Handler sync barrier

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

   /**
     *
     * @hide
     */
    public int postSyncBarrier() {
        return postSyncBarrier(SystemClock.uptimeMillis());
    }

    private int postSyncBarrier(long when) {
        synchronized (this) {
            final int token = mNextBarrierToken++;
            //1、屏障消息和普通消息的区别是屏障消息没有tartget。
            final Message msg = Message.obtain();
            msg.markInUse();
            msg.when = when;
            msg.arg1 = token;

            Message prev = null;
            Message p = mMessages;
            //2、根据时间顺序将屏障插入到消息链表中适当的位置
            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;
            }
            //3、返回一个序号，通过这个序号可以撤销屏障
            return token;
        }
    }

    private boolean enqueueMessage(MessageQueue queue, Message msg, long uptimeMillis) {
        msg.target = this;
        if (mAsynchronous) {
            msg.setAsynchronous(true);
        }
        return queue.enqueueMessage(msg, uptimeMillis);
    }

Message next() 

        .....//省略
        int pendingIdleHandlerCount = -1; // -1 only during first iteration
        // 1.如果nextPollTimeoutMillis=-1，一直阻塞不会超时。
        // 2.如果nextPollTimeoutMillis=0，不会阻塞，立即返回。
        // 3.如果nextPollTimeoutMillis>0，最长阻塞nextPollTimeoutMillis毫秒(超时)
        //   如果期间有程序唤醒会立即返回。
        int nextPollTimeoutMillis = 0;
        for (;;) {
            if (nextPollTimeoutMillis != 0) {
                Binder.flushPendingCommands();
            }
            nativePollOnce(ptr, nextPollTimeoutMillis);
            synchronized (this) {
                //获取系统开机到现在的时间
                final long now = SystemClock.uptimeMillis();
                Message prevMsg = null;
                Message msg = mMessages; //当前链表的头结点
                //如果target==null，那么它就是屏障，需要循环遍历，一直往后找到第一个异步的消息

                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());
                }
                if (msg != null) {
                    //如果有消息需要处理，先判断时间有没有到，如果没到的话设置一下阻塞时间，
                    //场景如常用的postDelay
                    if (now < msg.when) {
                       //计算出离执行时间还有多久赋值给nextPollTimeoutMillis，
                       //表示nativePollOnce方法要等待nextPollTimeoutMillis时长后返回
                        nextPollTimeoutMillis = (int) Math.min(msg.when - now, Integer.MAX_VALUE);
                    } else {
                        // 获取到消息
                        mBlocked = false;
                       //链表操作，获取msg并且删除该节点 
                        if (prevMsg != null) 
                            prevMsg.next = msg.next;
                        } else {
                            mMessages = msg.next;
                        }
                        msg.next = null；
                        msg.markInUse();
                        //返回拿到的消息
                        return msg;
                    }
                } else {
                    //没有消息，nextPollTimeoutMillis复位
                    nextPollTimeoutMillis = -1;
                }
                .....//省略

    }

public Handler(boolean async);
public Handler(Callback callback, boolean async);
public Handler(Looper looper, Callback callback, boolean async);

void scheduleTraversals() {
    if (!mTraversalScheduled) {
        mTraversalScheduled = true;
        //设置同步障碍，确保mTraversalRunnable优先被执行
        mTraversalBarrier = mHandler.getLooper().getQueue().postSyncBarrier();
        //内部通过Handler发送了一个异步消息
        mChoreographer.postCallback(
                Choreographer.CALLBACK_TRAVERSAL, mTraversalRunnable, null);
        if (!mUnbufferedInputDispatch) {
            scheduleConsumeBatchedInput();
        }
        notifyRendererOfFramePending();
        pokeDrawLockIfNeeded();
    }
}

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);
            //delayMillis传的是0，故此处进入条件
            if (dueTime <= now) {
               //实际上单单从这个方法的名字我们就能意识到做的是跟帧有关的工作。
                scheduleFrameLocked(now);
            } else {
                Message msg = mHandler.obtainMessage(MSG_DO_SCHEDULE_CALLBACK, action);
                msg.arg1 = callbackType;
                msg.setAsynchronous(true);
                mHandler.sendMessageAtTime(msg, dueTime);
            }
        }
    }

    void unscheduleTraversals() {
        if (mTraversalScheduled) {
            mTraversalScheduled = false;
            //移除内存屏障
            mHandler.getLooper().getQueue().removeSyncBarrier(mTraversalBarrier);
            mChoreographer.removeCallbacks(
                    Choreographer.CALLBACK_TRAVERSAL, mTraversalRunnable, null);
        }
    }