Handler是更新UI界面的機制，也是消息處理的機制,我們可以發送消息，也可以處理消息。

（1）為什么要有Handler？

Android在設計的時候，封裝了一套消息創建、傳遞、處理機制，如果不遵循這樣的機制就沒辦法更新UI信息，就會拋出異常。如果一個耗時的任務在主線程中操作，那么就會出現界面卡死，也就是Android的卡頓，耗時的操作需要在子線程中完成，但是這些操作完成之后還需要通知UI，Handler可以很好的處理這個工作。

Handler是Google工程師為我們設計好的處理高并發的工具類，這個類用于線程間通信。

（2）handler基本使用

首先，看一下代碼：

    /**
     * 處理事件
     */
    private Handler handler = new Handler(){

        @Override
        public void handleMessage(Message msg) {
            super.handleMessage(msg);

        }
    };

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);

        new Thread(new Runnable() {
            @Override
            public void run() {
                Message message = new Message();
                //發送數據，當一個耗時任務完成時，通知主線程
                handler.sendMessage(message);
            }
        }).start();

    }
}

handler.sendMessage(message)一般執行在子線程，用于通知主線程任務已完成，message是數據，可以將數據傳遞到主線程。主線程的handleMessage的回調方法中有一個參數message，這個message就是從子線程中傳遞過來的，handleMessage主要用于處理UI。

子線程通知主線程的方法或者子線程傳遞數據到主線程的方法還有很多，如下：

sendMessage(Message msg)
sendMessageDelayed(Message msg, long delayMillis)
sendEmptyMessage(int what)
sendEmptyMessageAtTime(int what, long uptimeMillis)
sendEmptyMessageDelayed(int what, long delayMillis)
sendMessageAtFrontOfQueue(Message msg)
 sendMessageAtTime(Message msg, long uptimeMillis)

除此之外，handler還有一些post方法：

post(Runnable r)
postAtFrontOfQueue(Runnable r)
postAtTime(Runnable r, long uptimeMillis)
postAtTime(Runnable r, Object token, long uptimeMillis)
postDelayed(Runnable r, long delayMillis)
postDelayed(Runnable r, Object token, long delayMillis)

post方法將callback傳入Message，callback就是子線程本身，使用代碼如下：

    new Handler().post(new Runnable() {
        @Override
        public void run() {
            
        }
    });

類似于

    new Thread(new Runnable() {
        @Override
        public void run() {
            
        }
    }).start();

（3）Android為什么要設置只能通過Handler機制更新UI？

最根本的問題解決多線程并發的問題；

假設如果在一個Activity中，有多個線程去更新UI，并且都沒有加鎖機制，那么會產生生么樣的問題？——更新界面混亂；
如果對更新UI 的操作都加鎖處理的話會產生什么樣子的問題？——性能下降
對于上述問題的考慮，Android提供了一套更新UI的機制，我們只需要遵循這樣的機制就好了。
不用關心多線程的問題，更新UI的操作，都是在主線程的消息隊列當中輪詢處理的。

（4）從源碼分析Handler

在上層，一般使用

sendMessage(Message msg)

來發送消息，所以研究Handler的源碼還是從sendMessage 開始。

【第一步】 跟蹤sendMessage 方法

/**
 * Pushes a message onto the end of the message queue after all pending messages
 * before the current time. It will be received 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.
 */
public final boolean sendMessage(Message msg)
{
    return sendMessageDelayed(msg, 0);
}

【第二步】 跟蹤sendMessageDelayed 方法

/**
 * 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)
{
    if (delayMillis < 0) {
        delayMillis = 0;
    }
    return sendMessageAtTime(msg, SystemClock.uptimeMillis() + delayMillis);
}

【第三步】 跟蹤sendMessageAtTime 方法

/**
 * Enqueue a message into the message queue after all pending messages
 * before the absolute time (in milliseconds) <var>uptimeMillis</var>.
 * <b>The time-base is {@link android.os.SystemClock#uptimeMillis}.</b>
 * Time spent in deep sleep will add an additional delay to execution.
 * You will receive it in {@link #handleMessage}, in the thread attached
 * to this handler.
 * 
 * @param uptimeMillis The absolute time at which the message should be
 *         delivered, using the
 *         {@link android.os.SystemClock#uptimeMillis} time-base.
 *         
 * @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 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);
}

源碼中的MessageQueue類就是消息隊列類，先看下如下代碼片段：

    MessageQueue queue = mQueue;

我們需要搞清楚一個問題，mQueue變量哪里來，消息隊列對象是怎么傳入Handler對象里的？

萬幸的是，Handler類中只有一個地方對mQueue變量賦值

/**
 * Use the {@link Looper} for the current thread with the specified callback interface
 * and set whether the handler should be asynchronous.
 *
 * Handlers are synchronous by default unless this constructor is used to make
 * one that is strictly asynchronous.
 *
 * Asynchronous messages represent interrupts or events that do not require global ordering
 * with respect to synchronous messages.  Asynchronous messages are not subject to
 * the synchronization barriers introduced by {@link MessageQueue#enqueueSyncBarrier(long)}.
 *
 * @param callback The callback interface in which to handle messages, or null.
 * @param async If true, the handler calls {@link Message#setAsynchronous(boolean)} for
 * each {@link Message} that is sent to it or {@link Runnable} that is posted to it.
 *
 * @hide
 */
public Handler(Callback callback, boolean async) {
    if (FIND_POTENTIAL_LEAKS) {
        final Class<? extends Handler> klass = getClass();
        if ((klass.isAnonymousClass() || klass.isMemberClass() || klass.isLocalClass()) &&
                (klass.getModifiers() & Modifier.STATIC) == 0) {
            Log.w(TAG, "The following Handler class should be static or leaks might occur: " +
                klass.getCanonicalName());
        }
    }

    mLooper = Looper.myLooper();
    if (mLooper == null) {
        throw new RuntimeException(
            "Can't create handler inside thread " + Thread.currentThread()
                    + " that has not called Looper.prepare()");
    }
    mQueue = mLooper.mQueue;
    mCallback = callback;
    mAsynchronous = async;
}

以上代碼是Handler的構造方法，其中我們只需要看mQueue變量相關的代碼即可

    mLooper = Looper.myLooper();
    mQueue = mLooper.mQueue;

所以消息隊列對象mQueue是從mLooper中拿到的，Looper是一個消息輪詢器。

【第四步】 跟蹤enqueueMessage 方法

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

【第五步】 跟蹤MessageQueue 類中的enqueueMessage 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;
}

代碼比較多，同樣，只需要看懂核心代碼即可

        msg.next = p; // invariant: p == prev.next
        prev.next = msg;

以上代碼就是核心代碼了，意思就是說，將消息插入消息隊列的隊尾，也就是入隊的操作。

源碼跟蹤到這里就可以總結出：handler.sendMessage(message)方法的作用是將Message插入到消息隊列（MessageQueue）中，換句話說，Handler的作用是將Message放入消息隊列（MessageQueue）中。

那么，Handler不僅僅可以發送消息，還可以接收消息：

private Handler handler = new Handler(){

    @Override
    public void handleMessage(Message msg) {
        super.handleMessage(msg);
    }
};

以上代碼大家都很熟悉了，Message將由handleMessage方法接收，那么Message是哪里來的呢？

在上面跟蹤發送消息流程的源碼中，我們已經知道了消息的插入其實采用了鏈表的插入方式，如下代碼：

        msg.next = p; // invariant: p == prev.next
        prev.next = msg;

那么，出隊（刪除隊列的某元素）的代碼怎么寫呢？

msg.next == null;

以上代碼就是將msg的next節點斷開，從而達到從MessageQueue中刪除next節點的目的。

在MessageQueue類中搜索相關的代碼之后終于找到了出隊方法。

【第六步】 尋找出隊方法

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;
    }
}

【第七步】 Message出隊之后做了什么事？

消息出隊之后又去干嘛了呢？next方法中沒有發現，但是next方法的返回值是Message，所以需要往上跟蹤。
上面已經提到，消息隊列對象是由Looper提供的，既然消息隊列對象是由Looper提供的，那么看看Looper類中有沒有調用next方法吧。

【第八步】 Looper中實現出隊

/**
 * 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();

    // Allow overriding a threshold with a system prop. e.g.
    // adb shell 'setprop log.looper.1000.main.slow 1 && stop && start'
    final int thresholdOverride =
            SystemProperties.getInt("log.looper."
                    + Process.myUid() + "."
                    + Thread.currentThread().getName()
                    + ".slow", 0);

    boolean slowDeliveryDetected = false;

    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;
        long slowDispatchThresholdMs = me.mSlowDispatchThresholdMs;
        long slowDeliveryThresholdMs = me.mSlowDeliveryThresholdMs;
        if (thresholdOverride > 0) {
            slowDispatchThresholdMs = thresholdOverride;
            slowDeliveryThresholdMs = thresholdOverride;
        }
        final boolean logSlowDelivery = (slowDeliveryThresholdMs > 0) && (msg.when > 0);
        final boolean logSlowDispatch = (slowDispatchThresholdMs > 0);

        final boolean needStartTime = logSlowDelivery || logSlowDispatch;
        final boolean needEndTime = logSlowDispatch;

        if (traceTag != 0 && Trace.isTagEnabled(traceTag)) {
            Trace.traceBegin(traceTag, msg.target.getTraceName(msg));
        }

        final long dispatchStart = needStartTime ? SystemClock.uptimeMillis() : 0;
        final long dispatchEnd;
        try {
            msg.target.dispatchMessage(msg);
            dispatchEnd = needEndTime ? SystemClock.uptimeMillis() : 0;
        } finally {
            if (traceTag != 0) {
                Trace.traceEnd(traceTag);
            }
        }
        if (logSlowDelivery) {
            if (slowDeliveryDetected) {
                if ((dispatchStart - msg.when) <= 10) {
                    Slog.w(TAG, "Drained");
                    slowDeliveryDetected = false;
                }
            } else {
                if (showSlowLog(slowDeliveryThresholdMs, msg.when, dispatchStart, "delivery",
                        msg)) {
                    // Once we write a slow delivery log, suppress until the queue drains.
                    slowDeliveryDetected = true;
                }
            }
        }
        if (logSlowDispatch) {
            showSlowLog(slowDispatchThresholdMs, dispatchStart, dispatchEnd, "dispatch", msg);
        }

        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();
    }
}

我們最終發現，消息出隊的操作其實由Looper類的loop()方法實現的。

那么，消息是怎么傳遞到Handler的handleMessage方法的呢？loop()方法中有一句核心代碼：

            msg.target.dispatchMessage(msg);

這個target就是Hander，最終調用了Handler中的dispatchMessage方法：

/**
 * Handle system messages here.
 */
public void dispatchMessage(Message msg) {
    if (msg.callback != null) {
        handleCallback(msg);
    } else {
        if (mCallback != null) {
            if (mCallback.handleMessage(msg)) {
                return;
            }
        }
        handleMessage(msg);
    }
}

這樣，Message出隊之后，將Message傳遞到了handleMessage方法，由Hander的handleMessage方法接收并處理Message。

最后，還需要說明一下一個for循環的細節，當消息出隊時，有一段代碼：

    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;
        long slowDispatchThresholdMs = me.mSlowDispatchThresholdMs;
        long slowDeliveryThresholdMs = me.mSlowDeliveryThresholdMs;
        if (thresholdOverride > 0) {
            slowDispatchThresholdMs = thresholdOverride;
            slowDeliveryThresholdMs = thresholdOverride;
        }
        final boolean logSlowDelivery = (slowDeliveryThresholdMs > 0) && (msg.when > 0);
        final boolean logSlowDispatch = (slowDispatchThresholdMs > 0);

        final boolean needStartTime = logSlowDelivery || logSlowDispatch;
        final boolean needEndTime = logSlowDispatch;

        if (traceTag != 0 && Trace.isTagEnabled(traceTag)) {
            Trace.traceBegin(traceTag, msg.target.getTraceName(msg));
        }

        final long dispatchStart = needStartTime ? SystemClock.uptimeMillis() : 0;
        final long dispatchEnd;
        try {
            msg.target.dispatchMessage(msg);
            dispatchEnd = needEndTime ? SystemClock.uptimeMillis() : 0;
        } finally {
            if (traceTag != 0) {
                Trace.traceEnd(traceTag);
            }
        }
        if (logSlowDelivery) {
            if (slowDeliveryDetected) {
                if ((dispatchStart - msg.when) <= 10) {
                    Slog.w(TAG, "Drained");
                    slowDeliveryDetected = false;
                }
            } else {
                if (showSlowLog(slowDeliveryThresholdMs, msg.when, dispatchStart, "delivery",
                        msg)) {
                    // Once we write a slow delivery log, suppress until the queue drains.
                    slowDeliveryDetected = true;
                }
            }
        }
        if (logSlowDispatch) {
            showSlowLog(slowDispatchThresholdMs, dispatchStart, dispatchEnd, "dispatch", msg);
        }

        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();
    }

for (;;) {}是無限循環，也就是說無限出隊，直至無消息可以出隊時線程掛起等待。

這種機制其實就是：生產者消費者設計模式。

[本章完...]