Android VSYNC （Choreographer）与UI刷新原理分析

private void postCallbackDelayedInternal(int callbackType,
        Object action, Object token, long delayMillis) {
    synchronized (mLock) {
        final long now = SystemClock.uptimeMillis();
        final long dueTime = now + delayMillis;
        mCallbackQueues[callbackType].addCallbackLocked(dueTime, action, token);
        if (dueTime <= now) {
        <!--申请VSYNC同步信号-->
            scheduleFrameLocked(now);
        } 
    }
}

// mFrameScheduled保证16ms内，只会申请一次垂直同步信号
// scheduleFrameLocked可以被调用多次，但是mFrameScheduled保证下一个vsync到来之前，不会有新的请求发出
// 多余的scheduleFrameLocked调用被无效化
private void scheduleFrameLocked(long now) {
    if (!mFrameScheduled) {
        mFrameScheduled = true;
        if (USE_VSYNC) {
            if (isRunningOnLooperThreadLocked()) {
                scheduleVsyncLocked();
            } else {
                // 因为invalid已经有了同步栅栏，所以必须mFrameScheduled，消息才能被UI线程执行
                Message msg = mHandler.obtainMessage(MSG_DO_SCHEDULE_VSYNC);
                msg.setAsynchronous(true);
                mHandler.sendMessageAtFrontOfQueue(msg);
            }
        }  
    }
}

  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) {
            long now = System.nanoTime();
            if (timestampNanos > now) {
            <!--正常情况，timestampNanos不应该大于now，一般是上传vsync的机制出了问题-->
                timestampNanos = now;
            }
            <!--如果上一个vsync同步信号没执行，那就不应该相应下一个（可能是其他线程通过某种方式请求的）-->
              if (mHavePendingVsync) {
                Log.w(TAG, "Already have a pending vsync event.  There should only be "
                        + "one at a time.");
            } else {
                mHavePendingVsync = true;
            }
            <!--timestampNanos其实是本次vsync产生的时间，从服务端发过来-->
            mTimestampNanos = timestampNanos;
            mFrame = frame;
            Message msg = Message.obtain(mHandler, this);
            <!--由于已经存在同步栅栏，所以VSYNC到来的Message需要作为异步消息发送过去-->
            msg.setAsynchronous(true);
            mHandler.sendMessageAtTime(msg, timestampNanos / TimeUtils.NANOS_PER_MS);
        }
        @Override
        public void run() {
            mHavePendingVsync = false;
            <!--这里的mTimestampNanos其实就是本次Vynsc同步信号到来的时候，但是执行这个消息的时候，可能延迟了-->
            doFrame(mTimestampNanos, mFrame);
        }
    }

void doFrame(long frameTimeNanos, int frame) {
    final long startNanos;
    synchronized (mLock) {
    <!--做了很多东西，都是为了保证一次16ms有一次垂直同步信号，有一次input 、刷新、重绘-->
        if (!mFrameScheduled) {
            return; // no work to do
        }
       long intendedFrameTimeNanos = frameTimeNanos;
        startNanos = System.nanoTime();
        final long jitterNanos = startNanos - frameTimeNanos;
        <!--检查是否因为延迟执行掉帧，每大于16ms，就多掉一帧-->
        if (jitterNanos >= mFrameIntervalNanos) {
            final long skippedFrames = jitterNanos / mFrameIntervalNanos;
            <!--跳帧，其实就是上一次请求刷新被延迟的时间，但是这里skippedFrames为0不代表没有掉帧-->
            if (skippedFrames >= SKIPPED_FRAME_WARNING_LIMIT) {
            <!--skippedFrames很大一定掉帧，但是为 0，去并非没掉帧-->
                Log.i(TAG, "Skipped " + skippedFrames + " frames!  "
                        + "The application may be doing too much work on its main thread.");
            }
            final long lastFrameOffset = jitterNanos % mFrameIntervalNanos;
                <!--开始doFrame的真正有效时间戳-->
            frameTimeNanos = startNanos - lastFrameOffset;
        }
        if (frameTimeNanos < mLastFrameTimeNanos) {
            <!--这种情况一般是生成vsync的机制出现了问题，那就再申请一次-->
            scheduleVsyncLocked();
            return;
        }
          <!--intendedFrameTimeNanos是本来要绘制的时间戳，frameTimeNanos是真正的，可以在渲染工具中标识延迟VSYNC多少-->
        mFrameInfo.setVsync(intendedFrameTimeNanos, frameTimeNanos);
        <!--移除mFrameScheduled判断，说明处理开始了，-->
        mFrameScheduled = false;
        <!--更新mLastFrameTimeNanos-->
        mLastFrameTimeNanos = frameTimeNanos;
    }
    try {
         <!--真正开始处理业务-->
        Trace.traceBegin(Trace.TRACE_TAG_VIEW, "Choreographer#doFrame");
        <!--处理打包的move事件-->
        mFrameInfo.markInputHandlingStart();
        doCallbacks(Choreographer.CALLBACK_INPUT, frameTimeNanos);
        <!--处理动画-->
        mFrameInfo.markAnimationsStart();
        doCallbacks(Choreographer.CALLBACK_ANIMATION, frameTimeNanos);
        <!--处理重绘-->
        mFrameInfo.markPerformTraversalsStart();
        doCallbacks(Choreographer.CALLBACK_TRAVERSAL, frameTimeNanos);
        <!--不知道干啥的-->
        doCallbacks(Choreographer.CALLBACK_COMMIT, frameTimeNanos);
    } finally {
        Trace.traceEnd(Trace.TRACE_TAG_VIEW);
    }
}