结论写在前面:Choreographer就是一个消息处理器,根据vsync 信号 来计算frame,而计算frame的方式就是处理三种回调,包括事件回调、动画回调、绘制回调。这三种事件在消息输入、加入动画、准备绘图layout 等动作时均会发给Choreographer。
下面来看分析过程
看过一些源码后,发现ui 绘制的管理,调度都是通过Choreographer这个类。
Choreographer 是个普通类,final 表示不能被继承修改其行为。
public final class Choreographer
单例模式持有一个本地进程的单例对象,
private static final ThreadLocal sThreadInstance =
new ThreadLocal()
该对象必须持有looper,意味着将使用消息队列
protected Choreographer initialValue() {
Looper looper = Looper.myLooper();
if (looper == null) {
throw new IllegalStateException("The current thread must have a looper!");
}
return new Choreographer(looper);
}
持有一个handler 对象
private final FrameHandler mHandler;
这个handler对象仅处理3种事件:
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;
}
}
}
在scheduleFrameLocked中事件被静态变量USE_VSYNC分开,也就是系统仅使用MSG_DO_SCHEDULE_VSYNC或MSG_DO_FRAME。
查一下概念,的确符合,DO_FRAME 是帧刷新,SCHEDULE_VSYNC是 垂直同步刷新。
在android4.1上加入的VSYNC 特性,并使用三重缓冲大幅改善了android 图像方面的性能。
这里有篇文章讲的很清楚:http://www.androidpolice.com/2012/07/12/getting-to-know-android-4-1-part-3-project-butter-how-it-works-and-what-it-added/
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);
}
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);
}
}
一路调用到nativeScheduleVsync();
native的东西先不往下看,回头来看mDisplayEventReceiver
private final class FrameDisplayEventReceiver extends DisplayEventReceiver
implements Runnable
观察一下方法名都是onXX ,一看就是回调的节奏
// Called from native code.
@SuppressWarnings("unused")
private void dispatchVsync(long timestampNanos, int builtInDisplayId, int frame) {
onVsync(timestampNanos, builtInDisplayId, frame);
}
看到这里的注释,就明确了这个类就是被native回调了。回调时调用onVsync();而这个方法时在FrameDisplayEventReceiver中重写的。
@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);
}
在onsync中把自己的runnable 加到消息队列中执行,这里使用异步消息,调用绘制do_frame()方法。
void doFrame(long frameTimeNanos, int frame) {
synchronized (mLock) {
//省略一些赋值
//可能时跳帧的情况,直接调用vsync,并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;
}
}
//先后处理事件回调、动画回调、绘制回调
doCallbacks(Choreographer.CALLBACK_INPUT, frameTimeNanos);
doCallbacks(Choreographer.CALLBACK_ANIMATION, frameTimeNanos);
doCallbacks(Choreographer.CALLBACK_TRAVERSAL, frameTimeNanos);
}
在do_frame()中判断有跳帧可能性,直接继续vsync。
否则开始新的一帧的绘制,依次处理事件回调、动画回调、绘制回调
在doCallbacks() 中先根据当前时间去除队列中第一个有效的回调,然后依次处理这些回调。
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();
//按时间取队列头
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));
}
//回调事件
c.run(frameTimeNanos);
}
} finally {
synchronized (mLock) {
mCallbacksRunning = false;
do {
final CallbackRecord next = callbacks.next;
recycleCallbackLocked(callbacks);
callbacks = next;
} while (callbacks != null);
}
}
}
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();
}
}
}
从这里可以看到,处理的事件分为两类:一类是doframe 事件,另一类是runnable 方法。接着,就来看一下到底是加的这些事件。
在《Android 动画animation 深入分析》http://blog.csdn.net/farmer_cc/article/details/18259117中分析到scheduleAnimation 的时候就是调用的android.view.Choreographer.postCallback(int, Runnable, Object) 方法。查看该方法的调用,在UI 绘制、和其他和动画相关的类中均有调用。这里并没有全部列出来。
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 把新来的事件加入到队列中,并根据时间来判断是去frame 还是直接回调
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;
// 入队列
mCallbackQueues[callbackType].addCallbackLocked(dueTime, action, token);
if (dueTime <= now) {
scheduleFrameLocked(now);
} else {
// 直接回调
Message msg = mHandler.obtainMessage(MSG_DO_SCHEDULE_CALLBACK, action);
msg.arg1 = callbackType;
msg.setAsynchronous(true);
mHandler.sendMessageAtTime(msg, dueTime);
}
}
写在后面:结合前文《Android 动画animation 深入分析》http://blog.csdn.net/farmer_cc/article/details/18259117中可以更多了解动画相关内容