Android Handler之消息同步屏障

概述

同步屏障：即消息的同步阻碍，指阻碍同步消息，只让异步消息通过。

平时我们通过 Handler 发送到消息大部分都是同步消息，当然也可以设置为异步消息。

源码分析

开启消息屏障

通过 MessageQueue#postSyncBarrier() 开启消息屏障。

// MessageQueue.java

public int postSyncBarrier() {
    return postSyncBarrier(SystemClock.uptimeMillis());
}

private int postSyncBarrier(long when) {    
    synchronized (this) {
        final int token = mNextBarrierToken++;
        // 从消息池中获取Message
        final Message msg = Message.obtain();
        msg.markInUse();
        // 没有给target赋值，没有绑定Handler
        msg.when = when;
        msg.arg1 = token;

        Message prev = null;
        Message p = mMessages;
        if (when != 0) {
            while (p != null && p.when <= when) {
                // 如果开启同步屏障的时间（假设记为T）T不为0，且当前的同步消息里有时间小于T，则prev也不为null
                prev = p;
                p = p.next;
            }
        }
        
        // 将msg插入到消息队列中
        if (prev != null) {  
            msg.next = p;
            prev.next = msg;
        } else {
            msg.next = p;
            mMessages = msg;
        }
        return token;
    }
}

取异步消息

Message next() {
    int nextPollTimeoutMillis = 0;
    // 死循环
    for (;;) {
        if (nextPollTimeoutMillis != 0) {
            Binder.flushPendingCommands();
        }

        nativePollOnce(ptr, nextPollTimeoutMillis);

        synchronized (this) {
            final long now = SystemClock.uptimeMillis();
            Message prevMsg = null;
            Message msg = mMessages;
            // 如果msg.target为null，表示收到消息同步屏障，循环遍历查找第一个异步消息
            if (msg != null && msg.target == null) {
                do {
                    prevMsg = msg;
                    msg = msg.next;
                } while (msg != null && !msg.isAsynchronous());
            }
            // 查找到的消息继续往下执行
            if (msg != null) {
                if (now < msg.when) {
                    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 (DEBUG) Log.v(TAG, "Returning message: " + msg);
                    msg.markInUse();
                    return msg;
                }
            } else {
                nextPollTimeoutMillis = -1;
            }
            // ...
        }
    }
}

删除消息屏障

通过 MessageQueue#removeSyncBarrier() 删除消息屏障，并唤醒。

public void removeSyncBarrier(int token) {
    // Remove a sync barrier token from the queue.
    // If the queue is no longer stalled by a barrier then wake it.
    synchronized (this) {
        Message prev = null;
        Message p = mMessages;
        while (p != null && (p.target != null || p.arg1 != token)) {
            prev = p;
            p = p.next;
        }
        if (p == null) {
            throw new IllegalStateException("The specified message queue synchronization "
                    + " barrier token has not been posted or has already been removed.");
        }
        final boolean needWake;
        if (prev != null) {
            prev.next = p.next;
            needWake = false;
        } else {
            mMessages = p.next;
            needWake = mMessages == null || mMessages.target != null;
        }
        p.recycleUnchecked();

        // If the loop is quitting then it is already awake.
        // We can assume mPtr != 0 when mQuitting is false.
        if (needWake && !mQuitting) {
            nativeWake(mPtr);
        }
    }
}

应用场景

日常的应用开发中，很少会用到同步屏障。主要是 Android 系统中的 UI 更新相关的消息即为异步消息，需要优先处理。如，在 View 更新时，draw、requestLayout、invalidate 等很多地方都调用了。

// ViewRootImpl.java

void scheduleTraversals() {
    if (!mTraversalScheduled) {
        mTraversalScheduled = true;
        // 开启同步屏障
        mTraversalBarrier = mHandler.getLooper().getQueue().postSyncBarrier();
        // 最终调用Choreographer#postCallbackDelayedInternal()
        mChoreographer.postCallback(
            Choreographer.CALLBACK_TRAVERSAL, mTraversalRunnable, null);
        notifyRendererOfFramePending();
        pokeDrawLockIfNeeded();
    }
}

// Choreographer.java

private void postCallbackDelayedInternal(int callbackType,
                                         Object action, Object token, long delayMillis) {
    if (DEBUG_FRAMES) {
        Log.d(TAG, "PostCallback: type=" + callbackType
              + ", action=" + action + ", token=" + token
              + ", delayMillis=" + 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 {
            // 发送异步消息
            Message msg = mHandler.obtainMessage(MSG_DO_SCHEDULE_CALLBACK, action);
            msg.arg1 = callbackType;
            msg.setAsynchronous(true);
            mHandler.sendMessageAtTime(msg, dueTime);
        }
    }
}

// ViewRootImpl.java

void unscheduleTraversals() {
    if (mTraversalScheduled) {
        mTraversalScheduled = false;
        // 移除消息同步屏障
        mHandler.getLooper().getQueue().removeSyncBarrier(mTraversalBarrier);
        mChoreographer.removeCallbacks(
            Choreographer.CALLBACK_TRAVERSAL, mTraversalRunnable, null);
    }
}