xref: /frameworks/base/core/java/android/view/Choreographer.java

/*
 * Copyright (C) 2011 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.hardware.display.DisplayManagerGlobal;
import android.os.Handler;
import android.os.Looper;
import android.os.Message;
import android.os.SystemClock;
import android.os.SystemProperties;
import android.util.Log;

/**
 * Coordinates the timing of animations, input and drawing.
 * <p>
 * The choreographer receives timing pulses (such as vertical synchronization)
 * from the display subsystem then schedules work to occur as part of rendering
 * the next display frame.
 * </p><p>
 * Applications typically interact with the choreographer indirectly using
 * higher level abstractions in the animation framework or the view hierarchy.
 * Here are some examples of things you can do using the higher-level APIs.
 * </p>
 * <ul>
 * <li>To post an animation to be processed on a regular time basis synchronized with
 * display frame rendering, use {@link android.animation.ValueAnimator#start}.</li>
 * <li>To post a {@link Runnable} to be invoked once at the beginning of the next display
 * frame, use {@link View#postOnAnimation}.</li>
 * <li>To post a {@link Runnable} to be invoked once at the beginning of the next display
 * frame after a delay, use {@link View#postOnAnimationDelayed}.</li>
 * <li>To post a call to {@link View#invalidate()} to occur once at the beginning of the
 * next display frame, use {@link View#postInvalidateOnAnimation()} or
 * {@link View#postInvalidateOnAnimation(int, int, int, int)}.</li>
 * <li>To ensure that the contents of a {@link View} scroll smoothly and are drawn in
 * sync with display frame rendering, do nothing.  This already happens automatically.
 * {@link View#onDraw} will be called at the appropriate time.</li>
 * </ul>
 * <p>
 * However, there are a few cases where you might want to use the functions of the
 * choreographer directly in your application.  Here are some examples.
 * </p>
 * <ul>
 * <li>If your application does its rendering in a different thread, possibly using GL,
 * or does not use the animation framework or view hierarchy at all
 * and you want to ensure that it is appropriately synchronized with the display, then use
 * {@link Choreographer#postFrameCallback}.</li>
 * <li>... and that's about it.</li>
 * </ul>
 * <p>
 * Each {@link Looper} thread has its own choreographer.  Other threads can
 * post callbacks to run on the choreographer but they will run on the {@link Looper}
 * to which the choreographer belongs.
 * </p>
 */
public final class Choreographer {
    private static final String TAG = "Choreographer";
    private static final boolean DEBUG = false;

    // The default amount of time in ms between animation frames.
    // When vsync is not enabled, we want to have some idea of how long we should
    // wait before posting the next animation message.  It is important that the
    // default value be less than the true inter-frame delay on all devices to avoid
    // situations where we might skip frames by waiting too long (we must compensate
    // for jitter and hardware variations).  Regardless of this value, the animation
    // and display loop is ultimately rate-limited by how fast new graphics buffers can
    // be dequeued.
    private static final long DEFAULT_FRAME_DELAY = 10;

    // The number of milliseconds between animation frames.
    private static volatile long sFrameDelay = DEFAULT_FRAME_DELAY;

    // Thread local storage for the choreographer.
    private static final ThreadLocal<Choreographer> sThreadInstance =
            new ThreadLocal<Choreographer>() {
        @Override
        protected Choreographer initialValue() {
            Looper looper = Looper.myLooper();
            if (looper == null) {
                throw new IllegalStateException("The current thread must have a looper!");
            }
            return new Choreographer(looper);
        }
    };

    // Enable/disable vsync for animations and drawing.
    private static final boolean USE_VSYNC = SystemProperties.getBoolean(
            "debug.choreographer.vsync", true);

    // Enable/disable using the frame time instead of returning now.
    private static final boolean USE_FRAME_TIME = SystemProperties.getBoolean(
            "debug.choreographer.frametime", true);

    // Set a limit to warn about skipped frames.
    // Skipped frames imply jank.
    private static final int SKIPPED_FRAME_WARNING_LIMIT = SystemProperties.getInt(
            "debug.choreographer.skipwarning", 30);

    private static final long NANOS_PER_MS = 1000000;

    private static final int MSG_DO_FRAME = 0;
    private static final int MSG_DO_SCHEDULE_VSYNC = 1;
    private static final int MSG_DO_SCHEDULE_CALLBACK = 2;

    // All frame callbacks posted by applications have this token.
    private static final Object FRAME_CALLBACK_TOKEN = new Object() {
        public String toString() { return "FRAME_CALLBACK_TOKEN"; }
    };

    private final Object mLock = new Object();

    private final Looper mLooper;
    private final FrameHandler mHandler;

    // The display event receiver can only be accessed by the looper thread to which
    // it is attached.  We take care to ensure that we post message to the looper
    // if appropriate when interacting with the display event receiver.
    private final FrameDisplayEventReceiver mDisplayEventReceiver;

    private CallbackRecord mCallbackPool;

    private final CallbackQueue[] mCallbackQueues;

    private boolean mFrameScheduled;
    private boolean mCallbacksRunning;
    private long mLastFrameTimeNanos;
    private long mFrameIntervalNanos;

    /**
     * Callback type: Input callback.  Runs first.
     * @hide
     */
    public static final int CALLBACK_INPUT = 0;

    /**
     * Callback type: Animation callback.  Runs before traversals.
     * @hide
     */
    public static final int CALLBACK_ANIMATION = 1;

    /**
     * Callback type: Traversal callback.  Handles layout and draw.  Runs last
     * after all other asynchronous messages have been handled.
     * @hide
     */
    public static final int CALLBACK_TRAVERSAL = 2;

    private static final int CALLBACK_LAST = CALLBACK_TRAVERSAL;

    private Choreographer(Looper looper) {
        mLooper = looper;
        mHandler = new FrameHandler(looper);
        mDisplayEventReceiver = USE_VSYNC ? new FrameDisplayEventReceiver(looper) : null;
        mLastFrameTimeNanos = Long.MIN_VALUE;

        mFrameIntervalNanos = (long)(1000000000 / getRefreshRate());

        mCallbackQueues = new CallbackQueue[CALLBACK_LAST + 1];
        for (int i = 0; i <= CALLBACK_LAST; i++) {
            mCallbackQueues[i] = new CallbackQueue();
        }
    }

    private static float getRefreshRate() {
        DisplayInfo di = DisplayManagerGlobal.getInstance().getDisplayInfo(
                Display.DEFAULT_DISPLAY);
        return di.refreshRate;
    }

    /**
     * Gets the choreographer for the calling thread.  Must be called from
     * a thread that already has a {@link android.os.Looper} associated with it.
     *
     * @return The choreographer for this thread.
     * @throws IllegalStateException if the thread does not have a looper.
     */
    public static Choreographer getInstance() {
        return sThreadInstance.get();
    }

    /**
     * The amount of time, in milliseconds, between each frame of the animation.
     * <p>
     * This is a requested time that the animation will attempt to honor, but the actual delay
     * between frames may be different, depending on system load and capabilities. This is a static
     * function because the same delay will be applied to all animations, since they are all
     * run off of a single timing loop.
     * </p><p>
     * The frame delay may be ignored when the animation system uses an external timing
     * source, such as the display refresh rate (vsync), to govern animations.
     * </p>
     *
     * @return the requested time between frames, in milliseconds
     * @hide
     */
    public static long getFrameDelay() {
        return sFrameDelay;
    }

    /**
     * The amount of time, in milliseconds, between each frame of the animation.
     * <p>
     * This is a requested time that the animation will attempt to honor, but the actual delay
     * between frames may be different, depending on system load and capabilities. This is a static
     * function because the same delay will be applied to all animations, since they are all
     * run off of a single timing loop.
     * </p><p>
     * The frame delay may be ignored when the animation system uses an external timing
     * source, such as the display refresh rate (vsync), to govern animations.
     * </p>
     *
     * @param frameDelay the requested time between frames, in milliseconds
     * @hide
     */
    public static void setFrameDelay(long frameDelay) {
        sFrameDelay = frameDelay;
    }

    /**
     * Subtracts typical frame delay time from a delay interval in milliseconds.
     * <p>
     * This method can be used to compensate for animation delay times that have baked
     * in assumptions about the frame delay.  For example, it's quite common for code to
     * assume a 60Hz frame time and bake in a 16ms delay.  When we call
     * {@link #postAnimationCallbackDelayed} we want to know how long to wait before
     * posting the animation callback but let the animation timer take care of the remaining
     * frame delay time.
     * </p><p>
     * This method is somewhat conservative about how much of the frame delay it
     * subtracts.  It uses the same value returned by {@link #getFrameDelay} which by
     * default is 10ms even though many parts of the system assume 16ms.  Consequently,
     * we might still wait 6ms before posting an animation callback that we want to run
     * on the next frame, but this is much better than waiting a whole 16ms and likely
     * missing the deadline.
     * </p>
     *
     * @param delayMillis The original delay time including an assumed frame delay.
     * @return The adjusted delay time with the assumed frame delay subtracted out.
     * @hide
     */
    public static long subtractFrameDelay(long delayMillis) {
        final long frameDelay = sFrameDelay;
        return delayMillis <= frameDelay ? 0 : delayMillis - frameDelay;
    }

    /**
     * Posts a callback to run on the next frame.
     * <p>
     * The callback runs once then is automatically removed.
     * </p>
     *
     * @param callbackType The callback type.
     * @param action The callback action to run during the next frame.
     * @param token The callback token, or null if none.
     *
     * @see #removeCallbacks
     * @hide
     */
    public void postCallback(int callbackType, Runnable action, Object token) {
        postCallbackDelayed(callbackType, action, token, 0);
    }

    /**
     * Posts a callback to run on the next frame after the specified delay.
     * <p>
     * The callback runs once then is automatically removed.
     * </p>
     *
     * @param callbackType The callback type.
     * @param action The callback action to run during the next frame after the specified delay.
     * @param token The callback token, or null if none.
     * @param delayMillis The delay time in milliseconds.
     *
     * @see #removeCallback
     * @hide
     */
    public void postCallbackDelayed(int callbackType,
            Runnable action, Object token, long delayMillis) {
        if (action == null) {
            throw new IllegalArgumentException("action must not be null");
        }
        if (callbackType < 0 || callbackType > CALLBACK_LAST) {
            throw new IllegalArgumentException("callbackType is invalid");
        }

        postCallbackDelayedInternal(callbackType, action, token, delayMillis);
    }

    private void postCallbackDelayedInternal(int callbackType,
            Object action, Object token, long delayMillis) {
        if (DEBUG) {
            Log.d(TAG, "PostCallback: type=" + callbackType
                    + ", action=" + action + ", token=" + token
                    + ", delayMillis=" + delayMillis);
        }

        synchronized (mLock) {
            final long now = SystemClock.uptimeMillis();
            final long dueTime = now + delayMillis;
            mCallbackQueues[callbackType].addCallbackLocked(dueTime, action, token);

            if (dueTime <= now) {
                scheduleFrameLocked(now);
            } else {
                Message msg = mHandler.obtainMessage(MSG_DO_SCHEDULE_CALLBACK, action);
                msg.arg1 = callbackType;
                msg.setAsynchronous(true);
                mHandler.sendMessageAtTime(msg, dueTime);
            }
        }
    }

    /**
     * Removes callbacks that have the specified action and token.
     *
     * @param callbackType The callback type.
     * @param action The action property of the callbacks to remove, or null to remove
     * callbacks with any action.
     * @param token The token property of the callbacks to remove, or null to remove
     * callbacks with any token.
     *
     * @see #postCallback
     * @see #postCallbackDelayed
     * @hide
     */
    public void removeCallbacks(int callbackType, Runnable action, Object token) {
        if (callbackType < 0 || callbackType > CALLBACK_LAST) {
            throw new IllegalArgumentException("callbackType is invalid");
        }

        removeCallbacksInternal(callbackType, action, token);
    }

    private void removeCallbacksInternal(int callbackType, Object action, Object token) {
        if (DEBUG) {
            Log.d(TAG, "RemoveCallbacks: type=" + callbackType
                    + ", action=" + action + ", token=" + token);
        }

        synchronized (mLock) {
            mCallbackQueues[callbackType].removeCallbacksLocked(action, token);
            if (action != null && token == null) {
                mHandler.removeMessages(MSG_DO_SCHEDULE_CALLBACK, action);
            }
        }
    }

    /**
     * Posts a frame callback to run on the next frame.
     * <p>
     * The callback runs once then is automatically removed.
     * </p>
     *
     * @param callback The frame callback to run during the next frame.
     *
     * @see #postFrameCallbackDelayed
     * @see #removeFrameCallback
     */
    public void postFrameCallback(FrameCallback callback) {
        postFrameCallbackDelayed(callback, 0);
    }

    /**
     * Posts a frame callback to run on the next frame after the specified delay.
     * <p>
     * The callback runs once then is automatically removed.
     * </p>
     *
     * @param callback The frame callback to run during the next frame.
     * @param delayMillis The delay time in milliseconds.
     *
     * @see #postFrameCallback
     * @see #removeFrameCallback
     */
    public void postFrameCallbackDelayed(FrameCallback callback, long delayMillis) {
        if (callback == null) {
            throw new IllegalArgumentException("callback must not be null");
        }

        postCallbackDelayedInternal(CALLBACK_ANIMATION,
                callback, FRAME_CALLBACK_TOKEN, delayMillis);
    }

    /**
     * Removes a previously posted frame callback.
     *
     * @param callback The frame callback to remove.
     *
     * @see #postFrameCallback
     * @see #postFrameCallbackDelayed
     */
    public void removeFrameCallback(FrameCallback callback) {
        if (callback == null) {
            throw new IllegalArgumentException("callback must not be null");
        }

        removeCallbacksInternal(CALLBACK_ANIMATION, callback, FRAME_CALLBACK_TOKEN);
    }

    /**
     * Gets the time when the current frame started.
     * <p>
     * This method provides the time in nanoseconds when the frame started being rendered.
     * The frame time provides a stable time base for synchronizing animations
     * and drawing.  It should be used instead of {@link SystemClock#uptimeMillis()}
     * or {@link System#nanoTime()} for animations and drawing in the UI.  Using the frame
     * time helps to reduce inter-frame jitter because the frame time is fixed at the time
     * the frame was scheduled to start, regardless of when the animations or drawing
     * callback actually runs.  All callbacks that run as part of rendering a frame will
     * observe the same frame time so using the frame time also helps to synchronize effects
     * that are performed by different callbacks.
     * </p><p>
     * Please note that the framework already takes care to process animations and
     * drawing using the frame time as a stable time base.  Most applications should
     * not need to use the frame time information directly.
     * </p><p>
     * This method should only be called from within a callback.
     * </p>
     *
     * @return The frame start time, in the {@link SystemClock#uptimeMillis()} time base.
     *
     * @throws IllegalStateException if no frame is in progress.
     * @hide
     */
    public long getFrameTime() {
        return getFrameTimeNanos() / NANOS_PER_MS;
    }

    /**
     * Same as {@link #getFrameTime()} but with nanosecond precision.
     *
     * @return The frame start time, in the {@link System#nanoTime()} time base.
     *
     * @throws IllegalStateException if no frame is in progress.
     * @hide
     */
    public long getFrameTimeNanos() {
        synchronized (mLock) {
            if (!mCallbacksRunning) {
                throw new IllegalStateException("This method must only be called as "
                        + "part of a callback while a frame is in progress.");
            }
            return USE_FRAME_TIME ? mLastFrameTimeNanos : System.nanoTime();
        }
    }

    private void scheduleFrameLocked(long now) {
        if (!mFrameScheduled) {
            mFrameScheduled = true;
            if (USE_VSYNC) {
                if (DEBUG) {
                    Log.d(TAG, "Scheduling next frame on vsync.");
                }

                // If running on the Looper thread, then schedule the vsync immediately,
                // otherwise post a message to schedule the vsync from the UI thread
                // as soon as possible.
                if (isRunningOnLooperThreadLocked()) {
                    scheduleVsyncLocked();
                } else {
                    Message msg = mHandler.obtainMessage(MSG_DO_SCHEDULE_VSYNC);
                    msg.setAsynchronous(true);
                    mHandler.sendMessageAtFrontOfQueue(msg);
                }
            } else {
                final long nextFrameTime = Math.max(
                        mLastFrameTimeNanos / NANOS_PER_MS + sFrameDelay, now);
                if (DEBUG) {
                    Log.d(TAG, "Scheduling next frame in " + (nextFrameTime - now) + " ms.");
                }
                Message msg = mHandler.obtainMessage(MSG_DO_FRAME);
                msg.setAsynchronous(true);
                mHandler.sendMessageAtTime(msg, nextFrameTime);
            }
        }
    }

    void doFrame(long frameTimeNanos, int frame) {
        final long startNanos;
        synchronized (mLock) {
            if (!mFrameScheduled) {
                return; // no work to do
            }

            startNanos = System.nanoTime();
            final long jitterNanos = startNanos - frameTimeNanos;
            if (jitterNanos >= mFrameIntervalNanos) {
                final long skippedFrames = jitterNanos / mFrameIntervalNanos;
                if (skippedFrames >= SKIPPED_FRAME_WARNING_LIMIT) {
                    Log.i(TAG, "Skipped " + skippedFrames + " frames!  "
                            + "The application may be doing too much work on its main thread.");
                }
                final long lastFrameOffset = jitterNanos % mFrameIntervalNanos;
                if (DEBUG) {
                    Log.d(TAG, "Missed vsync by " + (jitterNanos * 0.000001f) + " ms "
                            + "which is more than the frame interval of "
                            + (mFrameIntervalNanos * 0.000001f) + " ms!  "
                            + "Skipping " + skippedFrames + " frames and setting frame "
                            + "time to " + (lastFrameOffset * 0.000001f) + " ms in the past.");
                }
                frameTimeNanos = startNanos - lastFrameOffset;
            }

            if (frameTimeNanos < mLastFrameTimeNanos) {
                if (DEBUG) {
                    Log.d(TAG, "Frame time appears to be going backwards.  May be due to a "
                            + "previously skipped frame.  Waiting for next vsync.");
                }
                scheduleVsyncLocked();
                return;
            }

            mFrameScheduled = false;
            mLastFrameTimeNanos = frameTimeNanos;
        }

        doCallbacks(Choreographer.CALLBACK_INPUT, frameTimeNanos);
        doCallbacks(Choreographer.CALLBACK_ANIMATION, frameTimeNanos);
        doCallbacks(Choreographer.CALLBACK_TRAVERSAL, frameTimeNanos);

        if (DEBUG) {
            final long endNanos = System.nanoTime();
            Log.d(TAG, "Frame " + frame + ": Finished, took "
                    + (endNanos - startNanos) * 0.000001f + " ms, latency "
                    + (startNanos - frameTimeNanos) * 0.000001f + " ms.");
        }
    }

    void doCallbacks(int callbackType, long frameTimeNanos) {
        CallbackRecord callbacks;
        synchronized (mLock) {
            // We use "now" to determine when callbacks become due because it's possible
            // for earlier processing phases in a frame to post callbacks that should run
            // in a following phase, such as an input event that causes an animation to start.
            final long now = SystemClock.uptimeMillis();
            callbacks = mCallbackQueues[callbackType].extractDueCallbacksLocked(now);
            if (callbacks == null) {
                return;
            }
            mCallbacksRunning = true;
        }
        try {
            for (CallbackRecord c = callbacks; c != null; c = c.next) {
                if (DEBUG) {
                    Log.d(TAG, "RunCallback: type=" + callbackType
                            + ", action=" + c.action + ", token=" + c.token
                            + ", latencyMillis=" + (SystemClock.uptimeMillis() - c.dueTime));
                }
                c.run(frameTimeNanos);
            }
        } finally {
            synchronized (mLock) {
                mCallbacksRunning = false;
                do {
                    final CallbackRecord next = callbacks.next;
                    recycleCallbackLocked(callbacks);
                    callbacks = next;
                } while (callbacks != null);
            }
        }
    }

    void doScheduleVsync() {
        synchronized (mLock) {
            if (mFrameScheduled) {
                scheduleVsyncLocked();
            }
        }
    }

    void doScheduleCallback(int callbackType) {
        synchronized (mLock) {
            if (!mFrameScheduled) {
                final long now = SystemClock.uptimeMillis();
                if (mCallbackQueues[callbackType].hasDueCallbacksLocked(now)) {
                    scheduleFrameLocked(now);
                }
            }
        }
    }

    private void scheduleVsyncLocked() {
        mDisplayEventReceiver.scheduleVsync();
    }

    private boolean isRunningOnLooperThreadLocked() {
        return Looper.myLooper() == mLooper;
    }

    private CallbackRecord obtainCallbackLocked(long dueTime, Object action, Object token) {
        CallbackRecord callback = mCallbackPool;
        if (callback == null) {
            callback = new CallbackRecord();
        } else {
            mCallbackPool = callback.next;
            callback.next = null;
        }
        callback.dueTime = dueTime;
        callback.action = action;
        callback.token = token;
        return callback;
    }

    private void recycleCallbackLocked(CallbackRecord callback) {
        callback.action = null;
        callback.token = null;
        callback.next = mCallbackPool;
        mCallbackPool = callback;
    }

    /**
     * Implement this interface to receive a callback when a new display frame is
     * being rendered.  The callback is invoked on the {@link Looper} thread to
     * which the {@link Choreographer} is attached.
     */
    public interface FrameCallback {
        /**
         * Called when a new display frame is being rendered.
         * <p>
         * This method provides the time in nanoseconds when the frame started being rendered.
         * The frame time provides a stable time base for synchronizing animations
         * and drawing.  It should be used instead of {@link SystemClock#uptimeMillis()}
         * or {@link System#nanoTime()} for animations and drawing in the UI.  Using the frame
         * time helps to reduce inter-frame jitter because the frame time is fixed at the time
         * the frame was scheduled to start, regardless of when the animations or drawing
         * callback actually runs.  All callbacks that run as part of rendering a frame will
         * observe the same frame time so using the frame time also helps to synchronize effects
         * that are performed by different callbacks.
         * </p><p>
         * Please note that the framework already takes care to process animations and
         * drawing using the frame time as a stable time base.  Most applications should
         * not need to use the frame time information directly.
         * </p>
         *
         * @param frameTimeNanos The time in nanoseconds when the frame started being rendered,
         * in the {@link System#nanoTime()} timebase.  Divide this value by {@code 1000000}
         * to convert it to the {@link SystemClock#uptimeMillis()} time base.
         */
        public void doFrame(long frameTimeNanos);
    }

    private final class FrameHandler extends Handler {
        public FrameHandler(Looper looper) {
            super(looper);
        }

        @Override
        public void handleMessage(Message msg) {
            switch (msg.what) {
                case MSG_DO_FRAME:
                    doFrame(System.nanoTime(), 0);
                    break;
                case MSG_DO_SCHEDULE_VSYNC:
                    doScheduleVsync();
                    break;
                case MSG_DO_SCHEDULE_CALLBACK:
                    doScheduleCallback(msg.arg1);
                    break;
            }
        }
    }

    private final class FrameDisplayEventReceiver extends DisplayEventReceiver
            implements Runnable {
        private boolean mHavePendingVsync;
        private long mTimestampNanos;
        private int mFrame;

        public FrameDisplayEventReceiver(Looper looper) {
            super(looper);
        }

        @Override
        public void onVsync(long timestampNanos, int builtInDisplayId, int frame) {
            // Ignore vsync from secondary display.
            // This can be problematic because the call to scheduleVsync() is a one-shot.
            // We need to ensure that we will still receive the vsync from the primary
            // display which is the one we really care about.  Ideally we should schedule
            // vsync for a particular display.
            // At this time Surface Flinger won't send us vsyncs for secondary displays
            // but that could change in the future so let's log a message to help us remember
            // that we need to fix this.
            if (builtInDisplayId != SurfaceControl.BUILT_IN_DISPLAY_ID_MAIN) {
                Log.d(TAG, "Received vsync from secondary display, but we don't support "
                        + "this case yet.  Choreographer needs a way to explicitly request "
                        + "vsync for a specific display to ensure it doesn't lose track "
                        + "of its scheduled vsync.");
                scheduleVsync();
                return;
            }

            // Post the vsync event to the Handler.
            // The idea is to prevent incoming vsync events from completely starving
            // the message queue.  If there are no messages in the queue with timestamps
            // earlier than the frame time, then the vsync event will be processed immediately.
            // Otherwise, messages that predate the vsync event will be handled first.
            long now = System.nanoTime();
            if (timestampNanos > now) {
                Log.w(TAG, "Frame time is " + ((timestampNanos - now) * 0.000001f)
                        + " ms in the future!  Check that graphics HAL is generating vsync "
                        + "timestamps using the correct timebase.");
                timestampNanos = now;
            }

            if (mHavePendingVsync) {
                Log.w(TAG, "Already have a pending vsync event.  There should only be "
                        + "one at a time.");
            } else {
                mHavePendingVsync = true;
            }

            mTimestampNanos = timestampNanos;
            mFrame = frame;
            Message msg = Message.obtain(mHandler, this);
            msg.setAsynchronous(true);
            mHandler.sendMessageAtTime(msg, timestampNanos / NANOS_PER_MS);
        }

        @Override
        public void run() {
            mHavePendingVsync = false;
            doFrame(mTimestampNanos, mFrame);
        }
    }

    private static final class CallbackRecord {
        public CallbackRecord next;
        public long dueTime;
        public Object action; // Runnable or FrameCallback
        public Object token;

        public void run(long frameTimeNanos) {
            if (token == FRAME_CALLBACK_TOKEN) {
                ((FrameCallback)action).doFrame(frameTimeNanos);
            } else {
                ((Runnable)action).run();
            }
        }
    }

    private final class CallbackQueue {
        private CallbackRecord mHead;

        public boolean hasDueCallbacksLocked(long now) {
            return mHead != null && mHead.dueTime <= now;
        }

        public CallbackRecord extractDueCallbacksLocked(long now) {
            CallbackRecord callbacks = mHead;
            if (callbacks == null || callbacks.dueTime > now) {
                return null;
            }

            CallbackRecord last = callbacks;
            CallbackRecord next = last.next;
            while (next != null) {
                if (next.dueTime > now) {
                    last.next = null;
                    break;
                }
                last = next;
                next = next.next;
            }
            mHead = next;
            return callbacks;
        }

        public void addCallbackLocked(long dueTime, Object action, Object token) {
            CallbackRecord callback = obtainCallbackLocked(dueTime, action, token);
            CallbackRecord entry = mHead;
            if (entry == null) {
                mHead = callback;
                return;
            }
            if (dueTime < entry.dueTime) {
                callback.next = entry;
                mHead = callback;
                return;
            }
            while (entry.next != null) {
                if (dueTime < entry.next.dueTime) {
                    callback.next = entry.next;
                    break;
                }
                entry = entry.next;
            }
            entry.next = callback;
        }

        public void removeCallbacksLocked(Object action, Object token) {
            CallbackRecord predecessor = null;
            for (CallbackRecord callback = mHead; callback != null;) {
                final CallbackRecord next = callback.next;
                if ((action == null || callback.action == action)
                        && (token == null || callback.token == token)) {
                    if (predecessor != null) {
                        predecessor.next = next;
                    } else {
                        mHead = next;
                    }
                    recycleCallbackLocked(callback);
                } else {
                    predecessor = callback;
                }
                callback = next;
            }
        }
    }
}