消息處理機(jī)制主要對(duì)象:Looper,Handler,Message(還有MessageQueue和Runnable)

Looper不斷從MessageQueue消息隊(duì)列中取出一個(gè)Message，然后傳給Handle，如此循環(huán)往復(fù)，如果隊(duì)列為空，那么它會(huì)進(jìn)入休眠。

這些類的主要變量

Looper.java

static final ThreadLocal<Looper> sThreadLocal = new ThreadLocal<Looper>(); private static Looper sMainLooper; // guarded by Looper.class final MessageQueue mQueue; final Thread mThread;

Handler.java

final MessageQueue mQueue; final Looper mLooper; final Callback mCallback; final boolean mAsynchronous; IMessenger mMessenger;

Message.java

Handler target;每個(gè)消息只能對(duì)應(yīng)一個(gè)handlerRunnable callback;回調(diào)接口

MessageQueue.java

Message mMessages;

Runnable是一個(gè)空接口類，沒(méi)有變量

上一個(gè)書上的圖：

Handler和Thread沒(méi)有直接關(guān)系，但對(duì)應(yīng)關(guān)系可以推理得到

每個(gè)Thread只對(duì)應(yīng)一個(gè)Looper；

每個(gè)Looper只對(duì)應(yīng)一個(gè)MessageQueue;

每個(gè)MessageQueue對(duì)應(yīng)N個(gè)Message，每個(gè)Message只對(duì)應(yīng)一個(gè)Handler

==》每個(gè)Thread對(duì)應(yīng)N個(gè)Handler。

Handler是”真正處理事情“的地方，作用：處理消息，將Message壓入MessageQueue中

帶著一個(gè)問(wèn)題看源碼：創(chuàng)建handler對(duì)象的線程（ui/主線程除外）為什么，必須先調(diào)用Looper.prepare() ？

public Handler() { this(null, false); }public Handler(Callback callback) { this(callback, false); }public Handler(Looper looper) { this(looper, null, false); } public Handler(Looper looper, Callback callback) { this(looper, callback, false); } public Handler(boolean async) { this(null, async); }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 that has not called Looper.prepare()'); } mQueue = mLooper.mQueue; mCallback = callback; mAsynchronous = async; } public Handler(Looper looper, Callback callback, boolean async) { mLooper = looper; mQueue = looper.mQueue; mCallback = callback; mAsynchronous = async; }

初始化handler對(duì)象時(shí)(構(gòu)造方法是Handler(),Handler(Callback callback))都間接調(diào)用Handler(Callback callback, boolean async)構(gòu)造方法

主要代碼是Looper.myLooper();

static final ThreadLocal<Looper> sThreadLocal = new ThreadLocal<Looper>();//這是在Looper類中的定義public static Looper myLooper() { return sThreadLocal.get();//從當(dāng)前線程中獲得looper對(duì)象 }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));//為當(dāng)前線程設(shè)置looper對(duì)象}

我們自己創(chuàng)建線程必須通過(guò)Looper.prepare()方法為當(dāng)前線程設(shè)置looper對(duì)象才可以通過(guò)Looper.myLooper()方法返回looper對(duì)象，這樣在非UI線程創(chuàng)建handler對(duì)象時(shí)才不會(huì)報(bào)錯(cuò)。'Can’t create handler inside thread that has not called Looper.prepare()'

ps:prepare(boolean quitAllowed)(這個(gè)不用我們關(guān)心，略過(guò)。。)

這個(gè)quitAlowed參數(shù)是定義消息隊(duì)列用了，看的源代碼是android4.4

Looper.javaprivate Looper(boolean quitAllowed) { mQueue = new MessageQueue(quitAllowed); mRun = true; mThread = Thread.currentThread(); }MessageQueue.java// True if the message queue can be quit. private final boolean mQuitAllowed;//true消息隊(duì)列可以被quit，false消息隊(duì)列不能被quit。

主線程/UI線程的MessageQueue不能被銷毀掉。看源碼（銷毀調(diào)用Looper.quit()）

public static void prepareMainLooper() { prepare(false); synchronized (Looper.class) { if (sMainLooper != null) {throw new IllegalStateException('The main Looper has already been prepared.'); } sMainLooper = myLooper(); } }

偏離太遠(yuǎn)了

所以得出結(jié)論：創(chuàng)建handler對(duì)象的線程（ui/主線程除外），必須先調(diào)用Looper.prepare()

Handler作用1：處理消息

在Looper類中處理消息是通過(guò)msg.target.dispatchMessage(msg);target就是handler對(duì)象（Message類的內(nèi)部變量Handler target）將消息轉(zhuǎn)發(fā)到處理消息的對(duì)應(yīng)的handler對(duì)象上,然后這個(gè)target即handler對(duì)象會(huì)在處理消息前做一個(gè)檢查

public void dispatchMessage(Message msg) { if (msg.callback != null) {//如果msg有綁定callback回調(diào)接口Runaable不為空，則執(zhí)行Runnable的run方法 handleCallback(msg); } else { if (mCallback != null) {//如果handler的內(nèi)置接口類Callback不為空，則執(zhí)行boolean handleMessage(Message msg)這個(gè)方法if (mCallback.handleMessage(msg)) {執(zhí)行完成則return return;} } handleMessage(msg);//最后才執(zhí)行handler本身的方法 } } private static void handleCallback(Message message) { message.callback.run(); }public interface Callback {//handler的內(nèi)置接口類Callbackpublic boolean handleMessage(Message msg); }

Handler作用2：將Message壓入MessageQueue中

handler中提供的很多發(fā)送message的方法，除了sendMessageAtFrontOfQueue()方法（直接調(diào)用enqueueMessage(queue, msg, 0);）之外，其它的發(fā)送消息方法最終都會(huì)輾轉(zhuǎn)調(diào)用到sendMessageAtTime()方法

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); }

sendMessageAtTime()方法也是調(diào)用Handler中的enqueueMessage(queue, msg, uptimeMillis)方法

和sendMessageAtFrontOfQueue()方法兩者最后都會(huì)調(diào)用enqueueMessage(queue, msg, uptimeMillis)方法

區(qū)別是需要延遲uptimeMillis時(shí)間后才將Message壓入MessageQueue中

private boolean enqueueMessage(MessageQueue queue, Message msg, long uptimeMillis) { msg.target = this;//給msg的target賦值為handler自身然后加入MessageQueue中 if (mAsynchronous) { msg.setAsynchronous(true); } return queue.enqueueMessage(msg, uptimeMillis); }

最終所有的方法都是調(diào)用MessageQueue中的enqueueMessage(msg, uptimeMillis);方法，是不是感覺(jué)兩個(gè)方法差不多啊，注意參數(shù)！！

MessageQueue的使用是在Looper中

Handler的作用整理完畢（好像我現(xiàn)在已經(jīng)可以把Handler源碼完整默寫下來(lái)了。哈哈^.^記憶力真不行）

Looper類

作用：與當(dāng)前線程綁定，保證一個(gè)線程只會(huì)有一個(gè)Looper實(shí)例，同時(shí)一個(gè)Looper實(shí)例也只有一個(gè)MessageQueue。

對(duì)于Looper主要是prepare()和loop()兩個(gè)方法

prepare()將普通線程轉(zhuǎn)化為looper線程，

loop()方法，不斷從MessageQueue中去取消息，交給消息的target屬性的dispatchMessage去處理。

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 Printer 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.recycle(); } }

27行就是上面提到了，handler進(jìn)行消息處理的關(guān)鍵代碼了

看著上面的分析很復(fù)雜，總結(jié)下

1、首先Looper.prepare()為在當(dāng)前線程中保存一個(gè)Looper實(shí)例（sThreadLocal.set()），然后該實(shí)例中保存一個(gè)MessageQueue對(duì)象；因?yàn)長(zhǎng)ooper.prepare()在一個(gè)線程中只能調(diào)用一次，所以MessageQueue在一個(gè)線程中只會(huì)存在一個(gè)。

2、Looper.loop()會(huì)讓當(dāng)前線程進(jìn)入一個(gè)無(wú)限循環(huán)，不端從MessageQueue的實(shí)例中讀取消息，然后回調(diào)msg.target.dispatchMessage(msg)方法。

3、Handler的構(gòu)造方法，會(huì)首先得到當(dāng)前線程中保存的Looper實(shí)例，進(jìn)而與Looper實(shí)例中的MessageQueue想關(guān)聯(lián)。

4、Handler的sendMessage方法，會(huì)給msg的target賦值為handler自身，然后加入MessageQueue中。

5、在構(gòu)造Handler實(shí)例時(shí)，我們會(huì)重寫handleMessage方法，也就是msg.target.dispatchMessage(msg)最終調(diào)用的方法。

好了，總結(jié)完成，大家可能還會(huì)問(wèn)，那么在Activity中，我們并沒(méi)有顯示的調(diào)用Looper.prepare()和Looper.loop()方法，為啥Handler可以成功創(chuàng)建呢，這是因?yàn)樵贏ctivity的啟動(dòng)代碼中，已經(jīng)在當(dāng)前UI線程調(diào)用了Looper.prepare()和Looper.loop()方法。

以上就是本文的全部?jī)?nèi)容，希望對(duì)大家的學(xué)習(xí)有所幫助，也希望大家多多支持好吧啦網(wǎng)。