void scheduleTraversals() {
    if (!mTraversalScheduled) {
      mTraversalScheduled = true;
       //向消息隊列插入一個同步屏障的消息。msg.target==null的消息
             mTraversalBarrier = mHandler.getLooper().postSyncBarrier();
             mChoreographer.postCallback(
                  Choreographer.CALLBACK_TRAVERSAL, mTraversalRunnable, null);
    }
  }

 final TraversalRunnable mTraversalRunnable = new TraversalRunnable();
 final class TraversalRunnable implements Runnable {
            @Override
            public void run() {
                doTraversal();
            }
  }
   void doTraversal() {
      if (mTraversalScheduled) {
          mTraversalScheduled = false;
          //移除同步屏障消息
          mHandler.getLooper().removeSyncBarrier(mTraversalBarrier);
          //在這個方法中會調(diào)用 measure layout draw，view的繪制繪制流程的方法
          performTraversals();
      }
    }

int enqueueSyncBarrier(long when) {
        // Enqueue a new sync barrier token.
        // We don't need to wake the queue because the purpose of a barrier is to stall it.
        synchronized (this) {
            final int token = mNextBarrierToken++;
            // 這個msg.target沒有被賦值
            final Message msg = Message.obtain();
            msg.markInUse();
            msg.when = when;
            msg.arg1 = token;
            Message prev = null;
            Message p = mMessages;
            if (when != 0) {
                while (p != null && p.when <= when) {
                    prev = p;
                    p = p.next;
                }
            }
            if (prev != null) { // invariant: p == prev.next
                msg.next = p;
                prev.next = msg;
            } else {
                msg.next = p;
                mMessages = msg;
            }
            return token;
        }
    }

 boolean enqueueMessage(Message msg, long when) {
        if (msg.target == null) {
            throw new IllegalArgumentException("Message must have a target.");
        }
   ...........
}

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) {
                scheduleFrameLocked(now);
            } else {
                Message msg = mHandler.obtainMessage(MSG_DO_SCHEDULE_CALLBACK, action);
                msg.arg1 = callbackType;
                //將消息設(shè)置為異步消息
                msg.setAsynchronous(true);
                mHandler.sendMessageAtTime(msg, dueTime);
            }
        }
}

 Message next() {
            synchronized (this) {
                // Try to retrieve the next message.  Return if found.
                final long now = SystemClock.uptimeMillis();
                Message prevMsg = null;
                Message msg = mMessages;
                //如果msg.target==null說明我們已經(jīng)向消息隊里中插入了一條屏障消息。
                //此時會進入到這個循環(huán)中，找到msg.isAsynchronous==true的異步消息。
                //通常我們發(fā)送的都是同步消息isAsynchronous = false的，并且msg.target不能為null的。
                if (msg != null && msg.target == null) {
                    // Stalled by a barrier.  Find the next asynchronous message in the queue.
                    do {
                        prevMsg = msg;
                        msg = msg.next;
                    } while (msg != null && !msg.isAsynchronous());//msg.isAsynchronous==true時結(jié)束循環(huán)，說明找到了這個異步消息。
                }
                if (msg != null) {//找到了同步屏障的異步消息后，直接返回
                    if (now < msg.when) {
                        // Next message is not ready.  Set a timeout to wake up when it is ready.
                        nextPollTimeoutMillis = (int) Math.min(msg.when - now, Integer.MAX_VALUE);
                    } else {
                        // Got a message.
                        mBlocked = false;
                        if (prevMsg != null) {
                            prevMsg.next = msg.next;
                        } else {
                            mMessages = msg.next;
                        }
                        msg.next = null;
                        if (false) Log.v("MessageQueue", "Returning message: " + msg);
                        return msg;
                    }
                } else {//沒有找到的話則進入休眠直到下一次被喚醒
                    // No more messages.
                    nextPollTimeoutMillis = -1;
                }
        }
    }