简单来说，Choreographer主要作用是协调动画，输入和绘制的时间，它从显示子系统接收定时脉冲（例如垂直同步），然后安排渲染下一个frame的一部分工作。
自定义FrameCallback
FrameCallback是和Choreographer交互，在下一个frame被渲染时触发的接口类。开发者可以设置自己的FrameCallback。我们就从自定义FrameCallback作为切入口，尝试窥探一下Choreographer的实现原理。简单实现如下：
private static final String TAG = "Choreographer_test";
@Override
public void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
final ImageView imageView= (ImageView) findViewById(R.id.iv_anim);
imageView.setOnClickListener(new View.OnClickListener() {
@Override
public void onClick(View v) {
final long starTime=System.nanoTime();
Choreographer.getInstance().postFrameCallback(new Choreographer.FrameCallback() {
@Override
public void doFrame(long frameTimeNanos) {
Log.e(TAG,"starTime="+starTime+", frameTimeNanos="+frameTimeNanos+", frameDueTime="+(frameTimeNanos-starTime)/1000000);
}
});
}
});
}
在这里，我们自定义的FrameCallback只是简单把时间打印了一下。输出如下信息：
E/Choreographer_test: starTime=232157742945242, frameTimeNanos=232157744964255, frameDueTime=2
从log可以看出，这一帧�大概2ms就处理完毕。下面我们从源码角度窥探一下它具体的实现原理。
实现原理
1. 关键成员变量
构造函数
private Choreographer(Looper looper) {
mLooper = looper;
//1.创建Handler对象，用于处理消息
mHandler = new FrameHandler(looper);
//2.创建接收VSYNC信号的对象
mDisplayEventReceiver = USE_VSYNC ? new FrameDisplayEventReceiver(looper) : null;
//3.初始化上一次frame渲染的时间点
mLastFrameTimeNanos = Long.MIN_VALUE;
//4.帧率，也就是渲染一帧的时间，getRefreshRate是刷新率，一般是60
mFrameIntervalNanos = (long)(1000000000 / getRefreshRate());
//5.创建回调队列
mCallbackQueues = new CallbackQueue[CALLBACK_LAST + 1];
for (int i = 0; i <= CALLBACK_LAST; i++) {
mCallbackQueues[i] = new CallbackQueue();
}
}
FrameHandler
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:
//渲染下一个frame
doFrame(System.nanoTime(), 0);
break;
case MSG_DO_SCHEDULE_VSYNC:
//请求VSNYC信号
doScheduleVsync();
break;
case MSG_DO_SCHEDULE_CALLBACK:
//执行Callback
doScheduleCallback(msg.arg1);
break;
}
}
}
FrameDisplayEventReceiver
FrameDisplayEventReceiver是DisplayEventReceiver的子类，DisplayEventReceiver是接收VSYNC信息的java层实现。
public abstract class DisplayEventReceiver {
public void onVsync(long timestampNanos, int builtInDisplayId, int frame) {}
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);
}
}
private void dispatchVsync(long timestampNanos, int builtInDisplayId, int frame) {
onVsync(timestampNanos, builtInDisplayId, frame);
}
}
VSYNC信息一般由硬件中断产生，SurfaceFlinger处理。具体实现和监听机制可以参考链接，scheduleVsync方法用于请求VSNYC信号， Native方法接收到VSYNC信息处理后会调用java层dispatchVsync方法，最终调用到FrameDisplayEventReceiver的onVsync方法，具体实现我们一会再说。
CallbackQueue
CallbackQueue是个单链表实现，每种类型的callback(CallbackRecord)按照设置的执行时间(dueTime)顺序排序分别保存在其各自CallbackQueue。在Choreographer中有四种类型callback：Input、Animation、Draw，还有一种是用来解决动画启动问题的。
private final class CallbackQueue {
private CallbackRecord mHead;
public boolean hasDueCallbacksLocked(long now) {
return mHead != null && mHead.dueTime <= now;
}
//根据当前时间得到callback
public CallbackRecord extractDueCallbacksLocked(long now) {
....
....
}
//根据时间添加callback
public void addCallbackLocked(long dueTime, Object action, Object token) {
....
....
}
//移除callback
public void removeCallbacksLocked(Object action, Object token) {
....
....
}
}
}
2. 流程分析
大致分析完Choreographer关键的几个成员变量后，我们再回到postFrameCallback方法
public void postFrameCallbackDelayed(FrameCallback callback, long delayMillis) {
if (callback == null) {
throw new IllegalArgumentException("callback must not be null");
}
//默认为CALLBACK_ANIMATION类型
postCallbackDelayedInternal(CALLBACK_ANIMATION,
callback, FRAME_CALLBACK_TOKEN, delayMillis);
}
postCallbackDelayedInternal
private void postCallbackDelayedInternal(int callbackType,
Object action, Object token, long delayMillis) {
synchronized (mLock) {
final long now = SystemClock.uptimeMillis();
final long dueTime = now + delayMillis;
//添加callback到回调队列
mCallbackQueues[callbackType].addCallbackLocked(dueTime, action, token);
if (dueTime <= now) {
scheduleFrameLocked(now);
} else {
//设定的执行时间在当前时间之后,发送MSG_DO_SCHEDULE_CALLBACK，由FrameHanlder安排执行scheduleFrameLocked
Message msg = mHandler.obtainMessage(MSG_DO_SCHEDULE_CALLBACK, action);
msg.arg1 = callbackType;
msg.setAsynchronous(true);
mHandler.sendMessageAtTime(msg, dueTime);`
}
}
}
scheduleFrameLocked
private void scheduleFrameLocked(long now) {
....
if (isRunningOnLooperThreadLocked()) {
//若当前线程是UI线程，执行scheduleVsyncLocked请求VSYNC信号
scheduleVsyncLocked();
} else {
//非UI线程，发送MSG_DO_SCHEDULE_VSYNC消息到主线程
Message msg = mHandler.obtainMessage(MSG_DO_SCHEDULE_VSYNC);
msg.setAsynchronous(true);
mHandler.sendMessageAtFrontOfQueue(msg);
}
....
}
scheduleVsyncLocked最终调用FrameDisplayEventReceiver#scheduleVsync，收到Vsync信息后，调用FrameDisplayEventReceiver#onVsync
FrameDisplayEventReceiver#onVsync
private final class FrameDisplayEventReceiver extends DisplayEventReceiver
implements Runnable {
private boolean mHavePendingVsync;
private long mTimestampNanos;
private int mFrame;
public FrameDisplayEventReceiver(Looper looper) {
super(looper);
}
@Override
public void onVsync(long timestampNanos, int builtInDisplayId, int frame) {
....
....
mTimestampNanos = timestampNanos;
mFrame = frame;
//该消息的callback为当前对象FrameDisplayEventReceiver,收到消息调用其run方法,然后调用doFrame方法
Message msg = Message.obtain(mHandler, this);
msg.setAsynchronous(true);
mHandler.sendMessageAtTime(msg, timestampNanos / TimeUtils.NANOS_PER_MS);
}
@Override
public void run() {
mHavePendingVsync = false;
doFrame(mTimestampNanos, mFrame);
}
}
doFrame
void doFrame(long frameTimeNanos, int frame) {
....
//Vsync信号到来时间
long intendedFrameTimeNanos = frameTimeNanos;
//实际开始执行当前frame的时间
startNanos = System.nanoTime();
//时间差
final long jitterNanos = startNanos - frameTimeNanos;
//时间差大于帧率，则认为是跳帧
if (jitterNanos >= mFrameIntervalNanos) {
final long skippedFrames = jitterNanos / mFrameIntervalNanos;
if (skippedFrames >= SKIPPED_FRAME_WARNING_LIMIT) {
Log.i(TAG, "Skipped " + skippedFrames + " frames! "
+ "The application may be doing too much work on its main thread.");
}
....
....
//记录当前frame信息
mFrameInfo.setVsync(intendedFrameTimeNanos, frameTimeNanos);
mFrameScheduled = false;
//记录上一次frame渲染的时间点
mLastFrameTimeNanos = frameTimeNanos;
}
try {
//执行CallBack，优先级为：CALLBACK_INPUT>CALLBACK_ANIMATION>CALLBACK_TRAVERSAL>CALLBACK_COMMIT
Trace.traceBegin(Trace.TRACE_TAG_VIEW, "Choreographer#doFrame");
mFrameInfo.markInputHandlingStart();
doCallbacks(Choreographer.CALLBACK_INPUT, frameTimeNanos);
mFrameInfo.markAnimationsStart();
doCallbacks(Choreographer.CALLBACK_ANIMATION, frameTimeNanos);
mFrameInfo.markPerformTraversalsStart();
doCallbacks(Choreographer.CALLBACK_TRAVERSAL, frameTimeNanos);
doCallbacks(Choreographer.CALLBACK_COMMIT, frameTimeNanos);
} finally {
Trace.traceEnd(Trace.TRACE_TAG_VIEW);
}
....
}
doCallbacks
void doCallbacks(int callbackType, long frameTimeNanos) {
CallbackRecord callbacks;
synchronized (mLock) {
final long now = System.nanoTime();
// 从队列查找相应类型的CallbackRecord对象
callbacks = mCallbackQueues[callbackType].extractDueCallbacksLocked(
now / TimeUtils.NANOS_PER_MS);
if (callbacks == null) {
return;
}
mCallbacksRunning = true;
....
....
try {
Trace.traceBegin(Trace.TRACE_TAG_VIEW, CALLBACK_TRACE_TITLES[callbackType]);
for (CallbackRecord c = callbacks; c != null; c = c.next) {
....
//调用CallbackRecord的run方法
c.run(frameTimeNanos);
}
} finally {
synchronized (mLock) {
mCallbacksRunning = false;
//回收callbacks，加入mCallbackPool对象池
do {
final CallbackRecord next = callbacks.next;
recycleCallbackLocked(callbacks);
callbacks = next;
} while (callbacks != null);
}
Trace.traceEnd(Trace.TRACE_TAG_VIEW);
}
}
CallbackRecord#run
public void run(long frameTimeNanos) {
if (token == FRAME_CALLBACK_TOKEN) {
//调用自定义FrameCallback的doFrame方法
((FrameCallback)action).doFrame(frameTimeNanos);
} else {
((Runnable)action).run();
}
}
至此，关于Choreographer的整个调用流程及其原理已经分析完成。至于系统某些调用，如View的invalidate，触发ViewRootImpl#scheduleTraversals，最终调用
Choreographer#postCallback(Choreographer.CALLBACK_TRAVERSAL,mTraversalRunnable, null);，只是明确了Callbac的类型以及回调处理Runnable而已，基本流程和自定义FrameCallback一样。
总结
尽量避免在执行动画或渲染操作之后在主线程执行操作，在之前或之后都应该尽量避免发送消息到主线程looper
既然自定义FrameCallback可以在下一个frame被渲染的时候会被回调，那我们是不是可以根据这个原理实现应用的帧率监听呢，答案是肯定的，下面是我的简单实现:
1.自定义FrameCallback：FPSFrameCallback
public class FPSFrameCallback implements Choreographer.FrameCallback {
private static final String TAG = "FPS_TEST";
private long mLastFrameTimeNanos = 0;
private long mFrameIntervalNanos;
public FPSFrameCallback(long lastFrameTimeNanos) {
mLastFrameTimeNanos = lastFrameTimeNanos;
mFrameIntervalNanos = (long)(1000000000 / 60.0);
}
@Override
public void doFrame(long frameTimeNanos) {
//初始化时间
if (mLastFrameTimeNanos == 0) {
mLastFrameTimeNanos = frameTimeNanos;
}
final long jitterNanos = frameTimeNanos - mLastFrameTimeNanos;
if (jitterNanos >= mFrameIntervalNanos) {
final long skippedFrames = jitterNanos / mFrameIntervalNanos;
if(skippedFrames>30){
Log.i(TAG, "Skipped " + skippedFrames + " frames! "
+ "The application may be doing too much work on its main thread.");
}
}
mLastFrameTimeNanos=frameTimeNanos;
//注册下一帧回调
Choreographer.getInstance().postFrameCallback(this);
}
}
2.在Application中注册
@Override
public void onCreate() {
super.onCreate();
Choreographer.getInstance().postFrameCallback(new FPSFrameCallback(System.nanoTime()));
}
3.测试
public class MainActivity extends FragmentActivity {
@Override
public void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
}
@Override
protected void onResume() {
super.onResume();
try {
Thread.sleep(1000);
} catch (InterruptedException e) {
e.printStackTrace();
}
}
}
LOG输出如下：
I/Choreographer: Skipped 64 frames! The application may be doing too much work on its main thread.
I/FPS_TEST: Skipped 65 frames! The application may be doing too much work on its main thread.
基本和系统监控数值一致