Handler sendMessageDelayed()/postDelayed()机制详解

Thread/Hander/Looper是Android在Java线程基础之上提供的线程通信/消息处理机制，这个众所周知，不再细说。Handler提供了两个发送延迟处理任务的api：

/** * Enqueue a message into the message queue after all pending messages * before (current time + delayMillis). You will receive it in * {@link #handleMessage}, in the thread attached to this handler. *   * @return Returns true if the message was successfully placed in to the  *         message queue.  Returns false on failure, usually because the *         looper processing the message queue is exiting.  Note that a *         result of true does not mean the message will be processed -- if *         the looper is quit before the delivery time of the message *         occurs then the message will be dropped. */public final boolean sendMessageDelayed(Message msg, long delayMillis)/** * Causes the Runnable r to be added to the message queue, to be run * after the specified amount of time elapses. * The runnable will be run on the thread to which this handler * is attached. * <b>The time-base is {@link android.os.SystemClock#uptimeMillis}.</b> * Time spent in deep sleep will add an additional delay to execution. *   * @param r The Runnable that will be executed. * @param delayMillis The delay (in milliseconds) until the Runnable *        will be executed. *         * @return Returns true if the Runnable was successfully placed in to the  *         message queue.  Returns false on failure, usually because the *         looper processing the message queue is exiting.  Note that a *         result of true does not mean the Runnable will be processed -- *         if the looper is quit before the delivery time of the message *         occurs then the message will be dropped. */public final boolean postDelayed(Runnable r, long delayMillis)

问题在于，这两个delay的精度到底能有多大？如何理解？很多APP的定时处理机制都是使用这两个api递归抛延迟任务来实现的。所以有必要研究一下框架层的实现，心中有数。Android这套消息循环机制工作在最上层，距离Linux kernel的时间管理甚远。本文仍然采用跟踪分析代码的方式，基于android7.1.1。

postDelayed()实际上封装了sendMessageDelayed()，第一时间便殊途同归：

    public final boolean postDelayed(Runnable r, long delayMillis)    {        return sendMessageDelayed(getPostMessage(r), delayMillis);    }

    public final boolean sendMessageDelayed(Message msg, long delayMillis)    {        if (delayMillis < 0) {            delayMillis = 0;        }        return sendMessageAtTime(msg, SystemClock.uptimeMillis() + delayMillis);    }

postDelayed()首先通过getPostMessage()将传入的Runnable对象封装成一个Message，调用sendMessageDelayed()，而sendMessageDelayed()增加了一个delay时间参数的健壮性检查，然后转化成绝对时间，调用sendMessageAtTime()。至此，再多说一句：最简单的sendMessage()和post()实际上也是sendMessageDelayed(0)的封装。所以，Handler五花八门的post/send api们本质上无差别。只是为了让使用者在简单的情况下避免手动封装Message，只需提供一个Runnable即可。Handler调用关系整理如下：

post()/postDelayed()/sendMessage()->sendMessageDelayed()->sendMessageAtTime()->enqueueMessage()

postAtTime()->sendMessageAtTime()->enqueueMessage()

postAtFrontOfQueue()->sendMessageAtFrontOfQueue()->enqueueMessage()

最后都以enqueueMessage()告终

enqueueMessage()->MessageQueue.enqueueMessage(Message msg, long when)

如前所述，这时候when已经转化成绝对系统时间。转入消息队列类MessageQueue看一下enqueueMessage()这个方法：

    boolean enqueueMessage(Message msg, long when) {        if (msg.target == null) {            throw new IllegalArgumentException("Message must have a target.");        }        if (msg.isInUse()) {            throw new IllegalStateException(msg + " This message is already in use.");        }        synchronized (this) {            if (mQuitting) {                IllegalStateException e = new IllegalStateException(                        msg.target + " sending message to a Handler on a dead thread");                Log.w(TAG, e.getMessage(), e);                msg.recycle();                return false;            }            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;    }

这个方法比较简单，采用线程安全的方式将Message插入到消息队列中，插入的新消息有三种可能成为消息队列的head：

（1）消息队列为空;

（2）参数when为0，因为此时when已经转成绝对时间，所以只有AtFrontOfQueue系列的API才会满足这个条件;

（3）当前的head Message执行时间在when之后，即消息队列中无需要在此Message之前执行的Message。

接下来就要看看消息循环（Looper）如何使用when，这是本文问题的关键。关键的方法，Looper.loop()，启动线程消息循环：

    /**     * Run the message queue in this thread. Be sure to call     * {@link #quit()} to end the loop.     */    public static void loop() {        final Looper me = myLooper();        if (me == null) {            throw new RuntimeException("No Looper; Looper.prepare() wasn't called on this thread.");        }        final MessageQueue queue = me.mQueue;        // Make sure the identity of this thread is that of the local process,        // and keep track of what that identity token actually is.        Binder.clearCallingIdentity();        final long ident = Binder.clearCallingIdentity();        for (;;) {            Message msg = queue.next(); // might block            if (msg == null) {                // No message indicates that the message queue is quitting.                return;            }            // This must be in a local variable, in case a UI event sets the logger            final Printer logging = me.mLogging;            if (logging != null) {                logging.println(">>>>> Dispatching to " + msg.target + " " +                        msg.callback + ": " + msg.what);            }            final long traceTag = me.mTraceTag;            if (traceTag != 0 && Trace.isTagEnabled(traceTag)) {                Trace.traceBegin(traceTag, msg.target.getTraceName(msg));            }            try {                msg.target.dispatchMessage(msg);            } finally {                if (traceTag != 0) {                    Trace.traceEnd(traceTag);                }            }            if (logging != null) {                logging.println("<<<<< Finished to " + msg.target + " " + msg.callback);            }            // Make sure that during the course of dispatching the            // identity of the thread wasn't corrupted.            final long newIdent = Binder.clearCallingIdentity();            if (ident != newIdent) {                Log.wtf(TAG, "Thread identity changed from 0x"                        + Long.toHexString(ident) + " to 0x"                        + Long.toHexString(newIdent) + " while dispatching to "                        + msg.target.getClass().getName() + " "                        + msg.callback + " what=" + msg.what);            }            msg.recycleUnchecked();        }    }

从for(;;)可以看到一次循环开始于从消息队列中去取一个消息，MessageQueue.next()，如果next()返回null，则loop()会返回，本次消息循环结束。取出消息之后，通过Handler.dispatchMessage()处理消息：

msg.target.dispatchMessage(msg);

也就是说，取下一个消息的实际执行时间取决于上一个消息什么时候处理完。再看MessageQueue.next()做了什么：

    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;        }    }

看到next()实际上也有一个for(;;)，而出口只有两个：消息队列已经退出，返回null；找到了一个合适的消息，将其返回。如果没有合适的消息，或者消息队列为空，会block或者由IdleHandler处理，不在本文问题范畴，暂不展开。主要看找到合适的消息的逻辑：

                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;                }

可以看到，如果在消息队列中顺序找到了一个消息msg（前文分析过，消息队列的插入是由when顺序排列，所以如果当前的消息没有到执行时间，其后的也一定不会到），当前的系统时间小于msg.when，那么会计算一个timeout，以便在到执行时间时wake up；如果当前系统时间大于或等于msg.when，那么会返回msg给Looper.loop()。所以这个逻辑只能保证在when之前消息不被处理，不能够保证一定在when时被处理。很好理解：

（1）在Loop.loop()中是顺序处理消息，如果前一个消息处理耗时较长，完成之后已经超过了when，消息不可能在when时间点被处理。

（2）即使when的时间点没有被处理其他消息所占用，线程也有可能被调度失去cpu时间片。

（3）在等待时间点when的过程中有可能入队处理时间更早的消息，会被优先处理，又增加了（1）的可能性。

所以由上述三点可知，Handler提供的指定处理时间的api诸如postDelayed()/postAtTime()/sendMessageDelayed()/sendMessageAtTime()，只能保证在指定时间之前不被执行，不能保证在指定时间点被执行。