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Thread/Hander/Looper是Android在Java執行緒基礎之上提供的執行緒通訊/訊息處理機制，這個眾所周知，不再細說。Handler提供了兩個傳送延遲處理任務的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)

問題在於，這兩個delay的精度到底能有多大？如何理解？很多APP的定時處理機制都是使用這兩個api遞迴拋延遲任務來實現的。所以有必要研究一下框架層的實現，心中有數。Android這套訊息迴圈機制工作在最上層，距離Linux kernel的時間管理甚遠。本文仍然採用跟蹤分析程式碼的方式，基於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()將傳入的Runnable物件封裝成一個Message，呼叫sendMessageDelayed()，而sendMessageDelayed()增加了一個delay時間引數的健壯性檢查，然後轉化成絕對時間，呼叫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。所以，Handler五花八門的post/send api們本質上無差別。只是為了讓使用者在簡單的情況下避免手動封裝Message，只需提供一個Runnable即可。Handler呼叫關係整理如下：

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。轉入訊息佇列類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，採用執行緒安全的方式將Message插入到訊息佇列中，插入的新訊息有三種可能成為訊息佇列的head：

（1）訊息佇列為空;

（2）引數when為0，因為此時when已經轉成絕對時間，所以只有AtFrontOfQueue系列的API才會滿足這個條件;

（3）當前的head Message執行時間在when之後，即訊息佇列中無需要在此Message之前執行的Message。

接下來就要看看訊息迴圈（Looper）如何使用when，這是本文問題的關鍵。關鍵的方法，Looper.loop()，啟動執行緒訊息迴圈：

    /**
     * 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(;;)可以看到一次迴圈開始於從訊息佇列中去取一個訊息，MessageQueue.next()，如果next()返回null，則loop()會返回，本次訊息迴圈結束。取出訊息之後，通過Handler.dispatchMessage()處理訊息：

msg.target.dispatchMessage(msg);

也就是說，取下一個訊息的實際執行時間取決於上一個訊息什麼時候處理完。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(;;)，而出口只有兩個：訊息佇列已經退出，返回null；找到了一個合適的訊息，將其返回。如果沒有合適的訊息，或者訊息佇列為空，會block或者由IdleHandler處理，不在本文問題範疇，暫不展開。主要看找到合適的訊息的邏輯：

                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，如果在訊息佇列中順序找到了一個訊息msg（前文分析過，訊息佇列的插入是由when順序排列，所以如果當前的訊息沒有到執行時間，其後的也一定不會到），當前的系統時間小於msg.when，那麼會計算一個timeout，以便在到執行時間時wake up；如果當前系統時間大於或等於msg.when，那麼會返回msg給Looper.loop()。所以這個邏輯只能保證在when之前訊息不被處理，不能夠保證一定在when時被處理。很好理解：
（1）在Loop.loop()中是順序處理訊息，如果前一個訊息處理耗時較長，完成之後已經超過了when，訊息不可能在when時間點被處理。

（2）即使when的時間點沒有被處理其他訊息所佔用，執行緒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訊息，會被優先處理，又增加了（1）的可能性。

所以由上述三點可知，Handler提供的指定處理時間的api諸如postDelayed()/postAtTime()/sendMessageDelayed()/sendMessageAtTime()，只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。

from：https://blog.csdn.net/zhanglianyu00/article/details/70842494