总结 Android的消息机制的总体流程

ActivityThread是Android的主线程，主线程的入口方法为ActivityThread.java类中的main方法，main代码如下：

public static final void main(String[] args) {
SamplingProfilerIntegration.start();

Process.setArgV0("<pre-initialized>");

Looper.prepareMainLooper();
if (sMainThreadHandler == null) {
sMainThreadHandler = new Handler();
}

ActivityThread thread = new ActivityThread();
thread.attach(false);

if (false) {
Looper.myLooper().setMessageLogging(new
LogPrinter(Log.DEBUG, "ActivityThread"));
}

Looper.loop();

if (Process.supportsProcesses()) {
throw new RuntimeException("Main thread loop unexpectedly exited");
}

thread.detach();
String name = (thread.mInitialApplication != null)
? thread.mInitialApplication.getPackageName()
: "<unknown>";
Slog.i(TAG, "Main thread of " + name + " is now exiting");
}

 

从源码中可以看到，通过 Looper.prepareMainLooper 来给主线程创建了一个 Looper 对象以及 MessageQueue 对象，然后通过 Looper.loop(); 来开启消息循环。其中，主线程的 Handler 是mH这个
变量，mH是 ActivityThread.H 的一个实例，它内部定义了一组消息，主要包含了四大组建的启动和停止。

private final class H extends Handler {
public static final int LAUNCH_ACTIVITY = 100;
public static final int PAUSE_ACTIVITY = 101;
public static final int PAUSE_ACTIVITY_FINISHING= 102;
public static final int STOP_ACTIVITY_SHOW = 103;
public static final int STOP_ACTIVITY_HIDE = 104;
public static final int SHOW_WINDOW = 105;
public static final int HIDE_WINDOW = 106;
public static final int RESUME_ACTIVITY = 107;
public static final int SEND_RESULT = 108;
public static final int DESTROY_ACTIVITY = 109;
public static final int BIND_APPLICATION = 110;
public static final int EXIT_APPLICATION = 111;
public static final int NEW_INTENT = 112;
public static final int RECEIVER = 113;
public static final int CREATE_SERVICE = 114;
public static final int SERVICE_ARGS = 115;
public static final int STOP_SERVICE = 116;
public static final int REQUEST_THUMBNAIL = 117;
public static final int CONFIGURATION_CHANGED = 118;
public static final int CLEAN_UP_CONTEXT = 119;
public static final int GC_WHEN_IDLE = 120;
public static final int BIND_SERVICE = 121;
public static final int UNBIND_SERVICE = 122;
public static final int DUMP_SERVICE = 123;
public static final int LOW_MEMORY = 124;
public static final int ACTIVITY_CONFIGURATION_CHANGED = 125;
public static final int RELAUNCH_ACTIVITY = 126;
public static final int PROFILER_CONTROL = 127;
public static final int CREATE_BACKUP_AGENT = 128;
public static final int DESTROY_BACKUP_AGENT = 129;
public static final int SUICIDE = 130;
public static final int REMOVE_PROVIDER = 131;
public static final int ENABLE_JIT = 132;
public static final int DISPATCH_PACKAGE_BROADCAST = 133;
public static final int SCHEDULE_CRASH = 134;
String codeToString(int code) {
if (DEBUG_MESSAGES) {
switch (code) {
case LAUNCH_ACTIVITY: return "LAUNCH_ACTIVITY";
case PAUSE_ACTIVITY: return "PAUSE_ACTIVITY";
case PAUSE_ACTIVITY_FINISHING: return "PAUSE_ACTIVITY_FINISHING";
case STOP_ACTIVITY_SHOW: return "STOP_ACTIVITY_SHOW";
case STOP_ACTIVITY_HIDE: return "STOP_ACTIVITY_HIDE";
case SHOW_WINDOW: return "SHOW_WINDOW";
case HIDE_WINDOW: return "HIDE_WINDOW";
case RESUME_ACTIVITY: return "RESUME_ACTIVITY";
case SEND_RESULT: return "SEND_RESULT";
case DESTROY_ACTIVITY: return "DESTROY_ACTIVITY";
case BIND_APPLICATION: return "BIND_APPLICATION";
case EXIT_APPLICATION: return "EXIT_APPLICATION";
case NEW_INTENT: return "NEW_INTENT";
case RECEIVER: return "RECEIVER";
case CREATE_SERVICE: return "CREATE_SERVICE";
case SERVICE_ARGS: return "SERVICE_ARGS";
case STOP_SERVICE: return "STOP_SERVICE";
case REQUEST_THUMBNAIL: return "REQUEST_THUMBNAIL";
case CONFIGURATION_CHANGED: return "CONFIGURATION_CHANGED";
case CLEAN_UP_CONTEXT: return "CLEAN_UP_CONTEXT";
case GC_WHEN_IDLE: return "GC_WHEN_IDLE";
case BIND_SERVICE: return "BIND_SERVICE";
case UNBIND_SERVICE: return "UNBIND_SERVICE";
case DUMP_SERVICE: return "DUMP_SERVICE";
case LOW_MEMORY: return "LOW_MEMORY";
case ACTIVITY_CONFIGURATION_CHANGED: return "ACTIVITY_CONFIGURATION_CHANGED";
case RELAUNCH_ACTIVITY: return "RELAUNCH_ACTIVITY";
case PROFILER_CONTROL: return "PROFILER_CONTROL";
case CREATE_BACKUP_AGENT: return "CREATE_BACKUP_AGENT";
case DESTROY_BACKUP_AGENT: return "DESTROY_BACKUP_AGENT";
case SUICIDE: return "SUICIDE";
case REMOVE_PROVIDER: return "REMOVE_PROVIDER";
case ENABLE_JIT: return "ENABLE_JIT";
case DISPATCH_PACKAGE_BROADCAST: return "DISPATCH_PACKAGE_BROADCAST";
case SCHEDULE_CRASH: return "SCHEDULE_CRASH";
}
}
return "(unknown)";
}

}

 

ActivityThread通过 ApplicationThread 和 AMS（ActivityManagerService） 进行进程间通信，AMS以进程间通信的方式完成ActivityThread的请求后会回调ApplicationThread中的Binder方法
，然后ApplicationThread会向H发送消息，H收到消息后会将ApplicationThread中的逻辑切换到ActivityThread中去执行，即切换到主线程去执行。

总结
Android的消息机制的总体流程就是： Handler 向 MessageQueue 发送一条消息（即插入一条消息）， MessageQueue 通过 next 方法把消息传给 Looper ， Looper 收到消息后开始处理，然后最终交给
Handler 自己去处理。换句话说就是： Handler 给自己发送了一条消息，然后自己的 handleMessage 方法处理消息，只是中间过程经过了 MessageQueue 和 Looper 。调用的方法过程如下：
Handler.sendMessage 方法 –> Handler.enqueueMessage –> MessageQueue.next –> Looper.loop –> handler.dispatchMessage –> Handler.handleMessage (或者Runnable的run方法或者
Callback.handleMessage )。