// 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!");
        }
        Choreographer choreographer = new Choreographer(looper, VSYNC_SOURCE_APP);
        if (looper == Looper.getMainLooper()) {
            mMainInstance = choreographer;
        }
        return choreographer;
    }
};

private Choreographer(Looper looper, int vsyncSource) {
    mLooper = looper;
    mHandler = new FrameHandler(looper);　　　　　// 這里可以發(fā)現(xiàn)只有在為 true 的時候才會使用 vsync 
    mDisplayEventReceiver = USE_VSYNC
            ? new FrameDisplayEventReceiver(looper, vsyncSource)
            : null;
    mLastFrameTimeNanos = Long.MIN_VALUE;
　　　　 // 每一幀的間隔是根據(jù)刷新頻率來的
    mFrameIntervalNanos = (long)(1000000000 / getRefreshRate());

    mCallbackQueues = new CallbackQueue[CALLBACK_LAST + 1];　　　　　// 給每一種回調(diào)類型都創(chuàng)建了一個隊(duì)列
    for (int i = 0; i <= CALLBACK_LAST; i++) {
        mCallbackQueues[i] = new CallbackQueue();
    }
    // b/68769804: For low FPS experiments.
    setFPSDivisor(SystemProperties.getInt(ThreadedRenderer.DEBUG_FPS_DIVISOR, 1));
}

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, int vsyncSource) {
        super(looper, vsyncSource);
    }

    // TODO(b/116025192): physicalDisplayId is ignored because SF only emits VSYNC events for
    // the internal display and DisplayEventReceiver#scheduleVsync only allows requesting VSYNC
    // for the internal display implicitly.
    @Override
    public void onVsync(long timestampNanos, long physicalDisplayId, int frame) {
        // 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 / TimeUtils.NANOS_PER_MS);
    }

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

// android/view/Choreographer.java
private Choreographer(Looper looper, int vsyncSource) {
    mLooper = looper;
    mDisplayEventReceiver = USE_VSYNC
            ? new FrameDisplayEventReceiver(looper, vsyncSource)
            : null;
    ......
}
// android/view/Choreographer.java
public FrameDisplayEventReceiver(Looper looper, int vsyncSource) {
    super(looper, vsyncSource);
}
// android/view/DisplayEventReceiver.java
public DisplayEventReceiver(Looper looper, int vsyncSource) {
    ......
    mMessageQueue = looper.getQueue();
    mReceiverPtr = nativeInit(new WeakReference<DisplayEventReceiver>(this), mMessageQueue,
            vsyncSource);
}

@UnsupportedAppUsage
void doFrame(long frameTimeNanos, int frame) {
    final long startNanos;
    synchronized (mLock) {　　　　　　　// 為false, 說明還未開始
        if (!mFrameScheduled) {
            return; // no work to do
        }

        if (DEBUG_JANK && mDebugPrintNextFrameTimeDelta) {
            mDebugPrintNextFrameTimeDelta = false;
            Log.d(TAG, "Frame time delta: "
                    + ((frameTimeNanos - mLastFrameTimeNanos) * 0.000001f) + " ms");
        }

        long intendedFrameTimeNanos = frameTimeNanos;
        startNanos = System.nanoTime();　　　　　　　// 計(jì)算當(dāng)前時間與 vsync 信號的時間差
        final long jitterNanos = startNanos - frameTimeNanos;　　　　　　　// 說明出現(xiàn)掉幀情況,注意只有 jitterNanos 大于 16.6 ms 才說明掉幀，否則只是輕微的延遲。
        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.");
            }　　　　　　　　　　// 當(dāng)發(fā)生掉幀后，需要計(jì)算被耽誤的時間。比如處理了 36.6ms, 一個周期是 16.6 ms, 相當(dāng)于延遲了 3.4 ms 執(zhí)行
            final long lastFrameOffset = jitterNanos % mFrameIntervalNanos;
            if (DEBUG_JANK) {
                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.");
            }　　　　　　　　　 // 修正當(dāng)前幀的時間 = 開始時間 - 耽誤時間
            frameTimeNanos = startNanos - lastFrameOffset;
        }
　　　　　　　// 當(dāng)前時間小于前一幀時間，不執(zhí)行操作
        if (frameTimeNanos < mLastFrameTimeNanos) {
            if (DEBUG_JANK) {
                Log.d(TAG, "Frame time appears to be going backwards.  May be due to a "
                        + "previously skipped frame.  Waiting for next vsync.");
            }　　　　　　　　　　// 直接請求下一個 vsync 信號
            scheduleVsyncLocked();
            return;
        }
　　　　　　　// 大于 1 說明采用的默認(rèn)幀數(shù)的一半，因此需要根據(jù)時間間隔來判斷是否有必要執(zhí)行繪制
        if (mFPSDivisor > 1) {
            long timeSinceVsync = frameTimeNanos - mLastFrameTimeNanos;
            if (timeSinceVsync < (mFrameIntervalNanos * mFPSDivisor) && timeSinceVsync > 0) {　　　　　　　　　　　　// 時間間隔小于指定的時間，繼續(xù)請求下一個 vsync 信號
                scheduleVsyncLocked();
                return;
            }
        }
　　　　　　　// 保存當(dāng)前幀的相關(guān)信息
        mFrameInfo.setVsync(intendedFrameTimeNanos, frameTimeNanos);
        mFrameScheduled = false;
        mLastFrameTimeNanos = frameTimeNanos;
    }

    try {　　　　　　　// 執(zhí)行相關(guān) callbacks 
        Trace.traceBegin(Trace.TRACE_TAG_VIEW, "Choreographer#doFrame");
        AnimationUtils.lockAnimationClock(frameTimeNanos / TimeUtils.NANOS_PER_MS);

        mFrameInfo.markInputHandlingStart();
        doCallbacks(Choreographer.CALLBACK_INPUT, frameTimeNanos);

        mFrameInfo.markAnimationsStart();
        doCallbacks(Choreographer.CALLBACK_ANIMATION, frameTimeNanos);
        doCallbacks(Choreographer.CALLBACK_INSETS_ANIMATION, frameTimeNanos);

        mFrameInfo.markPerformTraversalsStart();
        doCallbacks(Choreographer.CALLBACK_TRAVERSAL, frameTimeNanos);

        doCallbacks(Choreographer.CALLBACK_COMMIT, frameTimeNanos);
    } finally {
        AnimationUtils.unlockAnimationClock();
        Trace.traceEnd(Trace.TRACE_TAG_VIEW);
    }

    if (DEBUG_FRAMES) {
        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 = System.nanoTime();　　　　　　　// 
        callbacks = mCallbackQueues[callbackType].extractDueCallbacksLocked(
                now / TimeUtils.NANOS_PER_MS);
        if (callbacks == null) {
            return;
        }
        mCallbacksRunning = true;

        // Update the frame time if necessary when committing the frame.
        // We only update the frame time if we are more than 2 frames late reaching
        // the commit phase.  This ensures that the frame time which is observed by the
        // callbacks will always increase from one frame to the next and never repeat.
        // We never want the next frame's starting frame time to end up being less than
        // or equal to the previous frame's commit frame time.  Keep in mind that the
        // next frame has most likely already been scheduled by now so we play it
        // safe by ensuring the commit time is always at least one frame behind.　　　　　　　// commit 類型是最后執(zhí)行的，如果此時發(fā)現(xiàn)前面處理時間過長，就會進(jìn)行糾正。
        if (callbackType == Choreographer.CALLBACK_COMMIT) {
            final long jitterNanos = now - frameTimeNanos;
            Trace.traceCounter(Trace.TRACE_TAG_VIEW, "jitterNanos", (int) jitterNanos);
            if (jitterNanos >= 2 * mFrameIntervalNanos) {
                final long lastFrameOffset = jitterNanos % mFrameIntervalNanos
                        + mFrameIntervalNanos;
                if (DEBUG_JANK) {
                    Log.d(TAG, "Commit callback delayed by " + (jitterNanos * 0.000001f)
                            + " ms which is more than twice the frame interval of "
                            + (mFrameIntervalNanos * 0.000001f) + " ms!  "
                            + "Setting frame time to " + (lastFrameOffset * 0.000001f)
                            + " ms in the past.");
                    mDebugPrintNextFrameTimeDelta = true;
                }
                frameTimeNanos = now - lastFrameOffset;
                mLastFrameTimeNanos = frameTimeNanos;
            }
        }
    }
    try {
        Trace.traceBegin(Trace.TRACE_TAG_VIEW, CALLBACK_TRACE_TITLES[callbackType]);
        for (CallbackRecord c = callbacks; c != null; c = c.next) {
            if (DEBUG_FRAMES) {
                Log.d(TAG, "RunCallback: type=" + callbackType
                        + ", action=" + c.action + ", token=" + c.token
                        + ", latencyMillis=" + (SystemClock.uptimeMillis() - c.dueTime));
            }　　　　　　　　　　// 運(yùn)行每一個 callback
            c.run(frameTimeNanos);
        }
    } finally {
        synchronized (mLock) {
            mCallbacksRunning = false;
            do {
                final CallbackRecord next = callbacks.next;
                recycleCallbackLocked(callbacks);
                callbacks = next;
            } while (callbacks != null);
        }
        Trace.traceEnd(Trace.TRACE_TAG_VIEW);
    }
}

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

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

// ViewRootImpl    
@UnsupportedAppUsage
void scheduleTraversals() {　　　　　
    // 如果已經(jīng)請求繪制了，就不會再次請求，因?yàn)槎啻握埱?，只有有一個執(zhí)行就滿足要求了
    if (!mTraversalScheduled) {
        mTraversalScheduled = true;// 同步
        mTraversalBarrier = mHandler.getLooper().getQueue().postSyncBarrier();
        // 發(fā)送一個 callback 用于在下一幀來臨時候處理
        mChoreographer.postCallback(
                Choreographer.CALLBACK_TRAVERSAL, mTraversalRunnable, null);
        if (!mUnbufferedInputDispatch) {
            scheduleConsumeBatchedInput();
        }　
        // 通知稍候繪制
        notifyRendererOfFramePending();
        pokeDrawLockIfNeeded();
    }
}

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_FRAMES) {
        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);
　　　　// 小于當(dāng)前時間，說明需要立即執(zhí)行
        if (dueTime <= now) {
            scheduleFrameLocked(now);
        } else {　　　　　　　　　 
           // 發(fā)送一個延遲 msg
            Message msg = mHandler.obtainMessage(MSG_DO_SCHEDULE_CALLBACK, action);
            msg.arg1 = callbackType;
            msg.setAsynchronous(true);
            mHandler.sendMessageAtTime(msg, dueTime);
        }
    }
}

private void scheduleFrameLocked(long now) {
        if (!mFrameScheduled) {
            mFrameScheduled = true;
            if (USE_VSYNC) {
                if (DEBUG_FRAMES) {
                    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.　　　　　　　　　
                // 如果是在一個 looper 線程中，那么直接執(zhí)行請求就好
                if (isRunningOnLooperThreadLocked()) {
                    scheduleVsyncLocked();
                } else {　　　　　　　　　　　　
                    // 如果是在主線程，那么需要發(fā)送一個請求 vsync 的消息，并插到最前面，需要確保前一個消息處理完后在開始請求
                    Message msg = mHandler.obtainMessage(MSG_DO_SCHEDULE_VSYNC);
                    msg.setAsynchronous(true);
                    mHandler.sendMessageAtFrontOfQueue(msg);
                }
            } else {　　　　　　　　　　
                // 如果不用 vsync 信號，那么就可以直接執(zhí)行，只是需要記錄每一幀的時間
                final long nextFrameTime = Math.max(
                        mLastFrameTimeNanos / TimeUtils.NANOS_PER_MS + sFrameDelay, now);
                if (DEBUG_FRAMES) {
                    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 doScheduleVsync() {
    synchronized (mLock) {
        if (mFrameScheduled) {
            scheduleVsyncLocked();
        }
    }
}


@UnsupportedAppUsage
private void scheduleVsyncLocked() {
    // 發(fā)起獲取 vsync 信號的請求
    mDisplayEventReceiver.scheduleVsync();
}   

/**
    * Schedules a single vertical sync pulse to be delivered when the next
    * display frame begins.
    */
@UnsupportedAppUsage
public void scheduleVsync() {
    if (mReceiverPtr == 0) {
        Log.w(TAG, "Attempted to schedule a vertical sync pulse but the display event "
                + "receiver has already been disposed.");
    } else {
        nativeScheduleVsync(mReceiverPtr);
    }
}