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：这个系列是本人写的第一篇比较完成的博客，肯定会有非常多的不足，希望大家能够多留言批评，希望能明确指出文章中可能有的错误，我会及时更正，谢谢。
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