深入分析UI 上層事件處理核心機制 Choreographer
結論寫在前面：Choreographer就是一個消息處理器，根據vsync 信號 來計算frame，而計算frame的方式就是處理三種回調，包括事件回調、動畫回調、繪製回調。這三種事件在消息輸入、加入動畫、準備繪圖layout 等動作時均會發給Choreographer。
下面來看分析過程
看過一些源碼後，發現ui 繪製的管理，調度都是通過Choreographer這個類。
1 Choreographer 是什麼？有什麼？
Choreographer 是個普通類，final 表示不能被繼承修改其行爲。
public final class Choreographer private static final ThreadLocal<Choreographer> sThreadInstance =
new ThreadLocal<Choreographer>() protected Choreographer initialValue() {
Looper looper = Looper.myLooper();
if (looper == null) {
throw new IllegalStateException("The current thread must have a looper!");
}
return new Choreographer(looper);
} private final FrameHandler mHandler; private final class FrameHandler extends Handler {
public FrameHandler(Looper looper) {
super(looper);
}
@Override
public void handleMessage(Message msg) {
switch (msg.what) {
case MSG_DO_FRAME:
doFrame(System.nanoTime(), 0);
break;
case MSG_DO_SCHEDULE_VSYNC:
doScheduleVsync();
break;
case MSG_DO_SCHEDULE_CALLBACK:
doScheduleCallback(msg.arg1);
break;
}
}
}查一下概念，的確符合，DO_FRAME 是幀刷新，SCHEDULE_VSYNC是 垂直同步刷新。
在android4.1上加入的VSYNC 特性，並使用三重緩衝大幅改善了android 圖像方面的性能。
private void scheduleFrameLocked(long now) {
if (USE_VSYNC) {
if (isRunningOnLooperThreadLocked()) {
scheduleVsyncLocked();
} else {
Message msg = mHandler.obtainMessage(MSG_DO_SCHEDULE_VSYNC);
msg.setAsynchronous(true);
mHandler.sendMessageAtFrontOfQueue(msg);
}
} else {
Message msg = mHandler.obtainMessage(MSG_DO_FRAME);
msg.setAsynchronous(true);
mHandler.sendMessageAtTime(msg, nextFrameTime);
}
2 從VSYNC 開始case MSG_DO_SCHEDULE_VSYNC:
void doScheduleVsync() {
synchronized (mLock) {
if (mFrameScheduled) {
scheduleVsyncLocked();
}
}
} private void scheduleVsyncLocked() {
mDisplayEventReceiver.scheduleVsync();
} 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 {
nativeScheduleVsync(mReceiverPtr);
}
}native的東西先不往下看，回頭來看mDisplayEventReceiver
private final class FrameDisplayEventReceiver extends DisplayEventReceiver
implements Runnable // Called from native code.
@SuppressWarnings("unused")
private void dispatchVsync(long timestampNanos, int builtInDisplayId, int frame) {
onVsync(timestampNanos, builtInDisplayId, frame);
} @Override
public void onVsync(long timestampNanos, int builtInDisplayId, int frame) {
mTimestampNanos = timestampNanos;
mFrame = frame;
Message msg = Message.obtain(mHandler, this);
msg.setAsynchronous(true);
mHandler.sendMessageAtTime(msg, timestampNanos / NANOS_PER_MS);
@Override
public void run() {
mHavePendingVsync = false;
doFrame(mTimestampNanos, mFrame);
}
void doFrame(long frameTimeNanos, int frame) {
synchronized (mLock) {
//省略一些賦值
//可能時跳幀的情況，直接調用vsync，並return
if (frameTimeNanos < mLastFrameTimeNanos) {
if (DEBUG) {
Log.d(TAG, "Frame time appears to be going backwards. May be due to a "
+ "previously skipped frame. Waiting for next vsync.");
}
scheduleVsyncLocked();
return;
}
}
//先後處理事件回調、動畫回調、繪製回調
doCallbacks(Choreographer.CALLBACK_INPUT, frameTimeNanos);
doCallbacks(Choreographer.CALLBACK_ANIMATION, frameTimeNanos);
doCallbacks(Choreographer.CALLBACK_TRAVERSAL, frameTimeNanos);
}
否則開始新的一幀的繪製，依次處理事件回調、動畫回調、繪製回調
在doCallbacks() 中先根據當前時間去除隊列中第一個有效的回調，然後依次處理這些回調。
void doCallbacks(int callbackType, long frameTimeNanos) {
CallbackRecord callbacks;
synchronized (mLock) {
// We use "now" to determine when callbacks become due because it's possible
// for earlier processing phases in a frame to post callbacks that should run
// in a following phase, such as an input event that causes an animation to start.
final long now = SystemClock.uptimeMillis();
//按時間取隊列頭
callbacks = mCallbackQueues[callbackType].extractDueCallbacksLocked(now);
if (callbacks == null) {
return;
}
mCallbacksRunning = true;
}
try {
for (CallbackRecord c = callbacks; c != null; c = c.next) {
if (DEBUG) {
Log.d(TAG, "RunCallback: type=" + callbackType
+ ", action=" + c.action + ", token=" + c.token
+ ", latencyMillis=" + (SystemClock.uptimeMillis() - c.dueTime));
}
//回調事件
c.run(frameTimeNanos);
}
} finally {
synchronized (mLock) {
mCallbacksRunning = false;
do {
final CallbackRecord next = callbacks.next;
recycleCallbackLocked(callbacks);
callbacks = next;
} while (callbacks != null);
}
}
}回調的數據結構是CallbackRecord，一個單向鏈表。根據token 的標誌，來回調doframe 或者runnable
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();
}
}
}3. 哪裏會使用Choreographer？
在《Android 動畫animation 深入分析》http://blog.csdn.net/farmer_cc/article/details/18259117中分析到scheduleAnimation
的時候就是調用的android.view.Choreographer.postCallback(int, Runnable, Object) 方法。查看該方法的調用，在UI 繪製、和其他和動畫相關的類中均有調用。這裏並沒有全部列出來。
scheduleTraversals() : void - android.view.ViewRootImpl
scheduleAnimation() : void - android.animation.ValueAnimator.AnimationHandler
scheduleAnimationLocked() : void - com.android.server.wm.WindowManagerService
postOnAnimation(Runnable) : void - android.view.View
在postCallbackDelayedInternal()中 把新來的事件加入到隊列中，並根據時間來判斷是去frame 還是直接回調
private void postCallbackDelayedInternal(int callbackType,
Object action, Object token, long delayMillis) {
if (DEBUG) {
Log.d(TAG, "PostCallback: type=" + callbackType
+ ", action=" + action + ", token=" + token
+ ", delayMillis=" + delayMillis);
}
synchronized (mLock) {
final long now = SystemClock.uptimeMillis();
final long dueTime = now + delayMillis;
// 入隊列
mCallbackQueues[callbackType].addCallbackLocked(dueTime, action, token);
if (dueTime <= now) {
scheduleFrameLocked(now);
} else {
// 直接回調
Message msg = mHandler.obtainMessage(MSG_DO_SCHEDULE_CALLBACK, action);
msg.arg1 = callbackType;
msg.setAsynchronous(true);
mHandler.sendMessageAtTime(msg, dueTime);
}
}至此，事件的來龍去脈就明確了。Choreographer就是一個消息處理器，根據vsync 信號 來計算frame，而計算frame的方式就是處理三種回調，包括事件回調、動畫回調、繪製回調。這三種事件在消息輸入、加入動畫、準備繪圖layout 等動作時均會發給Choreographer。
寫在後面：結合前文《Android 動畫animation 深入分析》http://blog.csdn.net/farmer_cc/article/details/18259117中可以更多瞭解動畫相關內容