Android系统Choreographer机制实现过程

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>() {@Overrideprotected 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HandlermHandler = 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 {@Overridepublic 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CallbackRecordcallbacks = 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CallbackRecordfor (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 FrameCallbackpublic 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VSyncnativeScheduleVsync(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：