Handler发送消息的delay可靠吗

MessageQueue如何处理消息

Handler的这些方法最终执行的都是MessageQueue.enqueueMessage()方法

boolean enqueueMessage(Message msg, long when) {
        synchronized (this) {
            msg.markInUse();
            msg.when = when;
            Message p = mMessages;
            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) {
                        break;
                    }
                    if (needWake && p.isAsynchronous()) {
                        needWake = false;
                    }
                }
                msg.next = p; // invariant: p == prev.next
                prev.next = msg;
            }

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

enqueueMessage()方法就是将handler发出的message插入messageQueue中。方法中走if条件的话就是当前queue中没有要处理的message，则当前插入的msg为message链表的头；如果走else方法，那么说明当前message链表中有message，插入的msg按照when的先后插入链表。

处理message链表中的消息是在Looper.loop()方法方法中进行的。loop()方法是个for(;;)循环，在每次循环中都会调用messageQueue.next()方法进行消息的获取，然后再分发处理。

messageQueue.next()

Message next() {
    // Return here if the message loop has already quit and been disposed.
    // This can happen if the application tries to restart a looper after quit
    // which is not supported.
    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);

        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) {
                // 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) {
                    // 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;
                    }
                    msg.next = null;
                    if (DEBUG) Log.v(TAG, "Returning message: " + msg);
                    msg.markInUse();
                    return msg;
                }
            } else {
                // No more messages.
                nextPollTimeoutMillis = -1;
            }

            // Process the quit message now that all pending messages have been handled.
            if (mQuitting) {
                dispose();
                return null;
            }

            // If first time idle, then get the number of idlers to run.
            // Idle handles only run if the queue is empty or if the first message
            // in the queue (possibly a barrier) is due to be handled in the future.
            if (pendingIdleHandlerCount < 0
                    && (mMessages == null || now < mMessages.when)) {
                pendingIdleHandlerCount = mIdleHandlers.size();
            }
            if (pendingIdleHandlerCount <= 0) {
                // No idle handlers to run.  Loop and wait some more.
                mBlocked = true;
                continue;
            }

            if (mPendingIdleHandlers == null) {
                mPendingIdleHandlers = new IdleHandler[Math.max(pendingIdleHandlerCount, 4)];
            }
            mPendingIdleHandlers = mIdleHandlers.toArray(mPendingIdleHandlers);
        }

        // Run the idle handlers.
        // We only ever reach this code block during the first iteration.
        for (int i = 0; i < pendingIdleHandlerCount; i++) {
            final IdleHandler idler = mPendingIdleHandlers[i];
            mPendingIdleHandlers[i] = null; // release the reference to the handler

            boolean keep = false;
            try {
                keep = idler.queueIdle();
            } catch (Throwable t) {
                Log.wtf(TAG, "IdleHandler threw exception", t);
            }

            if (!keep) {
                synchronized (this) {
                    mIdleHandlers.remove(idler);
                }
            }
        }

        // Reset the idle handler count to 0 so we do not run them again.
        pendingIdleHandlerCount = 0;

        // While calling an idle handler, a new message could have been delivered
        // so go back and look again for a pending message without waiting.
        nextPollTimeoutMillis = 0;
    }
}

在next()方法的for循环里面

nativePollOnce(ptr, nextPollTimeoutMillis);这句话nextPollTimeoutMillis=-1时会在native层阻塞，即message链表为空的情况（消息队列空闲）。当有消息立即执行或有消息将要执行都不会阻塞。

当阻塞在native层的时候，只有消息队列中插入了新的消息（messageQueue.enqueueMessage()），才能唤醒nativePollOnce方法，再往下执行next方法。

消息队列优化

• 过滤重复消息

  在handler.sendMessage()前，将之前发送过的相同what\object的message都remove掉。

• 取消互斥消息

• 复用消息

  复用消息可以避免重复GC

消息空闲IdleHandler

在消息队列空闲的时候会回调IdleHandler的queueIdle()方法。

idleHandler要在messageQueue中进行添加才能接收到回调。

Looper.myQueue().addIdleHandler(idleHandler)

使用独享的Looper——HandlerThread

在有Looper的线程就可以直接创建Handler，或者创建Handler的时候传入一个looper也可以。

delay大于Handler Looper的周期时，delay时间基本可靠

Looper负载越高，任务越容易积压，导致UI卡顿

在对时间要求高的时候，不要用Handler的delay作为计时的依据