承接上一篇Handler系列一，上篇主要總結(jié)了Handler如何通信，這篇來介紹Handler怎么通信。

• Handler的通信機(jī)制
• Handler，Looper，MessageQueue如何關(guān)聯(lián)

Handler 通信機(jī)制

• 創(chuàng)建Handler，并采用當(dāng)前線程的Looper創(chuàng)建消息循環(huán)系統(tǒng)；
• Handler通過sendMessage(Message)或Post(Runnable)發(fā)送消息，調(diào)用enqueueMessage把消息插入到消息鏈表中；
• Looper循環(huán)檢測消息隊(duì)列中的消息，若有消息則取出該消息，并調(diào)用該消息持有的handler的dispatchMessage方法，回調(diào)到創(chuàng)建Handler線程中重寫的handleMessage里執(zhí)行。

Handler如何關(guān)聯(lián)Looper、MessageQueue

Handler及其關(guān)聯(lián)的類圖

以上類圖可以快速幫助我們理清Handler與Looper、MessageQueue的關(guān)系，以下從源碼的角度慢慢分析：

1、Handler 發(fā)送消息

上一段很熟悉的代碼：

Message msg =Message.obtain(); //從全局池中返回一個(gè)message實(shí)例，避免多次創(chuàng)建message（如new Message）msg.obj = data;msg.what=1; //標(biāo)志消息的標(biāo)志handler.sendMessage(msg);

從sendMessageQueue開始追蹤，函數(shù)調(diào)用關(guān)系：sendMessage -> sendMessageDelayed ->sendMessageAtTime，在sendMessageAtTime中，攜帶者傳來的message與Handler的mQueue一起通過enqueueMessage進(jìn)入隊(duì)列了。

對于postRunnable而言，通過post投遞該runnable，調(diào)用getPostMessage，通過該runnable構(gòu)造一個(gè)message，再通過 sendMessageDelayed投遞，接下來和sendMessage的流程一樣了。

2、消息入隊(duì)列

在enqueueMessage中，通過MessageQueue入隊(duì)列，并為該message的target賦值為當(dāng)前的handler對象，記住msg.target很重要，之后Looper取出該消息時(shí)，還需要由msg.target.dispatchMessage回調(diào)到該handler中處理消息。

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

在MessageQueue中,由Message的消息鏈表進(jìn)行入隊(duì)列

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

3、Looper 處理消息

再說處理消息之前，先看Looper是如何構(gòu)建與獲取的：

• 構(gòu)造Looper時(shí)，構(gòu)建消息循環(huán)隊(duì)列，并獲取當(dāng)前線程

private Looper(boolean quitAllowed) {mQueue = new MessageQueue(quitAllowed);mThread = Thread.currentThread();}

• 但該函數(shù)是私有的，外界不能直接構(gòu)造一個(gè)Looper，而是通過Looper.prepare來構(gòu)造的：

public static void prepare() {prepare(true);}private static void prepare(boolean quitAllowed) {if (sThreadLocal.get() != null) {throw new RuntimeException("Only one Looper may be created per thread");}sThreadLocal.set(new Looper(quitAllowed));}

• 這里創(chuàng)建Looper，并把Looper對象保存在sThreadLocal中，那sThreadLocal是什么呢？

  static final ThreadLocal sThreadLocal = new ThreadLocal();

  它是一個(gè)保存Looper的TheadLocal實(shí)例，而ThreadLocal是線程私有的數(shù)據(jù)存儲(chǔ)類，可以來保存線程的Looper對象，這樣Handler就可以通過ThreadLocal來保存于獲取Looper對象了。
• TheadLocal 如何保存與獲取Looper？

public void set(T value) {Thread currentThread = Thread.currentThread();Values values = values(currentThread);if (values == null) {values = initializeValues(currentThread);}values.put(this, value);}public T get() {// Optimized for the fast path.Thread currentThread = Thread.currentThread();Values values = values(currentThread);if (values != null) {Object[] table = values.table;int index = hash & values.mask;if (this.reference == table[index]) {return (T) table[index + 1];}} else {values = initializeValues(currentThread);}return (T) values.getAfterMiss(this);} 在 set 中都是通過 `values.put` 保存當(dāng)前線程的 Looper 實(shí)例，通過 `values.getAfterMiss(this)`獲取，其中`put`和`getAfterMiss`都有`key`和`value`，都是由Value對象的table數(shù)組保存的，那么在table數(shù)組里怎么存的呢？table[index] = key.reference;table[index + 1] = value;很顯然在數(shù)組中，前一個(gè)保存著ThreadLocal對象引用的索引，后一個(gè)存儲(chǔ)傳入的Looper實(shí)例。

接下來看Looper在loop中如何處理消息

在loop中，一個(gè)循環(huán)，通過next取出MessageQueue中的消息

• 若取出的消息為null，則結(jié)束循環(huán)，返回。

  • 設(shè)置消息為空，可以通過MessageQueue的quit和quitSafely方法通知消息隊(duì)列退出。
• 若取出的消息不為空，則通過msg.target.dispatchMessage回調(diào)到handler中去。

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 blockif (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 loggerPrinter logging = me.mLogging;if (logging != null) {logging.println(">>>>> Dispatching to " + msg.target + " " +msg.callback + ": " + msg.what);}msg.target.dispatchMessage(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();}}

4、handler處理消息

Looper把消息回調(diào)到handler的dispatchMessage中進(jìn)行消息處理：

• 若該消息有callback，即通過Post(Runnable)的方式投遞消息，因?yàn)樵谕哆frunnable時(shí)，把runnable對象賦值給了message的callback。
• 若handler的mCallback不為空，則交由通過callback創(chuàng)建handler方式去處理。
• 否則，由最常見創(chuàng)建handler對象的方式，在重寫handlerMessage中處理。

public void dispatchMessage(Message msg) {if (msg.callback != null) {handleCallback(msg);} else {if (mCallback != null) {if (mCallback.handleMessage(msg)) {return;}}handleMessage(msg);}}

總結(jié)

以一個(gè)時(shí)序圖來總結(jié)handler的消息機(jī)制，包含上述如何關(guān)聯(lián)Looper和MessageQueue的過程。

Handler-Looper-MessageQueue時(shí)序圖

總結(jié)

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