在ViewRootImpl中有这么个方法scheduleTraversals，如果你深入过View的绘制流程，那你应该知道就是从这个方法开始触发performTraversals，来调出之后的measure，layout，draw。
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初见
我们先贴出这个方法：
void scheduleTraversals() {
if (!mTraversalScheduled) {
mTraversalScheduled = true;
mTraversalBarrier = mHandler.getLooper().getQueue().postSyncBarrier();
mChoreographer.postCallback(
Choreographer.CALLBACK_TRAVERSAL, mTraversalRunnable, null);
if (!mUnbufferedInputDispatch) {
scheduleConsumeBatchedInput();
}
notifyRendererOfFramePending();
pokeDrawLockIfNeeded();
}
}
抛开其他的诸如阻塞handler等代码，我们直接看```
mChoreographer.postCallback(
Choreographer.CALLBACK_TRAVERSAL, mTraversalRunnable, null);
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探索
我们继续跟着这个方法深究，看看它到底干了什么：
postCallback()->postCallbackDelayed()->postCallbackDelayedInternal():
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);
//delayMillis传的是0，故此处进入条件
if (dueTime <= now) {
//实际上单单从这个方法的名字我们就能意识到做的是跟帧有关的工作。
scheduleFrameLocked(now);
} else {
Message msg = mHandler.obtainMessage(MSG_DO_SCHEDULE_CALLBACK, action);
msg.arg1 = callbackType;
msg.setAsynchronous(true);
mHandler.sendMessageAtTime(msg, dueTime);
}
}
}
上锁，来调动帧的行为，很有可能就跟界面的一帧一帧的刷新有关，我们接着往下看：
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.
//是否是主线程，一般界面刷新能走到这一步都是在主线程中刷新的，所以进入条件
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 / 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);
}
}
}
这里有个常量USE_VSYNC，表示是否允许动画和绘制的垂直同步，默认是为true
// Enable/disable vsync for animations and drawing.
private static final boolean USE_VSYNC = SystemProperties.getBoolean(
"debug.choreographer.vsync", true);
紧接着scheduleVsyncLocked()-> mDisplayEventReceiver.scheduleVsync()->nativeScheduleVsync(mReceiverPtr)
走到一个native方法，底层我就暂时不分析，有兴趣的读者可以去看一下，最后会在底层处理垂直同步，然后回调onVsync方法，抽象类DisplayEventReceiver并没有实现这个方法，我们找到实现类是FrameDisplayEventReceiver。
@Override
public void onVsync(long timestampNanos, int builtInDisplayId, int frame) {
...
mTimestampNanos = timestampNanos;
mFrame = frame;
Message msg = Message.obtain(mHandler, this);
msg.setAsynchronous(true);
mHandler.sendMessageAtTime(msg, timestampNanos / TimeUtils.NANOS_PER_MS);
}
把自身包装成Message传给了handler，调用其run方法：
@Override
public void run() {
mHavePendingVsync = false;
doFrame(mTimestampNanos, mFrame);
}
void doFrame(long frameTimeNanos, int frame) {
final long startNanos;
synchronized (mLock) {
...
//是否有跳帧，如果有那么就打印log并且修正偏差
}
//执行callback
try {
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);
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.");
}
}
doFrame方法做的就是渲染下一帧，第一个痛不块中就是去检测是否卡顿并修补卡顿。然后开始做渲染工作，我们看几个doCallbacks方法的参数：
CALLBACK_INPUT：输入
CALLBACK_ANIMATION:动画
CALLBACK_TRAVERSAL:遍历，执行measure、layout、draw
CALLBACK_COMMIT：遍历完成的提交操作，用来修正动画启动时间
会从上往下的去执行一次
void doCallbacks(int callbackType, long frameTimeNanos) {
CallbackRecord callbacks;
synchronized (mLock) {
...
final long now = System.nanoTime();
callbacks = mCallbackQueues[callbackType].extractDueCallbacksLocked(
now / TimeUtils.NANOS_PER_MS);
if (callbacks == null) {
return;
}
mCallbacksRunning = true;
...
}
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));
}
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);
}
}
Choreographer内部维护了这四种链表，渲染每一帧的时候都会从上往下的去执行相应的渲染操作，有输入那么就先渲染输入队列，有动画就渲染动画，然后遍历，然后提交。
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总结
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控制外部输入事件处理，动画执行，UI变化，以及提交执行都是在同一个类中做的处理，即是Choreographer。
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在Choreographer对象中有四条链表，分别保存着待处理的输入事件，待处理的动画事件，待处理的遍历事件，以及待处理的提交时间。
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每次执行的时候，Choreographer会根据当前的时间，只处理事件链表中最后一个事件，当有耗时操作在主线程时，事件不能及时执行，就会出现所谓的“跳帧”，“卡顿”现象。
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Choreographer的共有方法postCallback(callbackType, Object)是往事件链表中放事件的方法。而doFrame()是消耗这些事件的方法。
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