core/java/android/view/ViewTreeObserver.java

/*
 * Copyright (C) 2008 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.view;

import android.annotation.NonNull;
import android.annotation.Nullable;
import android.compat.annotation.UnsupportedAppUsage;
import android.content.Context;
import android.graphics.Rect;
import android.graphics.Region;
import android.os.Build;
import android.util.Log;

import java.util.ArrayList;
import java.util.List;
import java.util.concurrent.CopyOnWriteArrayList;
import java.util.function.Consumer;

/**
 * A view tree observer is used to register listeners that can be notified of global
 * changes in the view tree. Such global events include, but are not limited to,
 * layout of the whole tree, beginning of the drawing pass, touch mode change....
 *
 * A ViewTreeObserver should never be instantiated by applications as it is provided
 * by the views hierarchy. Refer to {@link android.view.View#getViewTreeObserver()}
 * for more information.
 */
public final class ViewTreeObserver {
    // Recursive listeners use CopyOnWriteArrayList
    private CopyOnWriteArrayList<OnWindowFocusChangeListener> mOnWindowFocusListeners;
    private CopyOnWriteArrayList<OnWindowAttachListener> mOnWindowAttachListeners;
    private CopyOnWriteArrayList<OnGlobalFocusChangeListener> mOnGlobalFocusListeners;
    @UnsupportedAppUsage
    private CopyOnWriteArrayList<OnTouchModeChangeListener> mOnTouchModeChangeListeners;
    private CopyOnWriteArrayList<OnEnterAnimationCompleteListener>
            mOnEnterAnimationCompleteListeners;

    // Non-recursive listeners use CopyOnWriteArray
    // Any listener invoked from ViewRootImpl.performTraversals() should not be recursive
    @UnsupportedAppUsage
    private CopyOnWriteArray<OnGlobalLayoutListener> mOnGlobalLayoutListeners;
    @UnsupportedAppUsage
    private CopyOnWriteArray<OnComputeInternalInsetsListener> mOnComputeInternalInsetsListeners;
    @UnsupportedAppUsage
    private CopyOnWriteArray<OnScrollChangedListener> mOnScrollChangedListeners;
    private CopyOnWriteArray<OnPreDrawListener> mOnPreDrawListeners;
    private CopyOnWriteArray<OnWindowShownListener> mOnWindowShownListeners;
    private CopyOnWriteArray<Consumer<List<Rect>>> mGestureExclusionListeners;

    // These listeners cannot be mutated during dispatch
    private boolean mInDispatchOnDraw;
    private ArrayList<OnDrawListener> mOnDrawListeners;
    private static boolean sIllegalOnDrawModificationIsFatal;

    // These listeners are one-shot
    private ArrayList<Runnable> mOnFrameCommitListeners;

    /** Remains false until #dispatchOnWindowShown() is called. If a listener registers after
     * that the listener will be immediately called. */
    private boolean mWindowShown;

    private boolean mAlive = true;

    /**
     * Interface definition for a callback to be invoked when the view hierarchy is
     * attached to and detached from its window.
     */
    public interface OnWindowAttachListener {
        /**
         * Callback method to be invoked when the view hierarchy is attached to a window
         */
        public void onWindowAttached();

        /**
         * Callback method to be invoked when the view hierarchy is detached from a window
         */
        public void onWindowDetached();
    }

    /**
     * Interface definition for a callback to be invoked when the view hierarchy's window
     * focus state changes.
     */
    public interface OnWindowFocusChangeListener {
        /**
         * Callback method to be invoked when the window focus changes in the view tree.
         *
         * @param hasFocus Set to true if the window is gaining focus, false if it is
         * losing focus.
         */
        public void onWindowFocusChanged(boolean hasFocus);
    }

    /**
     * Interface definition for a callback to be invoked when the focus state within
     * the view tree changes.
     */
    public interface OnGlobalFocusChangeListener {
        /**
         * Callback method to be invoked when the focus changes in the view tree. When
         * the view tree transitions from touch mode to non-touch mode, oldFocus is null.
         * When the view tree transitions from non-touch mode to touch mode, newFocus is
         * null. When focus changes in non-touch mode (without transition from or to
         * touch mode) either oldFocus or newFocus can be null.
         *
         * @param oldFocus The previously focused view, if any.
         * @param newFocus The newly focused View, if any.
         */
        public void onGlobalFocusChanged(View oldFocus, View newFocus);
    }

    /**
     * Interface definition for a callback to be invoked when the global layout state
     * or the visibility of views within the view tree changes.
     */
    public interface OnGlobalLayoutListener {
        /**
         * Callback method to be invoked when the global layout state or the visibility of views
         * within the view tree changes
         */
        public void onGlobalLayout();
    }

    /**
     * Interface definition for a callback to be invoked when the view tree is about to be drawn.
     */
    public interface OnPreDrawListener {
        /**
         * Callback method to be invoked when the view tree is about to be drawn. At this point, all
         * views in the tree have been measured and given a frame. Clients can use this to adjust
         * their scroll bounds or even to request a new layout before drawing occurs.
         *
         * @return Return true to proceed with the current drawing pass, or false to cancel.
         *
         * @see android.view.View#onMeasure
         * @see android.view.View#onLayout
         * @see android.view.View#onDraw
         */
        public boolean onPreDraw();
    }

    /**
     * Interface definition for a callback to be invoked when the view tree is about to be drawn.
     */
    public interface OnDrawListener {
        /**
         * <p>Callback method to be invoked when the view tree is about to be drawn. At this point,
         * views cannot be modified in any way.</p>
         * 
         * <p>Unlike with {@link OnPreDrawListener}, this method cannot be used to cancel the
         * current drawing pass.</p>
         * 
         * <p>An {@link OnDrawListener} listener <strong>cannot be added or removed</strong>
         * from this method.</p>
         *
         * @see android.view.View#onMeasure
         * @see android.view.View#onLayout
         * @see android.view.View#onDraw
         */
        public void onDraw();
    }

    /**
     * Interface definition for a callback to be invoked when the touch mode changes.
     */
    public interface OnTouchModeChangeListener {
        /**
         * Callback method to be invoked when the touch mode changes.
         *
         * @param isInTouchMode True if the view hierarchy is now in touch mode, false  otherwise.
         */
        public void onTouchModeChanged(boolean isInTouchMode);
    }

    /**
     * Interface definition for a callback to be invoked when
     * something in the view tree has been scrolled.
     */
    public interface OnScrollChangedListener {
        /**
         * Callback method to be invoked when something in the view tree
         * has been scrolled.
         */
        public void onScrollChanged();
    }

    /**
     * Interface definition for a callback noting when a system window has been displayed.
     * This is only used for non-Activity windows. Activity windows can use
     * Activity.onEnterAnimationComplete() to get the same signal.
     * @hide
     */
    public interface OnWindowShownListener {
        /**
         * Callback method to be invoked when a non-activity window is fully shown.
         */
        void onWindowShown();
    }

    /**
     * Parameters used with OnComputeInternalInsetsListener.
     * 
     * We are not yet ready to commit to this API and support it, so
     * @hide
     */
    public final static class InternalInsetsInfo {

        @UnsupportedAppUsage
        public InternalInsetsInfo() {
        }

        /**
         * Offsets from the frame of the window at which the content of
         * windows behind it should be placed.
         */
        @UnsupportedAppUsage
        public final Rect contentInsets = new Rect();

        /**
         * Offsets from the frame of the window at which windows behind it
         * are visible.
         */
        @UnsupportedAppUsage
        public final Rect visibleInsets = new Rect();

        /**
         * Touchable region defined relative to the origin of the frame of the window.
         * Only used when {@link #setTouchableInsets(int)} is called with
         * the option {@link #TOUCHABLE_INSETS_REGION}.
         */
        @UnsupportedAppUsage
        public final Region touchableRegion = new Region();

        /**
         * Option for {@link #setTouchableInsets(int)}: the entire window frame
         * can be touched.
         */
        public static final int TOUCHABLE_INSETS_FRAME = 0;

        /**
         * Option for {@link #setTouchableInsets(int)}: the area inside of
         * the content insets can be touched.
         */
        public static final int TOUCHABLE_INSETS_CONTENT = 1;

        /**
         * Option for {@link #setTouchableInsets(int)}: the area inside of
         * the visible insets can be touched.
         */
        public static final int TOUCHABLE_INSETS_VISIBLE = 2;

        /**
         * Option for {@link #setTouchableInsets(int)}: the area inside of
         * the provided touchable region in {@link #touchableRegion} can be touched.
         */
        @UnsupportedAppUsage
        public static final int TOUCHABLE_INSETS_REGION = 3;

        /**
         * Set which parts of the window can be touched: either
         * {@link #TOUCHABLE_INSETS_FRAME}, {@link #TOUCHABLE_INSETS_CONTENT},
         * {@link #TOUCHABLE_INSETS_VISIBLE}, or {@link #TOUCHABLE_INSETS_REGION}.
         */
        @UnsupportedAppUsage
        public void setTouchableInsets(int val) {
            mTouchableInsets = val;
        }

        @UnsupportedAppUsage
        int mTouchableInsets;

        void reset() {
            contentInsets.setEmpty();
            visibleInsets.setEmpty();
            touchableRegion.setEmpty();
            mTouchableInsets = TOUCHABLE_INSETS_FRAME;
        }

        boolean isEmpty() {
            return contentInsets.isEmpty()
                    && visibleInsets.isEmpty()
                    && touchableRegion.isEmpty()
                    && mTouchableInsets == TOUCHABLE_INSETS_FRAME;
        }

        @Override
        public int hashCode() {
            int result = contentInsets.hashCode();
            result = 31 * result + visibleInsets.hashCode();
            result = 31 * result + touchableRegion.hashCode();
            result = 31 * result + mTouchableInsets;
            return result;
        }

        @Override
        public boolean equals(Object o) {
            if (this == o) return true;
            if (o == null || getClass() != o.getClass()) return false;

            InternalInsetsInfo other = (InternalInsetsInfo)o;
            return mTouchableInsets == other.mTouchableInsets &&
                    contentInsets.equals(other.contentInsets) &&
                    visibleInsets.equals(other.visibleInsets) &&
                    touchableRegion.equals(other.touchableRegion);
        }

        @UnsupportedAppUsage
        void set(InternalInsetsInfo other) {
            contentInsets.set(other.contentInsets);
            visibleInsets.set(other.visibleInsets);
            touchableRegion.set(other.touchableRegion);
            mTouchableInsets = other.mTouchableInsets;
        }
    }

    /**
     * Interface definition for a callback to be invoked when layout has
     * completed and the client can compute its interior insets.
     * 
     * We are not yet ready to commit to this API and support it, so
     * @hide
     */
    public interface OnComputeInternalInsetsListener {
        /**
         * Callback method to be invoked when layout has completed and the
         * client can compute its interior insets.
         *
         * @param inoutInfo Should be filled in by the implementation with
         * the information about the insets of the window.  This is called
         * with whatever values the previous OnComputeInternalInsetsListener
         * returned, if there are multiple such listeners in the window.
         */
        public void onComputeInternalInsets(InternalInsetsInfo inoutInfo);
    }

    /**
     * @hide
     */
    public interface OnEnterAnimationCompleteListener {
        public void onEnterAnimationComplete();
    }

    /**
     * Creates a new ViewTreeObserver. This constructor should not be called
     */
    ViewTreeObserver(Context context) {
        sIllegalOnDrawModificationIsFatal =
                context.getApplicationInfo().targetSdkVersion >= Build.VERSION_CODES.O;
    }

    /**
     * Merges all the listeners registered on the specified observer with the listeners
     * registered on this object. After this method is invoked, the specified observer
     * will return false in {@link #isAlive()} and should not be used anymore.
     *
     * @param observer The ViewTreeObserver whose listeners must be added to this observer
     */
    void merge(ViewTreeObserver observer) {
        if (observer.mOnWindowAttachListeners != null) {
            if (mOnWindowAttachListeners != null) {
                mOnWindowAttachListeners.addAll(observer.mOnWindowAttachListeners);
            } else {
                mOnWindowAttachListeners = observer.mOnWindowAttachListeners;
            }
        }

        if (observer.mOnWindowFocusListeners != null) {
            if (mOnWindowFocusListeners != null) {
                mOnWindowFocusListeners.addAll(observer.mOnWindowFocusListeners);
            } else {
                mOnWindowFocusListeners = observer.mOnWindowFocusListeners;
            }
        }

        if (observer.mOnGlobalFocusListeners != null) {
            if (mOnGlobalFocusListeners != null) {
                mOnGlobalFocusListeners.addAll(observer.mOnGlobalFocusListeners);
            } else {
                mOnGlobalFocusListeners = observer.mOnGlobalFocusListeners;
            }
        }

        if (observer.mOnGlobalLayoutListeners != null) {
            if (mOnGlobalLayoutListeners != null) {
                mOnGlobalLayoutListeners.addAll(observer.mOnGlobalLayoutListeners);
            } else {
                mOnGlobalLayoutListeners = observer.mOnGlobalLayoutListeners;
            }
        }

        if (observer.mOnPreDrawListeners != null) {
            if (mOnPreDrawListeners != null) {
                mOnPreDrawListeners.addAll(observer.mOnPreDrawListeners);
            } else {
                mOnPreDrawListeners = observer.mOnPreDrawListeners;
            }
        }

        if (observer.mOnDrawListeners != null) {
            if (mOnDrawListeners != null) {
                mOnDrawListeners.addAll(observer.mOnDrawListeners);
            } else {
                mOnDrawListeners = observer.mOnDrawListeners;
            }
        }

        if (observer.mOnFrameCommitListeners != null) {
            if (mOnFrameCommitListeners != null) {
                mOnFrameCommitListeners.addAll(observer.captureFrameCommitCallbacks());
            } else {
                mOnFrameCommitListeners = observer.captureFrameCommitCallbacks();
            }
        }

        if (observer.mOnTouchModeChangeListeners != null) {
            if (mOnTouchModeChangeListeners != null) {
                mOnTouchModeChangeListeners.addAll(observer.mOnTouchModeChangeListeners);
            } else {
                mOnTouchModeChangeListeners = observer.mOnTouchModeChangeListeners;
            }
        }

        if (observer.mOnComputeInternalInsetsListeners != null) {
            if (mOnComputeInternalInsetsListeners != null) {
                mOnComputeInternalInsetsListeners.addAll(observer.mOnComputeInternalInsetsListeners);
            } else {
                mOnComputeInternalInsetsListeners = observer.mOnComputeInternalInsetsListeners;
            }
        }

        if (observer.mOnScrollChangedListeners != null) {
            if (mOnScrollChangedListeners != null) {
                mOnScrollChangedListeners.addAll(observer.mOnScrollChangedListeners);
            } else {
                mOnScrollChangedListeners = observer.mOnScrollChangedListeners;
            }
        }

        if (observer.mOnWindowShownListeners != null) {
            if (mOnWindowShownListeners != null) {
                mOnWindowShownListeners.addAll(observer.mOnWindowShownListeners);
            } else {
                mOnWindowShownListeners = observer.mOnWindowShownListeners;
            }
        }

        if (observer.mGestureExclusionListeners != null) {
            if (mGestureExclusionListeners != null) {
                mGestureExclusionListeners.addAll(observer.mGestureExclusionListeners);
            } else {
                mGestureExclusionListeners = observer.mGestureExclusionListeners;
            }
        }

        observer.kill();
    }

    /**
     * Register a callback to be invoked when the view hierarchy is attached to a window.
     *
     * @param listener The callback to add
     *
     * @throws IllegalStateException If {@link #isAlive()} returns false
     */
    public void addOnWindowAttachListener(OnWindowAttachListener listener) {
        checkIsAlive();

        if (mOnWindowAttachListeners == null) {
            mOnWindowAttachListeners
                    = new CopyOnWriteArrayList<OnWindowAttachListener>();
        }

        mOnWindowAttachListeners.add(listener);
    }

    /**
     * Remove a previously installed window attach callback.
     *
     * @param victim The callback to remove
     *
     * @throws IllegalStateException If {@link #isAlive()} returns false
     *
     * @see #addOnWindowAttachListener(android.view.ViewTreeObserver.OnWindowAttachListener)
     */
    public void removeOnWindowAttachListener(OnWindowAttachListener victim) {
        checkIsAlive();
        if (mOnWindowAttachListeners == null) {
            return;
        }
        mOnWindowAttachListeners.remove(victim);
    }

    /**
     * Register a callback to be invoked when the window focus state within the view tree changes.
     *
     * @param listener The callback to add
     *
     * @throws IllegalStateException If {@link #isAlive()} returns false
     */
    public void addOnWindowFocusChangeListener(OnWindowFocusChangeListener listener) {
        checkIsAlive();

        if (mOnWindowFocusListeners == null) {
            mOnWindowFocusListeners
                    = new CopyOnWriteArrayList<OnWindowFocusChangeListener>();
        }

        mOnWindowFocusListeners.add(listener);
    }

    /**
     * Remove a previously installed window focus change callback.
     *
     * @param victim The callback to remove
     *
     * @throws IllegalStateException If {@link #isAlive()} returns false
     *
     * @see #addOnWindowFocusChangeListener(android.view.ViewTreeObserver.OnWindowFocusChangeListener)
     */
    public void removeOnWindowFocusChangeListener(OnWindowFocusChangeListener victim) {
        checkIsAlive();
        if (mOnWindowFocusListeners == null) {
            return;
        }
        mOnWindowFocusListeners.remove(victim);
    }

    /**
     * Register a callback to be invoked when the focus state within the view tree changes.
     *
     * @param listener The callback to add
     *
     * @throws IllegalStateException If {@link #isAlive()} returns false
     */
    public void addOnGlobalFocusChangeListener(OnGlobalFocusChangeListener listener) {
        checkIsAlive();

        if (mOnGlobalFocusListeners == null) {
            mOnGlobalFocusListeners = new CopyOnWriteArrayList<OnGlobalFocusChangeListener>();
        }

        mOnGlobalFocusListeners.add(listener);
    }

    /**
     * Remove a previously installed focus change callback.
     *
     * @param victim The callback to remove
     *
     * @throws IllegalStateException If {@link #isAlive()} returns false
     *
     * @see #addOnGlobalFocusChangeListener(OnGlobalFocusChangeListener)
     */
    public void removeOnGlobalFocusChangeListener(OnGlobalFocusChangeListener victim) {
        checkIsAlive();
        if (mOnGlobalFocusListeners == null) {
            return;
        }
        mOnGlobalFocusListeners.remove(victim);
    }

    /**
     * Register a callback to be invoked when the global layout state or the visibility of views
     * within the view tree changes
     *
     * @param listener The callback to add
     *
     * @throws IllegalStateException If {@link #isAlive()} returns false
     */
    public void addOnGlobalLayoutListener(OnGlobalLayoutListener listener) {
        checkIsAlive();

        if (mOnGlobalLayoutListeners == null) {
            mOnGlobalLayoutListeners = new CopyOnWriteArray<OnGlobalLayoutListener>();
        }

        mOnGlobalLayoutListeners.add(listener);
    }

    /**
     * Remove a previously installed global layout callback
     *
     * @param victim The callback to remove
     *
     * @throws IllegalStateException If {@link #isAlive()} returns false
     * 
     * @deprecated Use #removeOnGlobalLayoutListener instead
     *
     * @see #addOnGlobalLayoutListener(OnGlobalLayoutListener)
     */
    @Deprecated
    public void removeGlobalOnLayoutListener(OnGlobalLayoutListener victim) {
        removeOnGlobalLayoutListener(victim);
    }

    /**
     * Remove a previously installed global layout callback
     *
     * @param victim The callback to remove
     *
     * @throws IllegalStateException If {@link #isAlive()} returns false
     * 
     * @see #addOnGlobalLayoutListener(OnGlobalLayoutListener)
     */
    public void removeOnGlobalLayoutListener(OnGlobalLayoutListener victim) {
        checkIsAlive();
        if (mOnGlobalLayoutListeners == null) {
            return;
        }
        mOnGlobalLayoutListeners.remove(victim);
    }

    /**
     * Register a callback to be invoked when the view tree is about to be drawn
     *
     * @param listener The callback to add
     *
     * @throws IllegalStateException If {@link #isAlive()} returns false
     */
    public void addOnPreDrawListener(OnPreDrawListener listener) {
        checkIsAlive();

        if (mOnPreDrawListeners == null) {
            mOnPreDrawListeners = new CopyOnWriteArray<OnPreDrawListener>();
        }

        mOnPreDrawListeners.add(listener);
    }

    /**
     * Remove a previously installed pre-draw callback
     *
     * @param victim The callback to remove
     *
     * @throws IllegalStateException If {@link #isAlive()} returns false
     *
     * @see #addOnPreDrawListener(OnPreDrawListener)
     */
    public void removeOnPreDrawListener(OnPreDrawListener victim) {
        checkIsAlive();
        if (mOnPreDrawListeners == null) {
            return;
        }
        mOnPreDrawListeners.remove(victim);
    }

    /**
     * Register a callback to be invoked when the view tree window has been shown
     *
     * @param listener The callback to add
     *
     * @throws IllegalStateException If {@link #isAlive()} returns false
     * @hide
     */
    public void addOnWindowShownListener(OnWindowShownListener listener) {
        checkIsAlive();

        if (mOnWindowShownListeners == null) {
            mOnWindowShownListeners = new CopyOnWriteArray<OnWindowShownListener>();
        }

        mOnWindowShownListeners.add(listener);
        if (mWindowShown) {
            listener.onWindowShown();
        }
    }

    /**
     * Remove a previously installed window shown callback
     *
     * @param victim The callback to remove
     *
     * @throws IllegalStateException If {@link #isAlive()} returns false
     *
     * @see #addOnWindowShownListener(OnWindowShownListener)
     * @hide
     */
    public void removeOnWindowShownListener(OnWindowShownListener victim) {
        checkIsAlive();
        if (mOnWindowShownListeners == null) {
            return;
        }
        mOnWindowShownListeners.remove(victim);
    }

    /**
     * <p>Register a callback to be invoked when the view tree is about to be drawn.</p>
     * <p><strong>Note:</strong> this method <strong>cannot</strong> be invoked from
     * {@link android.view.ViewTreeObserver.OnDrawListener#onDraw()}.</p>
     *
     * @param listener The callback to add
     *
     * @throws IllegalStateException If {@link #isAlive()} returns false
     */
    public void addOnDrawListener(OnDrawListener listener) {
        checkIsAlive();

        if (mOnDrawListeners == null) {
            mOnDrawListeners = new ArrayList<OnDrawListener>();
        }

        if (mInDispatchOnDraw) {
            IllegalStateException ex = new IllegalStateException(
                    "Cannot call addOnDrawListener inside of onDraw");
            if (sIllegalOnDrawModificationIsFatal) {
                throw ex;
            } else {
                Log.e("ViewTreeObserver", ex.getMessage(), ex);
            }
        }
        mOnDrawListeners.add(listener);
    }

    /**
     * <p>Remove a previously installed pre-draw callback.</p>
     * <p><strong>Note:</strong> this method <strong>cannot</strong> be invoked from
     * {@link android.view.ViewTreeObserver.OnDrawListener#onDraw()}.</p>
     *
     * @param victim The callback to remove
     *
     * @throws IllegalStateException If {@link #isAlive()} returns false
     *
     * @see #addOnDrawListener(OnDrawListener)
     */
    public void removeOnDrawListener(OnDrawListener victim) {
        checkIsAlive();
        if (mOnDrawListeners == null) {
            return;
        }
        if (mInDispatchOnDraw) {
            IllegalStateException ex = new IllegalStateException(
                    "Cannot call removeOnDrawListener inside of onDraw");
            if (sIllegalOnDrawModificationIsFatal) {
                throw ex;
            } else {
                Log.e("ViewTreeObserver", ex.getMessage(), ex);
            }
        }
        mOnDrawListeners.remove(victim);
    }

    /**
     * Adds a frame commit callback. This callback will be invoked when the current rendering
     * content has been rendered into a frame and submitted to the swap chain. The frame may
     * not currently be visible on the display when this is invoked, but it has been submitted.
     * This callback is useful in combination with {@link PixelCopy} to capture the current
     * rendered content of the UI reliably.
     *
     * Note: Only works with hardware rendering. Does nothing otherwise.
     *
     * @param callback The callback to invoke when the frame is committed.
     */
    public void registerFrameCommitCallback(@NonNull Runnable callback) {
        checkIsAlive();
        if (mOnFrameCommitListeners == null) {
            mOnFrameCommitListeners = new ArrayList<>();
        }
        mOnFrameCommitListeners.add(callback);
    }

    @Nullable ArrayList<Runnable> captureFrameCommitCallbacks() {
        ArrayList<Runnable> ret = mOnFrameCommitListeners;
        mOnFrameCommitListeners = null;
        return ret;
    }

    /**
     * Attempts to remove the given callback from the list of pending frame complete callbacks.
     *
     * @param callback The callback to remove
     * @return Whether or not the callback was removed. If this returns true the callback will
     *         not be invoked. If false is returned then the callback was either never added
     *         or may already be pending execution and was unable to be removed
     */
    public boolean unregisterFrameCommitCallback(@NonNull Runnable callback) {
        checkIsAlive();
        if (mOnFrameCommitListeners == null) {
            return false;
        }
        return mOnFrameCommitListeners.remove(callback);
    }

    /**
     * Register a callback to be invoked when a view has been scrolled.
     *
     * @param listener The callback to add
     *
     * @throws IllegalStateException If {@link #isAlive()} returns false
     */
    public void addOnScrollChangedListener(OnScrollChangedListener listener) {
        checkIsAlive();

        if (mOnScrollChangedListeners == null) {
            mOnScrollChangedListeners = new CopyOnWriteArray<OnScrollChangedListener>();
        }

        mOnScrollChangedListeners.add(listener);
    }

    /**
     * Remove a previously installed scroll-changed callback
     *
     * @param victim The callback to remove
     *
     * @throws IllegalStateException If {@link #isAlive()} returns false
     *
     * @see #addOnScrollChangedListener(OnScrollChangedListener)
     */
    public void removeOnScrollChangedListener(OnScrollChangedListener victim) {
        checkIsAlive();
        if (mOnScrollChangedListeners == null) {
            return;
        }
        mOnScrollChangedListeners.remove(victim);
    }

    /**
     * Register a callback to be invoked when the invoked when the touch mode changes.
     *
     * @param listener The callback to add
     *
     * @throws IllegalStateException If {@link #isAlive()} returns false
     */
    public void addOnTouchModeChangeListener(OnTouchModeChangeListener listener) {
        checkIsAlive();

        if (mOnTouchModeChangeListeners == null) {
            mOnTouchModeChangeListeners = new CopyOnWriteArrayList<OnTouchModeChangeListener>();
        }

        mOnTouchModeChangeListeners.add(listener);
    }

    /**
     * Remove a previously installed touch mode change callback
     *
     * @param victim The callback to remove
     *
     * @throws IllegalStateException If {@link #isAlive()} returns false
     *
     * @see #addOnTouchModeChangeListener(OnTouchModeChangeListener)
     */
    public void removeOnTouchModeChangeListener(OnTouchModeChangeListener victim) {
        checkIsAlive();
        if (mOnTouchModeChangeListeners == null) {
            return;
        }
        mOnTouchModeChangeListeners.remove(victim);
    }

    /**
     * Register a callback to be invoked when the invoked when it is time to
     * compute the window's internal insets.
     *
     * @param listener The callback to add
     *
     * @throws IllegalStateException If {@link #isAlive()} returns false
     * 
     * We are not yet ready to commit to this API and support it, so
     * @hide
     */
    @UnsupportedAppUsage
    public void addOnComputeInternalInsetsListener(OnComputeInternalInsetsListener listener) {
        checkIsAlive();

        if (mOnComputeInternalInsetsListeners == null) {
            mOnComputeInternalInsetsListeners =
                    new CopyOnWriteArray<OnComputeInternalInsetsListener>();
        }

        mOnComputeInternalInsetsListeners.add(listener);
    }

    /**
     * Remove a previously installed internal insets computation callback
     *
     * @param victim The callback to remove
     *
     * @throws IllegalStateException If {@link #isAlive()} returns false
     *
     * @see #addOnComputeInternalInsetsListener(OnComputeInternalInsetsListener)
     * 
     * We are not yet ready to commit to this API and support it, so
     * @hide
     */
    @UnsupportedAppUsage
    public void removeOnComputeInternalInsetsListener(OnComputeInternalInsetsListener victim) {
        checkIsAlive();
        if (mOnComputeInternalInsetsListeners == null) {
            return;
        }
        mOnComputeInternalInsetsListeners.remove(victim);
    }

    /**
     * @hide
     */
    public void addOnEnterAnimationCompleteListener(OnEnterAnimationCompleteListener listener) {
        checkIsAlive();
        if (mOnEnterAnimationCompleteListeners == null) {
            mOnEnterAnimationCompleteListeners =
                    new CopyOnWriteArrayList<OnEnterAnimationCompleteListener>();
        }
        mOnEnterAnimationCompleteListeners.add(listener);
    }

    /**
     * @hide
     */
    public void removeOnEnterAnimationCompleteListener(OnEnterAnimationCompleteListener listener) {
        checkIsAlive();
        if (mOnEnterAnimationCompleteListeners == null) {
            return;
        }
        mOnEnterAnimationCompleteListeners.remove(listener);
    }

    /**
     * Add a listener to be notified when the tree's <em>transformed</em> gesture exclusion rects
     * change. This could be the result of an animation or other layout change, or a view calling
     * {@link View#setSystemGestureExclusionRects(List)}.
     *
     * @param listener listener to add
     * @see View#setSystemGestureExclusionRects(List)
     */
    public void addOnSystemGestureExclusionRectsChangedListener(
            @NonNull Consumer<List<Rect>> listener) {
        checkIsAlive();
        if (mGestureExclusionListeners == null) {
            mGestureExclusionListeners = new CopyOnWriteArray<>();
        }
        mGestureExclusionListeners.add(listener);
    }

    /**
     * Unsubscribe the given listener from gesture exclusion rect changes.
     * @see #addOnSystemGestureExclusionRectsChangedListener(Consumer)
     * @see View#setSystemGestureExclusionRects(List)
     */
    public void removeOnSystemGestureExclusionRectsChangedListener(
            @NonNull Consumer<List<Rect>> listener) {
        checkIsAlive();
        if (mGestureExclusionListeners == null) {
            return;
        }
        mGestureExclusionListeners.remove(listener);
    }

    private void checkIsAlive() {
        if (!mAlive) {
            throw new IllegalStateException("This ViewTreeObserver is not alive, call "
                    + "getViewTreeObserver() again");
        }
    }

    /**
     * Indicates whether this ViewTreeObserver is alive. When an observer is not alive,
     * any call to a method (except this one) will throw an exception.
     *
     * If an application keeps a long-lived reference to this ViewTreeObserver, it should
     * always check for the result of this method before calling any other method.
     *
     * @return True if this object is alive and be used, false otherwise.
     */
    public boolean isAlive() {
        return mAlive;
    }

    /**
     * Marks this ViewTreeObserver as not alive. After invoking this method, invoking
     * any other method but {@link #isAlive()} and {@link #kill()} will throw an Exception.
     *
     * @hide
     */
    private void kill() {
        mAlive = false;
    }

    /**
     * Notifies registered listeners that window has been attached/detached.
     */
    final void dispatchOnWindowAttachedChange(boolean attached) {
        // NOTE: because of the use of CopyOnWriteArrayList, we *must* use an iterator to
        // perform the dispatching. The iterator is a safe guard against listeners that
        // could mutate the list by calling the various add/remove methods. This prevents
        // the array from being modified while we iterate it.
        final CopyOnWriteArrayList<OnWindowAttachListener> listeners
                = mOnWindowAttachListeners;
        if (listeners != null && listeners.size() > 0) {
            for (OnWindowAttachListener listener : listeners) {
                if (attached) listener.onWindowAttached();
                else listener.onWindowDetached();
            }
        }
    }

    /**
     * Notifies registered listeners that window focus has changed.
     */
    final void dispatchOnWindowFocusChange(boolean hasFocus) {
        // NOTE: because of the use of CopyOnWriteArrayList, we *must* use an iterator to
        // perform the dispatching. The iterator is a safe guard against listeners that
        // could mutate the list by calling the various add/remove methods. This prevents
        // the array from being modified while we iterate it.
        final CopyOnWriteArrayList<OnWindowFocusChangeListener> listeners
                = mOnWindowFocusListeners;
        if (listeners != null && listeners.size() > 0) {
            for (OnWindowFocusChangeListener listener : listeners) {
                listener.onWindowFocusChanged(hasFocus);
            }
        }
    }

    /**
     * Notifies registered listeners that focus has changed.
     */
    @UnsupportedAppUsage
    final void dispatchOnGlobalFocusChange(View oldFocus, View newFocus) {
        // NOTE: because of the use of CopyOnWriteArrayList, we *must* use an iterator to
        // perform the dispatching. The iterator is a safe guard against listeners that
        // could mutate the list by calling the various add/remove methods. This prevents
        // the array from being modified while we iterate it.
        final CopyOnWriteArrayList<OnGlobalFocusChangeListener> listeners = mOnGlobalFocusListeners;
        if (listeners != null && listeners.size() > 0) {
            for (OnGlobalFocusChangeListener listener : listeners) {
                listener.onGlobalFocusChanged(oldFocus, newFocus);
            }
        }
    }

    /**
     * Notifies registered listeners that a global layout happened. This can be called
     * manually if you are forcing a layout on a View or a hierarchy of Views that are
     * not attached to a Window or in the GONE state.
     */
    public final void dispatchOnGlobalLayout() {
        // NOTE: because of the use of CopyOnWriteArrayList, we *must* use an iterator to
        // perform the dispatching. The iterator is a safe guard against listeners that
        // could mutate the list by calling the various add/remove methods. This prevents
        // the array from being modified while we iterate it.
        final CopyOnWriteArray<OnGlobalLayoutListener> listeners = mOnGlobalLayoutListeners;
        if (listeners != null && listeners.size() > 0) {
            CopyOnWriteArray.Access<OnGlobalLayoutListener> access = listeners.start();
            try {
                int count = access.size();
                for (int i = 0; i < count; i++) {
                    access.get(i).onGlobalLayout();
                }
            } finally {
                listeners.end();
            }
        }
    }

    /**
     * Returns whether there are listeners for on pre-draw events.
     */
    final boolean hasOnPreDrawListeners() {
        return mOnPreDrawListeners != null && mOnPreDrawListeners.size() > 0;
    }

    /**
     * Notifies registered listeners that the drawing pass is about to start. If a
     * listener returns true, then the drawing pass is canceled and rescheduled. This can
     * be called manually if you are forcing the drawing on a View or a hierarchy of Views
     * that are not attached to a Window or in the GONE state.
     *
     * @return True if the current draw should be canceled and rescheduled, false otherwise.
     */
    @SuppressWarnings("unchecked")
    public final boolean dispatchOnPreDraw() {
        boolean cancelDraw = false;
        final CopyOnWriteArray<OnPreDrawListener> listeners = mOnPreDrawListeners;
        if (listeners != null && listeners.size() > 0) {
            CopyOnWriteArray.Access<OnPreDrawListener> access = listeners.start();
            try {
                int count = access.size();
                for (int i = 0; i < count; i++) {
                    cancelDraw |= !(access.get(i).onPreDraw());
                }
            } finally {
                listeners.end();
            }
        }
        return cancelDraw;
    }

    /**
     * Notifies registered listeners that the window is now shown
     * @hide
     */
    @SuppressWarnings("unchecked")
    public final void dispatchOnWindowShown() {
        mWindowShown = true;
        final CopyOnWriteArray<OnWindowShownListener> listeners = mOnWindowShownListeners;
        if (listeners != null && listeners.size() > 0) {
            CopyOnWriteArray.Access<OnWindowShownListener> access = listeners.start();
            try {
                int count = access.size();
                for (int i = 0; i < count; i++) {
                    access.get(i).onWindowShown();
                }
            } finally {
                listeners.end();
            }
        }
    }

    /**
     * Notifies registered listeners that the drawing pass is about to start.
     */
    public final void dispatchOnDraw() {
        if (mOnDrawListeners != null) {
            mInDispatchOnDraw = true;
            final ArrayList<OnDrawListener> listeners = mOnDrawListeners;
            int numListeners = listeners.size();
            for (int i = 0; i < numListeners; ++i) {
                listeners.get(i).onDraw();
            }
            mInDispatchOnDraw = false;
        }
    }

    /**
     * Notifies registered listeners that the touch mode has changed.
     *
     * @param inTouchMode True if the touch mode is now enabled, false otherwise.
     */
    @UnsupportedAppUsage
    final void dispatchOnTouchModeChanged(boolean inTouchMode) {
        final CopyOnWriteArrayList<OnTouchModeChangeListener> listeners =
                mOnTouchModeChangeListeners;
        if (listeners != null && listeners.size() > 0) {
            for (OnTouchModeChangeListener listener : listeners) {
                listener.onTouchModeChanged(inTouchMode);
            }
        }
    }

    /**
     * Notifies registered listeners that something has scrolled.
     */
    @UnsupportedAppUsage
    final void dispatchOnScrollChanged() {
        // NOTE: because of the use of CopyOnWriteArrayList, we *must* use an iterator to
        // perform the dispatching. The iterator is a safe guard against listeners that
        // could mutate the list by calling the various add/remove methods. This prevents
        // the array from being modified while we iterate it.
        final CopyOnWriteArray<OnScrollChangedListener> listeners = mOnScrollChangedListeners;
        if (listeners != null && listeners.size() > 0) {
            CopyOnWriteArray.Access<OnScrollChangedListener> access = listeners.start();
            try {
                int count = access.size();
                for (int i = 0; i < count; i++) {
                    access.get(i).onScrollChanged();
                }
            } finally {
                listeners.end();
            }
        }
    }

    /**
     * Returns whether there are listeners for computing internal insets.
     */
    @UnsupportedAppUsage
    final boolean hasComputeInternalInsetsListeners() {
        final CopyOnWriteArray<OnComputeInternalInsetsListener> listeners =
                mOnComputeInternalInsetsListeners;
        return (listeners != null && listeners.size() > 0);
    }

    /**
     * Calls all listeners to compute the current insets.
     */
    @UnsupportedAppUsage
    final void dispatchOnComputeInternalInsets(InternalInsetsInfo inoutInfo) {
        // NOTE: because of the use of CopyOnWriteArrayList, we *must* use an iterator to
        // perform the dispatching. The iterator is a safe guard against listeners that
        // could mutate the list by calling the various add/remove methods. This prevents
        // the array from being modified while we iterate it.
        final CopyOnWriteArray<OnComputeInternalInsetsListener> listeners =
                mOnComputeInternalInsetsListeners;
        if (listeners != null && listeners.size() > 0) {
            CopyOnWriteArray.Access<OnComputeInternalInsetsListener> access = listeners.start();
            try {
                int count = access.size();
                for (int i = 0; i < count; i++) {
                    access.get(i).onComputeInternalInsets(inoutInfo);
                }
            } finally {
                listeners.end();
            }
        }
    }

    /**
     * @hide
     */
    public final void dispatchOnEnterAnimationComplete() {
        // NOTE: because of the use of CopyOnWriteArrayList, we *must* use an iterator to
        // perform the dispatching. The iterator is a safe guard against listeners that
        // could mutate the list by calling the various add/remove methods. This prevents
        // the array from being modified while we iterate it.
        final CopyOnWriteArrayList<OnEnterAnimationCompleteListener> listeners =
                mOnEnterAnimationCompleteListeners;
        if (listeners != null && !listeners.isEmpty()) {
            for (OnEnterAnimationCompleteListener listener : listeners) {
                listener.onEnterAnimationComplete();
            }
        }
    }

    void dispatchOnSystemGestureExclusionRectsChanged(@NonNull List<Rect> rects) {
        final CopyOnWriteArray<Consumer<List<Rect>>> listeners = mGestureExclusionListeners;
        if (listeners != null && listeners.size() > 0) {
            CopyOnWriteArray.Access<Consumer<List<Rect>>> access = listeners.start();
            try {
                final int count = access.size();
                for (int i = 0; i < count; i++) {
                    access.get(i).accept(rects);
                }
            } finally {
                listeners.end();
            }
        }
    }

    /**
     * Copy on write array. This array is not thread safe, and only one loop can
     * iterate over this array at any given time. This class avoids allocations
     * until a concurrent modification happens.
     * 
     * Usage:
     * 
     * CopyOnWriteArray.Access<MyData> access = array.start();
     * try {
     *     for (int i = 0; i < access.size(); i++) {
     *         MyData d = access.get(i);
     *     }
     * } finally {
     *     access.end();
     * }
     */
    static class CopyOnWriteArray<T> {
        private ArrayList<T> mData = new ArrayList<T>();
        private ArrayList<T> mDataCopy;

        private final Access<T> mAccess = new Access<T>();

        private boolean mStart;

        static class Access<T> {
            private ArrayList<T> mData;
            private int mSize;

            T get(int index) {
                return mData.get(index);
            }

            int size() {
                return mSize;
            }
        }

        CopyOnWriteArray() {
        }

        private ArrayList<T> getArray() {
            if (mStart) {
                if (mDataCopy == null) mDataCopy = new ArrayList<T>(mData);
                return mDataCopy;
            }
            return mData;
        }

        Access<T> start() {
            if (mStart) throw new IllegalStateException("Iteration already started");
            mStart = true;
            mDataCopy = null;
            mAccess.mData = mData;
            mAccess.mSize = mData.size();
            return mAccess;
        }

        void end() {
            if (!mStart) throw new IllegalStateException("Iteration not started");
            mStart = false;
            if (mDataCopy != null) {
                mData = mDataCopy;
                mAccess.mData.clear();
                mAccess.mSize = 0;
            }
            mDataCopy = null;
        }

        int size() {
            return getArray().size();
        }

        void add(T item) {
            getArray().add(item);
        }

        void addAll(CopyOnWriteArray<T> array) {
            getArray().addAll(array.mData);
        }

        void remove(T item) {
            getArray().remove(item);
        }

        void clear() {
            getArray().clear();
        }
    }
}