安卓 消息队列 优先级 顺序

韩梦飞沙 yue31313 韩亚飞 han_meng_fei_sha 313134555@qq.com

消息队列排序

一般的像我们普通调用Handler发送消息，最后都会调用MessageQueue的enqueueMessage。

public boolean sendMessageAtTime(Message msg, long uptimeMillis) {
    MessageQueue queue = mQueue;
    if (queue == null) {
        RuntimeException e = new RuntimeException(
                this + " sendMessageAtTime() called with no mQueue");
        Log.w("Looper", e.getMessage(), e);
        return false;
    }
    return enqueueMessage(queue, msg, uptimeMillis);
}

像sendMessageAtFrontOfQueue这样只是最后的时间设置为0，自然就排在队列的前面了。

public final boolean sendMessageAtFrontOfQueue(Message msg) {
    MessageQueue queue = mQueue;
    if (queue == null) {
        RuntimeException e = new RuntimeException(
            this + " sendMessageAtTime() called with no mQueue");
        Log.w("Looper", e.getMessage(), e);
        return false;
    }
    return enqueueMessage(queue, msg, 0);
}

private boolean enqueueMessage(MessageQueue queue, Message msg, long uptimeMillis) {
    msg.target = this;//一般讲target设置为Handler
    if (mAsynchronous) {
        msg.setAsynchronous(true);
    }
    return queue.enqueueMessage(msg, uptimeMillis);
}

最后就到MessageQueue的enqueueMessage函数中去了，该函数就是根据when把这个msg插入到合适的消息队列中。

boolean enqueueMessage(Message msg, long when) {
    ......

    synchronized (this) {
        ......

        msg.markInUse();
        msg.when = when;
        Message p = mMessages;//当前message
        boolean needWake;
        if (p == null || when == 0 || when < p.when) {
            // New head, wake up the event queue if blocked.
            msg.next = p;
            mMessages = msg;
            needWake = mBlocked;
        } else {
            // Inserted within the middle of the queue.  Usually we don't have to wake
            // up the event queue unless there is a barrier at the head of the queue
            // and the message is the earliest asynchronous message in the queue.
            needWake = mBlocked && p.target == null && msg.isAsynchronous();
            Message prev;
            for (;;) {
                prev = p;
                p = p.next;
                if (p == null || when < p.when) {//找到一个Message，when小于该message的when break
                    break;
                }
                if (needWake && p.isAsynchronous()) {
                    needWake = false;
                }
            }
            msg.next = p; // invariant: p == prev.next
            prev.next = msg;//这其实就是把该message插入合适的位置
        }

        // We can assume mPtr != 0 because mQuitting is false.
        if (needWake) {
            nativeWake(mPtr);
        }
    }
    return true;
}

最后我们再看下MessageQueue的next函数，就是消息线程循环时会不断调用MessageQueue的next来获取当前消息。

Message next() {
    final long ptr = mPtr;
    if (ptr == 0) {
        return null;
    }

    int pendingIdleHandlerCount = -1; // -1 only during first iteration
    int nextPollTimeoutMillis = 0;
    for (;;) {
        if (nextPollTimeoutMillis != 0) {
            Binder.flushPendingCommands();
        }

        nativePollOnce(ptr, nextPollTimeoutMillis);//c层的epoll函数会阻塞

        synchronized (this) {
            // Try to retrieve the next message.  Return if found.
            final long now = SystemClock.uptimeMillis();
            Message prevMsg = null;
            Message msg = mMessages;// 当前msg
            if (msg != null && msg.target == null) {//这里就是和异步消息有关，下节分析
                // 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) {
                if (now < msg.when) {//当前消息还没到，后续可以处理空闲处理器等
                    // 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;.//下个消息赋值为mMessages
                    }
                    msg.next = null;
                    if (DEBUG) Log.v(TAG, "Returning message: " + msg);
                    msg.markInUse();
                    return msg;
                }
            } else {
                // No more messages.
                nextPollTimeoutMillis = -1;//没有消息无限阻塞
            }
            ......