Handler源碼分析

2019-11-06 09:34:15

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Handler主要用于線程切換，一個(gè)典型的應(yīng)用場(chǎng)景是：子線程通過(guò)Handler更新主線程UI 本文將從源碼上來(lái)介紹Handler的實(shí)現(xiàn)原理簡(jiǎn)書地址：http://www.jianshu.com/p/bb078eac0190

Handler的工作流程

先看一張圖：

Handler工作流程

主要流程如下： * 構(gòu)造Message對(duì)象 * 通過(guò)Handler將Message發(fā)送到MessageQueue * Looper從MessageQueue里取出Message對(duì)象 * Looper調(diào)用Message對(duì)象里保存的Handler對(duì)象的dispatchMessage方法將Message的處理移交給Handler

那么Looper和MessageQueue是哪來(lái)的呢？看一下Handler的構(gòu)造方法：

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();//獲取Handler所在線程的Looper。 if (mLooper == null) { throw new RuntimeException( "Can't create handler inside thread that has not called Looper.如果mLooper為空則直接拋異常了，所以如果不是在主線程創(chuàng)建Handler之前一定要在子線程里調(diào)用Looper.prepare()準(zhǔn)備好一個(gè)Looper。Looper.prepare()會(huì)調(diào)用Looper的構(gòu)造方法創(chuàng)建一個(gè)Looper，在Looper的構(gòu)造方法中又創(chuàng)建了一個(gè)MessageQueue。

下面通過(guò)源碼來(lái)看這幾個(gè)步驟是如何實(shí)現(xiàn)的

1.構(gòu)造Message對(duì)象

通過(guò)Handler對(duì)象構(gòu)造

通過(guò)Message的靜態(tài)方法構(gòu)造

或者，直接

Message message = new Message();

各種方式在使用效果上最后的差別不大，任選其一即可。

2.通過(guò)Handler將Message發(fā)送到MessageQueue

//Handlerprivate boolean enqueueMessage(MessageQueue queue, Message msg, long uptimeMillis) { msg.target = this;//注意這一句，將target對(duì)象指向當(dāng)前handler if (mAsynchronous) { msg.setAsynchronous(true); } return queue.enqueueMessage(msg, uptimeMillis);//將Message入消息隊(duì)列 }

左右的發(fā)送消息的方法最終都調(diào)用到了這個(gè)方法，顧名思義，這個(gè)方法將Message對(duì)象添加到MessageQueue隊(duì)列，在入隊(duì)之前，將message的target對(duì)象賦值為當(dāng)前handler對(duì)象，最后會(huì)通過(guò)這個(gè)target對(duì)象來(lái)處理這個(gè)message。

//MessageQueueboolean 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; }

可以看到，MessageQueue其實(shí)是一個(gè)單鏈表，所以很多操作都是單鏈表的操作。如果p==null（當(dāng)前隊(duì)列為空）或者when==0（通過(guò)sendMessageAtFrontOfQueue方法發(fā)送的消息）或者when

//Handlerpublic final boolean sendMessageDelayed(Message msg, long delayMillis) { if (delayMillis < 0) { delayMillis = 0; } return sendMessageAtTime(msg, SystemClock.uptimeMillis() + delayMillis); }

3.Looper從MessageQueue里取出Message對(duì)象

當(dāng)創(chuàng)建好Looper后，會(huì)調(diào)用Looper.loop()方法不斷的從MessageQueue里讀取Message，如果是主線程，那么Looper.loop()方法在系統(tǒng)創(chuàng)建進(jìn)程的時(shí)候就已經(jīng)調(diào)用過(guò)了，如果在子線程則需要自己調(diào)用。

//Looperpublic 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; ...省略 for (;;) { //從隊(duì)列里取出消息 Message msg = queue.next(); // might block if (msg == null) { // No message indicates that the message queue is quitting. return; } ...省略 try { //將msg的處理移交給Handler msg.target.dispatchMessage(msg); } finally { if (traceTag != 0) { Trace.traceEnd(traceTag); } } ...省略 } }

Message msg = queue.next(); // might block 通過(guò)一個(gè)無(wú)限的for循環(huán)中通過(guò)MessageQueue.next()讀消息，如果隊(duì)列沒(méi)有消息則阻塞。在next方法中，如果隊(duì)首的消息執(zhí)行時(shí)間還沒(méi)到，就設(shè)置一個(gè)等待時(shí)間，如果到了就從鏈表里取出來(lái)，然后返回。

//MessageQueueMessage 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. ... for(;;){ if (nextPollTimeoutMillis != 0) { Binder.flushPendingCommands(); } nativePollOnce(ptr, nextPollTimeoutMillis); synchronized (this) { ... 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; } ... } } }

4.調(diào)用Handler的dispatchMessage方法將Message的處理移交給Handler

在Loopermsg.target.dispatchMessage(msg)讓Handler去處理，這里的target就是在調(diào)用Handler.的enqueueMessage方法時(shí)賦值得，忘記了可以去步驟2里再看一下。

至此，處理流程又回到了Handler的dispatchMessage方法里，邏輯很簡(jiǎn)單，一個(gè)細(xì)節(jié)要注意，如果mCallback不為空，是不會(huì)調(diào)用handleMessage，這里mCallback是在創(chuàng)建Handler的時(shí)候就傳進(jìn)來(lái)的，所以使用Handler處理消息，要么在創(chuàng)建Handler的時(shí)候傳入一個(gè)Callback，要么重寫handleMessage方法。

//Handler /** * 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); } } private static void handleCallback(Message message) { message.callback.run(); } /** * Subclasses must implement this to receive messages. */ public void handleMessage(Message msg) { }

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