Android的postOnAnimation分析

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转自：http://blog.csdn.net/oujunli/article/details/8571078

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);

...

try {
for (CallbackRecord c = callbacks; c != null; c = c.next) {
if (DEBUG) {
Xlog.d(TAG, "RunCallback: type=" + callbackType
+ ", action=" + c.action + ", token=" + c.token
+ ", latencyMillis=" + (SystemClock.uptimeMillis() - c.dueTime)
+ ",this = " + this);
}
c.run(frameTimeNanos);
}
} finally {

...

doCallbacks遍历postCallbackDelayedInternal里面添加的元素，并执行元素里的run函数，整个流程也就跑了一遍。

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