----》類名:ViewRootImpl

  @Override
  public void requestLayout() {
    if (!mHandlingLayoutInLayoutRequest) {
      checkThread();
      mLayoutRequested = true;
      //重點(diǎn)
      scheduleTraversals();
    }
  }

----》類名:ViewRootImpl

  void scheduleTraversals() {
    if (!mTraversalScheduled) {
      mTraversalScheduled = true;
      mTraversalBarrier = mHandler.getLooper().getQueue().postSyncBarrier();
      mChoreographer.postCallback(
          Choreographer.CALLBACK_TRAVERSAL, mTraversalRunnable, null);
      ......
    }
  }

 public static Choreographer getInstance() {
    return sThreadInstance.get();
  }
  
    // Thread local storage for the choreographer.
  private static final ThreadLocal<Choreographer> sThreadInstance =
      new ThreadLocal<Choreographer>() {
    @Override
    protected Choreographer initialValue() {
      Looper looper = Looper.myLooper();
      if (looper == null) {
        throw new IllegalStateException("The current thread must have a looper!");
      }
      Choreographer choreographer = new Choreographer(looper, VSYNC_SOURCE_APP);
      if (looper == Looper.getMainLooper()) {
        mMainInstance = choreographer;
      }
      return choreographer;
    }
  };

 ----》類名:Choreographer
 //1
  public void postCallback(int callbackType, Runnable action, Object token) {
    postCallbackDelayed(callbackType, action, token, 0);
  }
 //2 
   public void postCallbackDelayed(int callbackType,
      Runnable action, Object token, long delayMillis) {
    ....
    postCallbackDelayedInternal(callbackType, action, token, delayMillis);
  }
  //3
   private void postCallbackDelayedInternal(int callbackType,
      Object action, Object token, long delayMillis) {
        ...
        mCallbackQueues[callbackType].addCallbackLocked(dueTime, action, token);
        if (dueTime <= now) {
        scheduleFrameLocked(now);
      } else {
        ...
       }
      }

 ----》類名:Choreographer
 private void scheduleFrameLocked(long now) {
    ...
    // If running on the Looper thread, then schedule the vsync immediately,
        // otherwise post a message to schedule the vsync from the UI thread
        // as soon as possible.
        if (isRunningOnLooperThreadLocked()) {
          scheduleVsyncLocked();
        } else {
          Message msg = mHandler.obtainMessage(MSG_DO_SCHEDULE_VSYNC);
          msg.setAsynchronous(true);
          mHandler.sendMessageAtFrontOfQueue(msg);
        }
      } else {
        ...
      }
    }
  }

 ----》類名:Choreographer
 
  private void scheduleVsyncLocked() {
    mDisplayEventReceiver.scheduleVsync();
  }
  
 ----》類名:DisplayEventReceiver
 
    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 {
    //mReceiverPtr是Native層一個(gè)類的指針地址
    //這里這個(gè)類指的是底層NativeDisplayEventReceiver這個(gè)類
    //nativeScheduleVsync底層會(huì)調(diào)用到requestNextVsync（）去請(qǐng)求下一個(gè)Vsync，
    //具體不跟蹤了，native層代碼更長(zhǎng)，還涉及到各種描述符監(jiān)聽以及跨進(jìn)程數(shù)據(jù)傳輸
      nativeScheduleVsync(mReceiverPtr);
    }
  }

 ----》類名:DisplayEventReceiver
 
  // Called from native code. <---注釋還是很良心的
  private void dispatchVsync(long timestampNanos, int builtInDisplayId, int frame) {
    onVsync(timestampNanos, builtInDisplayId, frame);
  }

 public void onVsync(long timestampNanos, int builtInDisplayId, int frame) {
  }

 ----》類名:Choreographer
 
private final class FrameDisplayEventReceiver extends DisplayEventReceiver
      implements Runnable {
      ....
      }

 ----》類名:Choreographer
 
 private final class FrameDisplayEventReceiver extends DisplayEventReceiver
      implements Runnable {

    @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 / TimeUtils.NANOS_PER_MS);
    }

    @Override
    public void run() {
      ....
      doFrame(mTimestampNanos, mFrame);
    }
  }

  void doFrame(long frameTimeNanos, int frame) {
    final long startNanos;
    synchronized (mLock) {
      //1、階段一
      long intendedFrameTimeNanos = frameTimeNanos;
      startNanos = System.nanoTime();
      final long jitterNanos = startNanos - frameTimeNanos;
      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.");
        }
        ...
      }
      ...
    }

 void doFrame(long frameTimeNanos, int frame) {
 ...
try {
//階段2
      Trace.traceBegin(Trace.TRACE_TAG_VIEW, "Choreographer#doFrame");
      AnimationUtils.lockAnimationClock(frameTimeNanos / TimeUtils.NANOS_PER_MS);

      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);
    } 
    ...
    }

 ----》類名:ViewRootImpl

  final class TraversalRunnable implements Runnable {
    @Override
    public void run() {
      doTraversal();
    }
  }
  
  void doTraversal() {
    if (mTraversalScheduled) {
     ...
      performTraversals();
     ...
    }
  }
  
  private void performTraversals() {
   ...
   //開始真正的界面繪制
    performDraw();
   ...
  }