一定搞懂Handler消息处理机制系列之「04.Message是如何触发的」

2019-11-06 09:37:04

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Message是如何触发的

还是ActivityThread这段代码。来自Android中为什么主线程不会因为Looper.loop()里的死循环阻塞？

我们知道APP的入口是在ActivityThread,一个java类,有着main方法,而且main方法中的代码也不是很多.

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与Looper的由来我们得知这段代码中的Looper.prepareMainLooper()在主线程中创建了一个Looper对象，然后在这段代码的末尾处，调用了Looper.loop()方法，我们来看看Looper.loop()源码：

//删除部分代码public static void loop() { //获取当前线程的Looper对象 final Looper me = myLooper(); if (me == null) { throw new RuntimeException("No Looper; Looper.prepare() wasn't called on this thread."); } //获取Looper对象中的消息队列 final MessageQueue queue = me.mQueue; ... for (;;) { //不断的从消息队列拿出消息队列的第一条消息，直到没有消息为止 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(); } }

从这个loop()方法中逻辑就比较明了了，里面有一个死循环，从消息队列中不断的取出消息，然后调用这个方法msg.target.dispatchMessage(msg)，msg.tagre为当初你创建的Handler对象，因为在Handler把消息放入消息队列的时候执行了以下代码:

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

可以看到这里把创建的Handler赋值给了msg.target。然后msg.target.dispatchMessage(msg)就相当于handler.dispatchMessage(msg),所以我们来看看Handler的dispatchMessage()方法：

public void dispatchMessage(Message msg) { if (msg.callback != null) { handleCallback(msg); } else { //mCallback为当初创建Handler时传入的Callback对象 if (mCallback != null) { if (mCallback.handleMessage(msg)) { return; } } handleMessage(msg); } } private static void handleCallback(Message message) { message.callback.run(); }

如果看过第一篇Handler消息发送我们就可以了解到Handler是如何发送消息的，所以看到这段代码就应该知道这些消息或者是回调是如何发送的了。

根据自己的理解，绘制了以下流程图，如有错误，请及时提醒我改正：