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一、JAVA层Handler、MessageQueue和Looper

1.三个类关系

  MessageQueue类是在Looper类中被实例化的，并被记录下来：

  frameworks/base/core/java/android/os/Looper.java

private Looper(boolean quitAllowed) {  mQueue = new MessageQueue(quitAllowed);  mRun = true;  mThread = Thread.currentThread();}

  Looper类又是以参数的形式传递给Handler类的：

frameworks/base/core/java/android/os/Handler.java

public Handler(Looper looper, Callback callback) {  mLooper = looper;  mQueue = looper.mQueue;  mCallback = callback;}

2.处理流程

  首先会调用Handler的sendMessage：

  frameworks/base/core/java/android/os/Handler.java

public final boolean sendMessage(Message msg){  return sendMessageDelayed(msg, 0);}public final boolean sendMessageDelayed(Message msg, long delayMillis){  if (delayMillis < 0) {    delayMillis = 0;  }  return sendMessageAtTime(msg, SystemClock.uptimeMillis() + delayMillis);}public boolean sendMessageAtTime(Message msg, long uptimeMillis){  boolean sent = false;  MessageQueue queue = mQueue;  if (queue != null) {    msg.target = this;    sent = queue.enqueueMessage(msg, uptimeMillis);  }  else {    RuntimeException e = new RuntimeException(                this + " sendMessageAtTime() called with no mQueue");            Log.w("Looper", e.getMessage(), e);  }  return sent;}

  接着我们看Looper类的死循环：

  frameworks/base/core/java/android/os/Looper.java

public static void loop() {  final Looper me = myLooper();  ......  final MessageQueue queue = me.mQueue;  ......  for (;;) {    Message msg = queue.next(); // might block    if (msg == null) {      return;    }    Printer logging = me.mLogging;    if (logging != null) {      logging.println(">>>>> Dispatching to " + msg.target + " " +                        msg.callback + ": " + msg.what);    }    msg.target.dispatchMessage(msg);    ......  }}

msg.target.dispatchMessage(msg)其实就是下面：

frameworks/base/core/java/android/os/Handler.java

public void dispatchMessage(Message msg) {  if (msg.callback != null) {    handleCallback(msg);  } else {    if (mCallback != null) {      if (mCallback.handleMessage(msg)) {        return;      }    }    handleMessage(msg);  }}

说明：一般继承Handler类都会重新实现handleMessage函数，完成消息处理。

二、C++层Looper应用

  C++层Looper是在实例化时创建睡眠/唤醒管道、传入回调函数；当通过唤醒管道唤醒进程时，首先执行回调函数处理的。与JAVA层机制不一样！

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