在Android中绘图中，线程可以使用postOnAnimation函数，用于在系统进行下一次动画操作时，运行当前的线程，postOnAnimation(this)：
public void postOnAnimation(Runnable action) {
final AttachInfo attachInfo = mAttachInfo;
if (attachInfo != null) {
attachInfo.mViewRootImpl.mChoreographer.postCallback(
Choreographer.CALLBACK_ANIMATION, action, null);
} else {
// Assume that post will succeed later
ViewRootImpl.getRunQueue().post(action);
}
}
只看attachInfo不为空的情况，会执行if里面的语句，可以看到属性的Choreographer，看一下Choreographer的postCallback：
public void postCallback(int callbackType, Runnable action, Object token) {
postCallbackDelayed(callbackType, action, token, 0);
}
postCallbackDelayed：
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);
}
可见最终的实现是在postCallbackDelayedInternal里面，看一下postCallbackDelayedInternal的代码：
private void postCallbackDelayedInternal(int callbackType,
Object action, Object token, long delayMillis) {
if (DEBUG) {
Xlog.d(TAG, "PostCallback: type=" + callbackType
+ ", action=" + action + ", token=" + token
+ ", delayMillis=" + delayMillis + ",this = " + this);
}
synchronized (mLock) {
final long now = SystemClock.uptimeMillis();
final long dueTime = now + delayMillis;
mCallbackQueues[callbackType].addCallbackLocked(dueTime, action, token);
if (dueTime <= now) {
scheduleFrameLocked(now);
} else {
Message msg = mHandler.obtainMessage(MSG_DO_SCHEDULE_CALLBACK, action);
msg.arg1 = callbackType;
msg.setAsynchronous(true);
mHandler.sendMessageAtTime(msg, dueTime);
}
}
}
可以看到在postCallbackDelayedInternal里，在mCallbackQueues队列里面的callbackType添加一个元素。mCallbackQueues是一个CallbackQueue，从命名里就可以看出现，是一个用于回调的队列，并且是安时间先后顺序排好的，与闹钟原理一样，并且里面有一个CallbackRecord类，记录回调相关的信息。callbackType的类型有三种:CALLBACK_INPUT、CALLBACK_ANIMATION、CALLBACK_TRAVERSAL，这里我们用的是CALLBACK_ANIMATION。在postOnAnimation没有delay，所以只要收到Vsync信号，便立即会被执行 。当有Vsync到来是，会执行doFrame，doFrame里有：
...
doCallbacks(Choreographer.CALLBACK_INPUT, frameTimeNanos);
doCallbacks(Choreographer.CALLBACK_ANIMATION, frameTimeNanos);
doCallbacks(Choreographer.CALLBACK_TRAVERSAL, frameTimeNanos);
doCallbacks里面有如下语句：
...
callbacks = mCallbackQueues[callbackType].extractDueCallbacksLocked(now);