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.javapublic int postSyncBarrier() {return postSyncBarrier(SystemClock.uptimeMillis());
}private int postSyncBarrier(long when) {    synchronized (this) {final int token = mNextBarrierToken++;// 从消息池中获取Messagefinal Message msg = Message.obtain();msg.markInUse();// 没有给target赋值，没有绑定Handlermsg.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nullprev = 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.javavoid 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.javaprivate 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.javavoid 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);}
}