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从FrameCallback理解Choreographer原理及简单帧率监控应用

96
adison
2017.03.15 09:18* 字数 695

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

Android日常
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