Handler sendMessageDelayed()/postDelayed()機制詳解

Thread/Hander/Looper是Android在Java線程基礎之上提供的線程通信/消息處理機制，這個衆所周知，不再細說。Handler提供了兩個發送延遲處理任務的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)
           

問題在于，這兩個delay的精度到底能有多大？如何了解？很多APP的定時處理機制都是使用這兩個api遞歸抛延遲任務來實作的。是以有必要研究一下架構層的實作，心中有數。Android這套消息循環機制工作在最上層，距離Linux kernel的時間管理甚遠。本文仍然采用跟蹤分析代碼的方式，基于android7.1.1。

postDelayed()實際上封裝了sendMessageDelayed()，第一時間便殊途同歸：

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()首先通過getPostMessage()将傳入的Runnable對象封裝成一個Message，調用sendMessageDelayed()，而sendMessageDelayed()增加了一個delay時間參數的健壯性檢查，然後轉化成絕對時間，調用sendMessageAtTime()。至此，再多說一句：最簡單的sendMessage()和post()實際上也是sendMessageDelayed(0)的封裝。是以，Handler五花八門的post/send api們本質上無差别。隻是為了讓使用者在簡單的情況下避免手動封裝Message，隻需提供一個Runnable即可。Handler調用關系整理如下：

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

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

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

最後都以enqueueMessage()告終

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

如前所述，這時候when已經轉化成絕對系統時間。轉入消息隊列類MessageQueue看一下enqueueMessage()這個方法：

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;
    }
           

這個方法比較簡單，采用線程安全的方式将Message插入到消息隊列中，插入的新消息有三種可能成為消息隊列的head：

（1）消息隊列為空;

（2）參數when為0，因為此時when已經轉成絕對時間，是以隻有AtFrontOfQueue系列的API才會滿足這個條件;

（3）目前的head Message執行時間在when之後，即消息隊列中無需要在此Message之前執行的Message。

接下來就要看看消息循環（Looper）如何使用when，這是本文問題的關鍵。關鍵的方法，Looper.loop()，啟動線程消息循環：

/**
     * 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();
        }
    }
           

從for(;;)可以看到一次循環開始于從消息隊列中去取一個消息，MessageQueue.next()，如果next()傳回null，則loop()會傳回，本次消息循環結束。取出消息之後，通過Handler.dispatchMessage()處理消息：

msg.target.dispatchMessage(msg);
           

也就是說，取下一個消息的實際執行時間取決于上一個消息什麼時候處理完。再看MessageQueue.next()做了什麼：

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()實際上也有一個for(;;)，而出口隻有兩個：消息隊列已經退出，傳回null；找到了一個合适的消息，将其傳回。如果沒有合适的消息，或者消息隊列為空，會block或者由IdleHandler處理，不在本文問題範疇，暫不展開。主要看找到合适的消息的邏輯：

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，如果在消息隊列中順序找到了一個消息msg（前文分析過，消息隊列的插入是由when順序排列，是以如果目前的消息沒有到執行時間，其後的也一定不會到），目前的系統時間小于msg.when，那麼會計算一個timeout，以便在到執行時間時wake up；如果目前系統時間大于或等于msg.when，那麼會傳回msg給Looper.loop()。是以這個邏輯隻能保證在when之前消息不被處理，不能夠保證一定在when時被處理。很好了解：

（1）在Loop.loop()中是順序處理消息，如果前一個消息處理耗時較長，完成之後已經超過了when，消息不可能在when時間點被處理。

（2）即使when的時間點沒有被處理其他消息所占用，線程也有可能被排程失去cpu時間片。

（3）在等待時間點when的過程中有可能入隊處理時間更早的消息，會被優先處理，又增加了（1）的可能性。

是以由上述三點可知，Handler提供的指定處理時間的api諸如postDelayed()/postAtTime()/sendMessageDelayed()/sendMessageAtTime()，隻能保證在指定時間之前不被執行，不能保證在指定時間點被執行。