在Android中繪圖中，線程可以使用postOnAnimation函數，用於在系統進行下一次動畫操作時，運行當前的線程，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不為空的情況，會執行if裡面的語句，可以看到屬性的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);
}
可見最終的實現是在postCallbackDelayedInternal裡面，看一下postCallbackDelayedInternal的代碼：
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裡，在mCallbackQueues隊列裡面的callbackType添加一個元素。mCallbackQueues是一個CallbackQueue，從命名裡就可以看出現，是一個用於回調的隊列，並且是安時間先後順序排好的，與鬧鐘原理一樣，並且裡面有一個CallbackRecord類，記錄回調相關的信息。callbackType的類型有三種:CALLBACK_INPUT、CALLBACK_ANIMATION、CALLBACK_TRAVERSAL，這裡我們用的是CALLBACK_ANIMATION。在postOnAnimation沒有delay，所以只要收到Vsync信號，便立即會被執行 。當有Vsync到來是，會執行doFrame，doFrame裡有：
...
doCallbacks(Choreographer.CALLBACK_INPUT, frameTimeNanos);
doCallbacks(Choreographer.CALLBACK_ANIMATION, frameTimeNanos);
doCallbacks(Choreographer.CALLBACK_TRAVERSAL, frameTimeNanos);
doCallbacks裡面有如下語句：
...
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遍歷postCallbackDelayedInternal裡面添加的元素，並執行元素裡的run函數，整個流程也就跑了一遍。
更多Android相關信息見Android 專題頁面 http://www.linuxidc.com/topicnews.aspx?tid=11
