void scheduleTraversals() {
       if (!mTraversalScheduled) {
           mTraversalScheduled = true;
           //設(shè)置同步障礙，確保mTraversalRunnable優(yōu)先被執(zhí)行
           mTraversalBarrier = mHandler.getLooper().getQueue().postSyncBarrier();
           //內(nèi)部持有Handler關(guān)聯(lián)主looper, 然后通過Handler發(fā)送了一個(gè)異步消息到主線程messageQueue
           mChoreographer.postCallback(
                   Choreographer.CALLBACK_TRAVERSAL, mTraversalRunnable, null);
           。。。。
       }
   }

void doTraversal() {
        if (mTraversalScheduled) {
            mTraversalScheduled = false;
          //移除消息屏障  
         mHandler.getLooper().getQueue().removeSyncBarrier(mTraversalBarrier);
            if (mProfile) {
                Debug.startMethodTracing("ViewAncestor");
            }
            performTraversals();
            if (mProfile) {
                Debug.stopMethodTracing();
                mProfile = false;
            }
        }
    }

//Message.java
public boolean isAsynchronous() {
    return (flags & FLAG_ASYNCHRONOUS) != 0;
}

private int postSyncBarrier(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++;
        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;
    }
}

//Handler.java
private boolean enqueueMessage(MessageQueue queue, Message msg, long uptimeMillis) {
    msg.target = this;
    //...
    return queue.enqueueMessage(msg, uptimeMillis);
}

public static void loop() {
        final Looper me = myLooper();  
        final MessageQueue queue = me.mQueue;
        for (;;) {
           //取出消息，沒有消息則阻塞
            Message msg = queue.next();
            msg.target.dispatchMessage(msg);
       }
}

Message next() {
    //...
    int pendingIdleHandlerCount = -1; // -1 only during first iteration
    int nextPollTimeoutMillis = 0;
    for (;;) {
        //...
        synchronized (this) {
            // Try to retrieve the next message.  Return if found.
            final long now = SystemClock.uptimeMillis();
            Message prevMsg = null;
            Message msg = mMessages;
            if (msg != null && msg.target == null) {//碰到同步屏障，target==null
                // Stalled by a barrier.  Find the next asynchronous message in the queue.
                // do while循環(huán)遍歷消息鏈表
                // 跳出循環(huán)時(shí)，msg指向離表頭最近的一個(gè)“非同步消息”，沒有就會為null
                do {
                    prevMsg = msg;
                    msg = msg.next;
                } while (msg != null && !msg.isAsynchronous());
            }
            if (msg != null) {
                if (now < msg.when) {
                    //...
                } else {
                    // Got a message.
                    mBlocked = false;
                    if (prevMsg != null) {
                        //將msg從消息鏈表中移除
                        prevMsg.next = msg.next;
                    } else {
                        mMessages = msg.next;
                    }
                    msg.next = null;
                    if (DEBUG) Log.v(TAG, "Returning message: " + msg);
                    msg.markInUse();
                    //返回異步消息
                    return msg;
                }
            } else {
                // No more messages.
                nextPollTimeoutMillis = -1;
            }
            //...
        }
        //...
    }
}

public void removeSyncBarrier(int token) {
            // Remove a sync barrier token from the queue.  
            //....省略.......移除隊(duì)列中barrier的token消息
            //喚醒線程
            if (needWake && !mQuitting) {
                nativeWake(mPtr);
            }
        }
    }

public Handler(boolean async);
public Handler(Callback callback, boolean async);
public Handler(Looper looper, Callback callback, boolean async);
private boolean enqueueMessage(MessageQueue queue, Message msg, long uptimeMillis) {
        msg.target = this; //target 是不會為null的
        if (mAsynchronous) {// 默認(rèn)為false ，消息默認(rèn)是被標(biāo)記為同步(普通)消息
            msg.setAsynchronous(true);
        }
        return queue.enqueueMessage(msg, uptimeMillis);
}