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>() {
	@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：