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在Android4.1之后增加了Choreographer机制，用于同Vsync机制配合，实现统一调度界面绘图. Choreographer构造过程frameworks\base\core\java\android\view\Choreographer.java public static Choreographer getInstance() { return sThreadInstance.get(); }
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); } };
为调用线程创建一个Choreographer实例，调用线程必须具备消息循环功能，因为ViewRootImpl对象的构造是在应用程序进程的UI主线程中执行的，因此创建的Choreographer对象将使用UI线程消息队列。 private Choreographer(Looper looper) { mLooper = looper; //创建消息处理Handler mHandler = new FrameHandler(looper); //如果系统使用了Vsync机制，则注册一个FrameDisplayEventReceiver接收器 mDisplayEventReceiver = USE_VSYNC ? new FrameDisplayEventReceiver(looper) : null; mLastFrameTimeNanos = Long.MIN_VALUE; //屏幕刷新周期 mFrameIntervalNanos = (long)(1000000000 / new Display(Display.DEFAULT_DISPLAY, null).getRefreshRate()); //创建回调数组 mCallbackQueues = new CallbackQueue[CALLBACK_LAST + 1]; //初始化数组 for (int i = 0; i <= CALLBACK_LAST; i++) { mCallbackQueues[i] = new CallbackQueue(); } }
变量USE_VSYNC用于表示系统是否是用了Vsync同步机制，该值是通过读取系统属性debug.choreographer.vsync来获取的。如果系统使用了Vsync同步机制，则创建一个FrameDisplayEventReceiver对象用于请求并接收Vsync事件，最后Choreographer创建了一个大小为3的CallbackQueue队列数组，用于保存不同类型的Callback。 添加回调过程frameworks\base\core\java\android\view\Choreographer.java public void postCallback(int callbackType, Runnable action, Object token) { postCallbackDelayed(callbackType, action, token, 0); }
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) { synchronized (mLock) { final long now = SystemClock.uptimeMillis(); final long dueTime = now + delayMillis; //将要执行的回调封装成CallbackRecord对象，保存到mCallbackQueues数组中 mCallbackQueues[callbackType].addCallbackLocked(dueTime, action, token); //函数执行时间到 if (dueTime <= now) { scheduleFrameLocked(now); } else {//通过异步消息方式实现函数延时执行 Message msg = mHandler.obtainMessage(MSG_DO_SCHEDULE_CALLBACK, action); msg.arg1 = callbackType; msg.setAsynchronous(true); mHandler.sendMessageAtTime(msg, dueTime); } } }
private final class FrameHandler extends Handler { @Override public void handleMessage(Message msg) { switch (msg.what) { case MSG_DO_SCHEDULE_CALLBACK: doScheduleCallback(msg.arg1); break; } } }
void doScheduleCallback(int callbackType) { synchronized (mLock) { if (!mFrameScheduled) { final long now = SystemClock.uptimeMillis(); if (mCallbackQueues[callbackType].hasDueCallbacksLocked(now)) { scheduleFrameLocked(now); } } } }
private void scheduleFrameLocked(long now) { if (!mFrameScheduled) { mFrameScheduled = true; //检查是否使用了Vsync机制 if (USE_VSYNC) { //如果当前线程具备消息循环，则直接请求VSync信号 if (isRunningOnLooperThreadLocked()) { scheduleVsyncLocked(); } else {//如果当前线程不具备消息循环，则通过主线程请求VSync信号 Message msg = mHandler.obtainMessage(MSG_DO_SCHEDULE_VSYNC); msg.setAsynchronous(true); mHandler.sendMessageAtFrontOfQueue(msg); } } else { //如果系统没有使用VSync机制，则使用异步消息延时执行屏幕刷新 final long nextFrameTime = Math.max( mLastFrameTimeNanos / NANOS_PER_MS + sFrameDelay, now); Message msg = mHandler.obtainMessage(MSG_DO_FRAME); msg.setAsynchronous(true); mHandler.sendMessageAtTime(msg, nextFrameTime); } } }
在该函数中考虑了两种情况，一种是系统没有使用Vsync机制，在这种情况下，首先根据屏幕刷新频率计算下一次刷新时间，通过异步消息方式延时执行doFrame()函数实现屏幕刷新。如果系统使用了Vsync机制，并且当前线程具备消息循环，则直接请求Vsync信号，否则就通过主线程来请求Vsync信号。FrameDisplayEventReceiver对象用于请求并接收Vsync信号，当Vsync信号到来时，系统会自动调用其onVsync()函数，在该回调函数中执行doFrame()实现屏幕刷新。 
当VSYNC信号到达时，Choreographer doFrame()函数被调用 void doFrame(long frameTimeNanos, int frame) { final long startNanos; synchronized (mLock) { if (!mFrameScheduled) { return; // no work to do } //保存起始时间 startNanos = System.nanoTime(); //由于Vsync事件处理采用的是异步方式，因此这里计算消息发送与函数调用开始之间所花费的时间 final long jitterNanos = startNanos - frameTimeNanos; //如果线程处理该消息的时间超过了屏幕刷新周期 if (jitterNanos >= mFrameIntervalNanos) { //计算函数调用期间所错过的帧数 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; frameTimeNanos = startNanos - lastFrameOffset; } //如果frameTimeNanos小于一个屏幕刷新周期，则重新请求VSync信号 if (frameTimeNanos < mLastFrameTimeNanos) { scheduleVsyncLocked(); return; } mFrameScheduled = false; mLastFrameTimeNanos = frameTimeNanos; } //分别回调CALLBACK_INPUT、CALLBACK_ANIMATION、CALLBACK_TRAVERSAL事件 doCallbacks(Choreographer.CALLBACK_INPUT, frameTimeNanos); doCallbacks(Choreographer.CALLBACK_ANIMATION, frameTimeNanos); doCallbacks(Choreographer.CALLBACK_TRAVERSAL, frameTimeNanos); }
Choreographer类中分别定义了CallbackRecord、CallbackQueue内部类，CallbackQueue是一个按时间先后顺序保存CallbackRecord的单向循环链表。
在Choreographer中定义了三个CallbackQueue队列，用数组mCallbackQueues表示，用于分别保存CALLBACK_INPUT、CALLBACK_ANIMATION、CALLBACK_TRAVERSAL这三种类型的Callback，当调用Choreographer类的postCallback()函数时，就是往指定类型的CallbackQueue队列中通过addCallbackLocked()函数添加一个CallbackRecord项：首先构造一个CallbackRecord对象，然后按时间先后顺序插入到CallbackQueue链表中。从代码注释中，我们可以知道CALLBACK_INPUT是指输入回调，该回调优先级最高，首先得到执行，而CALLBACK_TRAVERSAL是指处理布局和绘图的回调，只有在所有异步消息都执行完后才得到执行，CALLBACK_ANIMATION是指动画回调，比CALLBACK_TRAVERSAL优先执行，从doFrame()函数中的doCallbacks调用就能印证这点。 doCallbacks(Choreographer.CALLBACK_INPUT, frameTimeNanos); doCallbacks(Choreographer.CALLBACK_ANIMATION, frameTimeNanos); doCallbacks(Choreographer.CALLBACK_TRAVERSAL, frameTimeNanos);
当Vsync事件到来时，顺序执行CALLBACK_INPUT、CALLBACK_ANIMATION和CALLBACK_TRAVERSAL对应CallbackQueue队列中注册的回调。 void doCallbacks(int callbackType, long frameTimeNanos) { CallbackRecord callbacks; synchronized (mLock) { final long now = SystemClock.uptimeMillis(); //从指定类型的CallbackQueue队列中查找执行时间到的CallbackRecord callbacks = mCallbackQueues[callbackType].extractDueCallbacksLocked(now); if (callbacks == null) { return; } mCallbacksRunning = true; } try { //由于CallbackQueues是按时间先后顺序排序的，因此遍历执行所有时间到的CallbackRecord for (CallbackRecord c = callbacks; c != null; c = c.next) { c.run(frameTimeNanos); } } finally { synchronized (mLock) { mCallbacksRunning = false; do { final CallbackRecord next = callbacks.next; recycleCallbackLocked(callbacks); callbacks = next; } while (callbacks != null); } } }
该函数就是按时间顺序先后执行到时的CallbackRecord 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(); } } }
我们知道Choreographer对外提供了两个接口函数用于注册指定的Callback，postCallback()用于注册Runnable对象，而postFrameCallback()函数用于注册FrameCallback对象，无论注册的是Runnable对象还是FrameCallback对象，在CallbackRecord对象中统一装箱为Object类型。在执行其回调函数时，就需要区别这两种对象类型，如果注册的是Runnable对象，则调用其run()函数，如果注册的是FrameCallback对象，则调用它的doFrame()函数。
Vsync请求过程我们知道在Choreographer构造函数中，构造了一个FrameDisplayEventReceiver对象，用于请求并接收Vsync信号，Vsync信号请求过程如下： private void scheduleVsyncLocked() { //申请Vsync信号 mDisplayEventReceiver.scheduleVsync(); }
FrameDisplayEventReceiver继承于DisplayEventReceiver类，Vsync请求在DisplayEventReceiver中实现。 frameworks\base\core\java\android\view\ DisplayEventReceiver.java 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 { //通过Jni方式调用native层的NativeDisplayEventReceiver对象来请求VSync nativeScheduleVsync(mReceiverPtr); } }
frameworks\base\core\jni\ android_view_DisplayEventReceiver.cpp static void nativeScheduleVsync(JNIEnv* env, jclass clazz, jint receiverPtr) { //得到NativeDisplayEventReceiver对象指针 sp<NativeDisplayEventReceiver> receiver = reinterpret_cast<NativeDisplayEventReceiver*>(receiverPtr); //通过NativeDisplayEventReceiver请求VSync status_t status = receiver->scheduleVsync(); if (status) { String8 message; message.appendFormat("Failed to schedule next vertical sync pulse. status=%d", status); jniThrowRuntimeException(env, message.string()); } }
status_t NativeDisplayEventReceiver::scheduleVsync() { if (!mWaitingForVsync) { ALOGV("receiver %p ~ Scheduling vsync.", this); // Drain all pending events. nsecs_t vsyncTimestamp; uint32_t vsyncCount; readLastVsyncMessage(&vsyncTimestamp, &vsyncCount); status_t status = mReceiver.requestNextVsync(); if (status) { ALOGW("Failed to request next vsync, status=%d", status); return status; } mWaitingForVsync = true; } return OK; }
VSync请求过程又转交给了DisplayEventReceiver frameworks\native\libs\gui\ DisplayEventReceiver.cpp status_t DisplayEventReceiver::requestNextVsync() { if (mEventConnection != NULL) { mEventConnection->requestNextVsync(); return NO_ERROR; } return NO_INIT; }
这里又通过IDisplayEventConnection接口来请求Vsync信号，IDisplayEventConnection实现了Binder通信框架，可以跨进程调用，因为Vsync信号请求进程和Vsync产生进程有可能不在同一个进程空间，因此这里就借助IDisplayEventConnection接口来实现。下面通过图来梳理Vsync请求的调用流程：
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