一定搞懂Handler消息處理機(jī)制系列之「04.Message是如何觸發(fā)的」

2019-11-06 09:37:04

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Message是如何觸發(fā)的

還是ActivityThread這段代碼。來(lái)自Android中為什么主線程不會(huì)因?yàn)長(zhǎng)ooper.loop()里的死循環(huán)阻塞？

我們知道APP的入口是在ActivityThread,一個(gè)java類,有著main方法,而且main方法中的代碼也不是很多.

public static void main(String[] args) { Trace.traceBegin(Trace.TRACE_TAG_ACTIVITY_MANAGER, "ActivityThreadMain"); SamplingPRofilerIntegration.start(); // CloseGuard defaults to true and can be quite spammy. We // disable it here, but selectively enable it later (via // StrictMode) on debug builds, but using DropBox, not logs. CloseGuard.setEnabled(false); Environment.initForCurrentUser(); // Set the reporter for event logging in libcore EventLogger.setReporter(new EventLoggingReporter()); AndroidKeyStoreProvider.install(); // Make sure TrustedCertificateStore looks in the right place for CA certificates final File configDir = Environment.getUserConfigDirectory(UserHandle.myUserId()); TrustedCertificateStore.setDefaultUserDirectory(configDir); Process.setArgV0("<pre-initialized>"); Looper.prepareMainLooper(); ActivityThread thread = new ActivityThread(); thread.attach(false); if (sMainThreadHandler == null) { sMainThreadHandler = thread.getHandler(); } if (false) { Looper.myLooper().setMessageLogging(new LogPrinter(Log.DEBUG, "ActivityThread")); } // End of event ActivityThreadMain. Trace.traceEnd(Trace.TRACE_TAG_ACTIVITY_MANAGER); Looper.loop(); throw new RuntimeException("Main thread loop unexpectedly exited"); }

在上篇文章MessageQueue與Looper的由來(lái)我們得知這段代碼中的Looper.prepareMainLooper()在主線程中創(chuàng)建了一個(gè)Looper對(duì)象，然后在這段代碼的末尾處，調(diào)用了Looper.loop()方法，我們來(lái)看看Looper.loop()源碼：

//刪除部分代碼public static void loop() { //獲取當(dāng)前線程的Looper對(duì)象 final Looper me = myLooper(); if (me == null) { throw new RuntimeException("No Looper; Looper.prepare() wasn't called on this thread."); } //獲取Looper對(duì)象中的消息隊(duì)列 final MessageQueue queue = me.mQueue; ... for (;;) { //不斷的從消息隊(duì)列拿出消息隊(duì)列的第一條消息，直到?jīng)]有消息為止 Message msg = queue.next(); // might block if (msg == null) { // No message indicates that the message queue is quitting. return; } ... try { msg.target.dispatchMessage(msg); } finally { if (traceTag != 0) { Trace.traceEnd(traceTag); } } ... msg.recycleUnchecked(); } }

從這個(gè)loop()方法中邏輯就比較明了了，里面有一個(gè)死循環(huán)，從消息隊(duì)列中不斷的取出消息，然后調(diào)用這個(gè)方法msg.target.dispatchMessage(msg)，msg.tagre為當(dāng)初你創(chuàng)建的Handler對(duì)象，因?yàn)樵贖andler把消息放入消息隊(duì)列的時(shí)候執(zhí)行了以下代碼:

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

可以看到這里把創(chuàng)建的Handler賦值給了msg.target。然后msg.target.dispatchMessage(msg)就相當(dāng)于handler.dispatchMessage(msg),所以我們來(lái)看看Handler的dispatchMessage()方法：

public void dispatchMessage(Message msg) { if (msg.callback != null) { handleCallback(msg); } else { //mCallback為當(dāng)初創(chuàng)建Handler時(shí)傳入的Callback對(duì)象 if (mCallback != null) { if (mCallback.handleMessage(msg)) { return; } } handleMessage(msg); } } private static void handleCallback(Message message) { message.callback.run(); }

如果看過(guò)第一篇Handler消息發(fā)送我們就可以了解到Handler是如何發(fā)送消息的，所以看到這段代碼就應(yīng)該知道這些消息或者是回調(diào)是如何發(fā)送的了。

根據(jù)自己的理解，繪制了以下流程圖，如有錯(cuò)誤，請(qǐng)及時(shí)提醒我改正：