader that has finished.
         * @param data The data generated by the Loader.
         */
        public void onLoadFinished(Loader<D> loader, D data);
    }
    
    /**
     * Ensures a loader is initialized and active.  If the loader doesn't
     * already exist, one is created and (if the activity/fragment is currently
     * started) starts the loader.  Otherwise the last created
     * loader is re-used.
     *
     * <p>In either case, the given callback is associated with the loader, and
     * will be called as the loader state changes.  If at the point of call
     * the caller is in its started state, and the requested loader
     * already exists and has generated its data, then
     * callback. {@link LoaderCallbacks#onLoadFinished} will
     * be called immediately (inside of this function), so you must be prepared
     * for this to happen.
     */
    public <D> Loader<D> initLoader(int id, Bundle args,
            LoaderManager.LoaderCallbacks<D> callback);

    /**
     * Creates a new loader in this manager, registers the callbacks to it,
     * and (if the activity/fragment is currently started) starts loading it.
     * If a loader with the same id has previously been
     * started it will automatically be destroyed when the new loader completes
     * its work. The callback will be delivered before the old loader
     * is destroyed.
     */
    public <D> Loader<D> restartLoader(int id, Bundle args,
            LoaderManager.LoaderCallbacks<D> callback);

    /**
     * Stops and removes the loader with the given ID.
     */
    public void stopLoader(int id);

    /**
     * Return the Loader with the given id or null if no matching Loader
     * is found.
     */
    public <D> Loader<D> getLoader(int id);
}

class LoaderManagerImpl implements LoaderManager {
    static final String TAG = "LoaderManagerImpl";
    static final boolean DEBUG = true;

    // These are the currently active loaders.  A loader is here
    // from the time its load is started until it has been explicitly
    // stopped or restarted by the application.
    final SparseArray<LoaderInfo> mLoaders = new SparseArray<LoaderInfo>();

    // These are previously run loaders.  This list is maintained internally
    // to avoid destroying a loader while an application is still using it.
    // It allows an application to restart a loader, but continue using its
    // previously run loader until the new loader's data is available.
    final SparseArray<LoaderInfo> mInactiveLoaders = new SparseArray<LoaderInfo>();

    boolean mStarted;
    boolean mRetaining;
    boolean mRetainingStarted;

    final class LoaderInfo implements Loader.OnLoadCompleteListener<Object> {
        final int mId;
        final Bundle mArgs;
        LoaderManager.LoaderCallbacks<Object> mCallbacks;
        Loader<Object> mLoader;
        Object mData;
        boolean mStarted;
        boolean mRetaining;
        boolean mRetainingStarted;
        boolean mDestroyed;
        boolean mListenerRegistered;
        
        public LoaderInfo(int id, Bundle args, LoaderManager.LoaderCallbacks<Object> callbacks) {
            mId = id;
            mArgs = args;
            mCallbacks = callbacks;
        }
        
        void start() {
            if (mRetaining && mRetainingStarted) {
                // Our owner is started, but we were being retained from a
                // previous instance in the started state...  so there is really
                // nothing to do here, since the loaders are still started.
                mStarted = true;
                return;
            }

            if (mStarted) {
                // If loader already started, don't restart.
                return;
            }

            if (DEBUG) Log.v(TAG, "  Starting: " + this);
            if (mLoader == null && mCallbacks != null) {
               mLoader = mCallbacks.onCreateLoader(mId, mArgs);
            }
            if (mLoader != null) {
                if (!mListenerRegistered) {
                    mLoader.registerListener(mId, this);
                    mListenerRegistered = true;
                }
                mLoader.startLoading();
                mStarted = true;
            }
        }
        
        void retain() {
            if (DEBUG) Log.v(TAG, "  Retaining: " + this);
            mRetaining = true;
            mRetainingStarted = mStarted;
            mStarted = false;
            mCallbacks = null;
        }
        
        void finishRetain() {
            if (mRetaining) {
                if (DEBUG) Log.v(TAG, "  Finished Retaining: " + this);
                mRetaining = false;
                if (mStarted != mRetainingStarted) {
                    if (!mStarted) {
                        // This loader was retained in a started state, but
                        // at the end of retaining everything our owner is
                        // no longer started...  so make it stop.
                        stop();
                    }
                }
                if (mStarted && mData != null && mCallbacks != null) {
                    // This loader was retained, and now at the point of
                    // finishing the retain we find we remain started, have
                    // our data, and the owner has a new callback...  so
                    // let's deliver the data now.
                    mCallbacks.onLoadFinished(mLoader, mData);
                }
            }
        }
        
        void stop() {
            if (DEBUG) Log.v(TAG, "  Stopping: " + this);
            mStarted = false;
            if (!mRetaining) {
                if (mLoader != null && mListenerRegistered) {
                    // Let the loader know we're done with it
                    mListenerRegistered = false;
                    mLoader.unregisterListener(this);
                    mLoader.stopLoading();
                }
                mData = null;
            }
        }
        
        void destroy() {
            if (DEBUG) Log.v(TAG, "  Destroying: " + this);
            mDestroyed = true;
            mCallbacks = null;
            if (mLoader != null) {
                if (mListenerRegistered) {
                    mListenerRegistered = false;
                    mLoader.unregisterListener(this);
                }
                mLoader.destroy();
            }
        }
        
        @Override public void onLoadComplete(Loader<Object> loader, Object data) {
            if (DEBUG) Log.v(TAG, "onLoadComplete: " + this + " mDestroyed=" + mDestroyed);

            if (mDestroyed) {
                return;
            }
            
            // Notify of the new data so the app can switch out the old data before
            // we try to destroy it.
            mData = data;
            if (mCallbacks != null) {
                mCallbacks.onLoadFinished(loader, data);
            }

            if (DEBUG) Log.v(TAG, "onLoadFinished returned: " + this);

            // We have now given the application the new loader with its
            // loaded data, so it should have stopped using the previous
            // loader.  If there is a previous loader on the inactive list,
            // clean it up.
            LoaderInfo info = mInactiveLoaders.get(mId);
            if (info != null && info != this) {
                info.destroy();
                mInactiveLoaders.remove(mId);
            }
        }

        @Override
        public String toString() {
            StringBuilder sb = new StringBuilder(64);
            sb.append("LoaderInfo{");
            sb.append(Integer.toHexString(System.identityHashCode(this)));
            sb.append(" #");
            sb.append(mId);
            if (mArgs != null) {
                sb.append(" ");
                sb.append(mArgs.toString());
            }
            sb.append("}");
            return sb.toString();
        }
    }
    
    LoaderManagerImpl(boolean started) {
        mStarted = started;
    }
    
    private LoaderInfo createLoader(int id, Bundle args,
            LoaderManager.LoaderCallbacks<Object> callback) {
        LoaderInfo info = new LoaderInfo(id, args,  (LoaderManager.LoaderCallbacks<Object>)callback);
        mLoaders.put(id, info);
        Loader<Object> loader = callback.onCreateLoader(id, args);
        info.mLoader = (Loader<Object>)loader;
        if (mStarted) {
            // The activity will start all existing loaders in it's onStart(),
            // so only start them here if we're past that point of the activitiy's
            // life cycle
            info.start();
        }
        return info;
    }
    
    @SuppressWarnings("unchecked")
    public <D> Loader<D> initLoader(int id, Bundle args, LoaderManager.LoaderCallbacks<D> callback) {
        LoaderInfo info = mLoaders.get(id);
        
        if (DEBUG) Log.v(TAG, "initLoader in " + this + ": cur=" + info);

        if (info == null) {
            // Loader doesn't already exist; create.
            info = createLoader(id, args,  (LoaderManager.LoaderCallbacks<Object>)callback);
        } else {
            info.mCallbacks = (LoaderManager.LoaderCallbacks<Object>)callback;
        }
        
        if (info.mData != null && mStarted) {
            // If the loader has already generated its data, report it now.
            info.mCallbacks.onLoadFinished(info.mLoader, info.mData);
        }
        
        return (Loader<D>)info.mLoader;
    }
    
    @SuppressWarnings("unchecked")
    public <D> Loader<D> restartLoader(int id, Bundle args, LoaderManager.LoaderCallbacks<D> callback) {
        LoaderInfo info = mLoaders.get(id);
        if (DEBUG) Log.v(TAG, "restartLoader in " + this + ": cur=" + info);
        if (info != null) {
            LoaderInfo inactive = mInactiveLoaders.get(id);
            if (inactive != null) {
                if (info.mData != null) {
                    // This loader now has data...  we are probably being
                    // called from within onLoadComplete, where we haven't
                    // yet destroyed the last inactive loader.  So just do
                    // that now.
                    if (DEBUG) Log.v(TAG, "  Removing last inactive loader in " + this);
                    inactive.destroy();
                    mInactiveLoaders.put(id, info);
                } else {
                    // We already have an inactive loader for this ID that we are
                    // waiting for!  Now we have three active loaders... let's just
                    // drop the one in the middle, since we are still waiting for
                    // its result but that result is already out of date.
                    if (DEBUG) Log.v(TAG, "  Removing intermediate loader in " + this);
                    info.destroy();
                }
            } else {
                // Keep track of the previous instance of this loader so we can destroy
                // it when the new one completes.
                if (DEBUG) Log.v(TAG, "  Making inactive: " + info);
                mInactiveLoaders.put(id, info);
            }
        }
        
        info = createLoader(id, args,  (LoaderManager.LoaderCallbacks<Object>)callback);
        return (Loader<D>)info.mLoader;
    }
    
    public void stopLoader(int id) {
        if (DEBUG) Log.v(TAG, "stopLoader in " + this + " of " + id);
        int idx = mLoaders.indexOfKey(id);
        if (idx >= 0) {
            LoaderInfo info = mLoaders.valueAt(idx);
            mLoaders.removeAt(idx);
            info.destroy();
        }
    }

    @SuppressWarnings("unchecked")
    public <D> Loader<D> getLoader(int id) {
        LoaderInfo loaderInfo = mLoaders.get(id);
        if (loaderInfo != null) {
            return (Loader<D>)mLoaders.get(id).mLoader;
        }
        return null;
    }
 
    void doStart() {
        if (DEBUG) Log.v(TAG, "Starting: " + this);

        // Call out to sub classes so they can start their loaders
        // Let the existing loaders know that we want to be notified when a load is complete
        for (int i = mLoaders.size()-1; i >= 0; i--) {
            mLoaders.valueAt(i).start();
        }
        mStarted = true;
    }
    
    void doStop() {
        if (DEBUG) Log.v(TAG, "Stopping: " + this);

        for (int i = mLoaders.size()-1; i >= 0; i--) {
            mLoaders.valueAt(i).stop();
        }
        mStarted = false;
    }
    
    void doRetain() {
        if (DEBUG) Log.v(TAG, "Retaining: " + this);

        mRetaining = true;
        mStarted = false;
        for (int i = mLoaders.size()-1; i >= 0; i--) {
            mLoaders.valueAt(i).retain();
        }
    }
    
    void finishRetain() {
        if (DEBUG) Log.v(TAG, "Finished Retaining: " + this);

        mRetaining = false;
        for (int i = mLoaders.size()-1; i >= 0; i--) {
            mLoaders.valueAt(i).finishRetain();
        }
    }
    
    void doDestroy() {
        if (!mRetaining) {
            if (DEBUG) Log.v(TAG, "Destroying Active: " + this);
            for (int i = mLoaders.size()-1; i >= 0; i--) {
                mLoaders.valueAt(i).destroy();
            }
        }
        
        if (DEBUG) Log.v(TAG, "Destroying Inactive: " + this);
        for (int i = mInactiveLoaders.size()-1; i >= 0; i--) {
            mInactiveLoaders.valueAt(i).destroy();
        }
        mInactiveLoaders.clear();
    }
}