Android中的Looper类&Handler &Message

Android中的Looper类，是用来封装消息循环和消息队列的一个类，用于在android线程中进行消息处理。handler其实可以看做是一个工具类，用来向消息队列中插入消息的。 

(1) Looper类用来为一个线程开启一个消息循环。 
    默认情况下android中新诞生的线程是没有开启消息循环的。（主线程除外，主线程系统会自动为其创建Looper对象，开启消息循环。） 
    Looper对象通过MessageQueue来存放消息和事件。一个线程只能有一个Looper，对应一个MessageQueue。 

(2) 通常是通过Handler对象来与Looper进行交互的。Handler可看做是Looper的一个接口，用来向指定的Looper发送消息及定义处理方法。 
    默认情况下Handler会与其被定义时所在线程的Looper绑定，比如，Handler在主线程中定义，那么它是与主线程的Looper绑定。 
mainHandler = new Handler() 等价于new Handler（Looper.myLooper()）. 
Looper.myLooper()：获取当前进程的looper对象，类似的 Looper.getMainLooper() 用于获取主线程的Looper对象。 

(3) 在非主线程中直接new Handler() 会报如下的错误: 
E/AndroidRuntime( 6173): Uncaught handler: thread Thread-8 exiting due to uncaught exception 
E/AndroidRuntime( 6173): java.lang.RuntimeException: Can't create handler inside thread that has not called Looper.prepare() 
原因是非主线程中默认没有创建Looper对象，需要先调用Looper.prepare()启用Looper。 

(4) Looper.loop(); 让Looper开始工作，从消息队列里取消息，处理消息。 

    注意：写在Looper.loop()之后的代码不会被执行，这个函数内部应该是一个循环，当调用mHandler.getLooper().quit()后，loop才会中止，其后的代码才能得以运行。 

(5) 基于以上知识，可实现主线程给子线程（非主线程）发送消息。 

    把下面例子中的mHandler声明成类成员，在主线程通过mHandler发送消息即可。 
    
    Android官方文档中Looper的介绍： 
Class used to run a message loop for a thread. Threads by default do not have a message loop associated with them; to create one, call prepare() in the thread that is to run the loop, and then loop() to have it process messages until the loop is stopped. 

Most interaction with a message loop is through the Handler class. 

This is a typical example of the implementation of a Looper thread, using the separation of prepare() and loop() to create an initial Handler to communicate with the Looper.

01 class LooperThread extends Thread {
02       public Handler mHandler;
03
04       public void run() {
05           Looper.prepare();
06
07           mHandler = new Handler() {
08               public void handleMessage(Message msg) {
09                   // process incoming messages here
10               }
11           };
12
13           Looper.loop();
14       }
15 }

转自： http://vinny-w.iteye.com/blog/1334641

下文转自：http://blog.csdn.net/innost/article/details/6055793

 //Looper类分析
 //没找到合适的分析代码的办法，只能这么来了。每个重要行的上面都会加上注释
 //功能方面的代码会在代码前加上一段分析
 public class Looper {
    //static变量，判断是否打印调试信息。
     private static final boolean DEBUG = false;
     private static final boolean localLOGV = DEBUG ? Config.LOGD : Config.LOGV;

     // sThreadLocal.get() will return null unless you've called prepare().
 //线程本地存储功能的封装，TLS，thread local storage,什么意思呢？因为存储要么在栈上，例如函数内定义的内部变量。要么在堆上，例如new或者malloc出来的东西
 //但是现在的系统比如Linux和windows都提供了线程本地存储空间，也就是这个存储空间是和线程相关的，一个线程内有一个内部存储空间，这样的话我把线程相关的东西就存储到
 //这个线程的TLS中，就不用放在堆上而进行同步操作了。
     private static final ThreadLocal sThreadLocal = new ThreadLocal();
 //消息队列，MessageQueue，看名字就知道是个queue..
     final MessageQueue mQueue;
     volatile boolean mRun;
 //和本looper相关的那个线程，初始化为null
     Thread mThread;
     private Printer mLogging = null;
 //static变量，代表一个UI Process（也可能是service吧，这里默认就是UI）的主线程
     private static Looper mMainLooper = null;

      /** Initialize the current thread as a looper.
       * This gives you a chance to create handlers that then reference
       * this looper, before actually starting the loop. Be sure to call
       * {@link #loop()} after calling this method, and end it by calling
       * {@link #quit()}.
       */
 //往TLS中设上这个Looper对象的，如果这个线程已经设过了ｌｏｏｐｅｒ的话就会报错
 //这说明，一个线程只能设一个looper
     public static final void prepare() {
         if (sThreadLocal.get() != null) {
             throw new RuntimeException("Only one Looper may be created per thread");
         }
         sThreadLocal.set(new Looper());
     }

     /** Initialize the current thread as a looper, marking it as an application's main
      *  looper. The main looper for your application is created by the Android environment,
      *  so you should never need to call this function yourself.
      * {@link #prepare()}
      */
  //由framework设置的UI程序的主消息循环，注意，这个主消息循环是不会主动退出的
 //
     public static final void prepareMainLooper() {
         prepare();
         setMainLooper(myLooper());
 //判断主消息循环是否能退出....
 //通过quit函数向looper发出退出申请
         if (Process.supportsProcesses()) {
             myLooper().mQueue.mQuitAllowed = false;
         }
     }

     private synchronized static void setMainLooper(Looper looper) {
         mMainLooper = looper;
     }

     /** Returns the application's main looper, which lives in the main thread of the application.
      */
     public synchronized static final Looper getMainLooper() {
         return mMainLooper;
     }

     /**
      *  Run the message queue in this thread. Be sure to call
      * {@link #quit()} to end the loop.
      */
 //消息循环，整个程序就在这里while了。
 //这个是static函数喔！
     public static final void loop() {
         Looper me = myLooper();//从该线程中取出对应的looper对象
         MessageQueue queue = me.mQueue;//取消息队列对象...
         while (true) {
             Message msg = queue.next(); // might block取消息队列中的一个待处理消息..
             //if (!me.mRun) {//是否需要退出？mRun是个volatile变量，跨线程同步的，应该是有地方设置它。
             //    break;
             //}
             if (msg != null) {
                 if (msg.target == null) {
                     // No target is a magic identifier for the quit message.
                     return;
                 }
                 if (me.mLogging!= null) me.mLogging.println(
                         ">>>>> Dispatching to " + msg.target + " "
                         + msg.callback + ": " + msg.what
                         );
                 msg.target.dispatchMessage(msg);
                 if (me.mLogging!= null) me.mLogging.println(
                         "<<<<< Finished to    " + msg.target + " "
                         + msg.callback);
                 msg.recycle();
             }
         }
     }

     /**
      * Return the Looper object associated with the current thread.  Returns
      * null if the calling thread is not associated with a Looper.
      */
 //返回和线程相关的looper
     public static final Looper myLooper() {
         return (Looper)sThreadLocal.get();
     }

     /**
      * Control logging of messages as they are processed by this Looper.  If
      * enabled, a log message will be written to <var>printer</var>
      * at the beginning and ending of each message dispatch, identifying the
      * target Handler and message contents.
      *
      * @param printer A Printer object that will receive log messages, or
      * null to disable message logging.
      */
 //设置调试输出对象，looper循环的时候会打印相关信息，用来调试用最好了。
     public void setMessageLogging(Printer printer) {
         mLogging = printer;
     }

     /**
      * Return the {@link MessageQueue} object associated with the current
      * thread.  This must be called from a thread running a Looper, or a
      * NullPointerException will be thrown.
      */
     public static final MessageQueue myQueue() {
         return myLooper().mQueue;
     }
 //创建一个新的looper对象，
 //内部分配一个消息队列，设置mRun为true
     private Looper() {
         mQueue = new MessageQueue();
         mRun = true;
         mThread = Thread.currentThread();
     }

     public void quit() {
         Message msg = Message.obtain();
         // NOTE: By enqueueing directly into the message queue, the
         // message is left with a null target.  This is how we know it is
         // a quit message.
         mQueue.enqueueMessage(msg, 0);
     }

     /**
      * Return the Thread associated with this Looper.
      */
     public Thread getThread() {
         return mThread;
     }
     //后面就简单了，打印，异常定义等。
     public void dump(Printer pw, String prefix) {
         pw.println(prefix + this);
         pw.println(prefix + "mRun=" + mRun);
         pw.println(prefix + "mThread=" + mThread);
         pw.println(prefix + "mQueue=" + ((mQueue != null) ? mQueue : "(null"));
         if (mQueue != null) {
             synchronized (mQueue) {
                 Message msg = mQueue.mMessages;
                 int n = 0;
                 while (msg != null) {
                     pw.println(prefix + "  Message " + n + ": " + msg);
                     n++;
                     msg = msg.next;
                 }
                 pw.println(prefix + "(Total messages: " + n + ")");
             }
         }
     }

     public String toString() {
         return "Looper{"
             + Integer.toHexString(System.identityHashCode(this))
             + "}";
     }

     static class HandlerException extends Exception {

         HandlerException(Message message, Throwable cause) {
             super(createMessage(cause), cause);
         }

         static String createMessage(Throwable cause) {
             String causeMsg = cause.getMessage();
             if (causeMsg == null) {
                 causeMsg = cause.toString();
             }
             return causeMsg;
         }
     }
 }

那怎么往这个消息队列中发送消息呢？？调用looper的static函数myQueue可以获得消息队列，这样你就可用自己往里边插入消息了。不过这种方法比较麻烦，这个时候handler类就发挥作用了。先来看看handler的代码，就明白了。

 class Handler{
 ..........
 //handler默认构造函数
 public Handler() {
 //这个if是干嘛用的暂时还不明白，涉及到java的深层次的内容了应该
         if (FIND_POTENTIAL_LEAKS) {
             final Class<? extends Handler> klass = getClass();
             if ((klass.isAnonymousClass() || klass.isMemberClass() || klass.isLocalClass()) &&
                     (klass.getModifiers() & Modifier.STATIC) == 0) {
                 Log.w(TAG, "The following Handler class should be static or leaks might occur: " +
                     klass.getCanonicalName());
             }
         }
 //获取本线程的looper对象
 //如果本线程还没有设置looper，这回抛异常
         mLooper = Looper.myLooper();
         if (mLooper == null) {
             throw new RuntimeException(
                 "Can't create handler inside thread that has not called Looper.prepare()");
         }
 //无耻啊，直接把looper的queue和自己的queue搞成一个了
 //这样的话，我通过handler的封装机制加消息的话，就相当于直接加到了looper的消息队列中去了
         mQueue = mLooper.mQueue;
         mCallback = null;
     }
 //还有好几种构造函数，一个是带callback的，一个是带looper的
 //由外部设置looper
     public Handler(Looper looper) {
         mLooper = looper;
         mQueue = looper.mQueue;
         mCallback = null;
     }
 // 带callback的，一个handler可以设置一个callback。如果有callback的话，
 //凡是发到通过这个handler发送的消息，都有callback处理，相当于一个总的集中处理
 //待会看dispatchMessage的时候再分析
 public Handler(Looper looper, Callback callback) {
         mLooper = looper;
         mQueue = looper.mQueue;
         mCallback = callback;
     }
 //
 //通过handler发送消息
 //调用了内部的一个sendMessageDelayed
 public final boolean sendMessage(Message msg)
     {
         return sendMessageDelayed(msg, 0);
     }
 //FT，又封装了一层，这回是调用sendMessageAtTime了
 //因为延时时间是基于当前调用时间的，所以需要获得绝对时间传递给sendMessageAtTime
 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) {
 //把消息的target设置为自己，然后加入到消息队列中
 //对于队列这种数据结构来说，操作比较简单了
             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;
     }
 //还记得looper中的那个消息循环处理吗
 //从消息队列中得到一个消息后，会调用它的target的dispatchMesage函数
 //message的target已经设置为handler了，所以
 //最后会转到handler的msg处理上来
 //这里有个处理流程的问题
 public void dispatchMessage(Message msg) {
 //如果msg本身设置了callback，则直接交给这个callback处理了
         if (msg.callback != null) {
             handleCallback(msg);
         } else {
 //如果该handler的callback有的话，则交给这个callback处理了---相当于集中处理
           if (mCallback != null) {
                 if (mCallback.handleMessage(msg)) {
                     return;
                 }
            }
 //否则交给派生处理,基类默认处理是什么都不干
             handleMessage(msg);
         }
     }
 ..........
 }

 讲了这么多，该怎么创建和使用一个带消息循环的线程呢？

 //假设在onCreate中创建一个线程
 //不花时间考虑代码的完整和严谨性了，以讲述原理为主。
 ....

 ... onCreate(...){

 //难点是如何把android中的looper和java的thread弄到一起去。
 //而且还要把随时取得这个looper用来创建handler
 //最简单的办法就是从Thread派生一个
 class ThreadWithMessageHandle extends Thread{
   //重载run函数
   Looper myLooper = null;
   run(){
   Looper.prepare();//将Looper设置到这个线程中
   myLooper = Looper.myLooper();
   Looper.loop();开启消息循环
 }

  ThreadWithMessageHandle  threadWithMgs = new ThreadWithMessageHandle();
  threadWithMsg.start();
  Looper looper = threadWithMsg.myLooper;//
 //这里有个问题.threadWithMgs中的myLooper可能此时为空
 //需要同步处理一下
 //或者像API文档中的那样，把handler定义到ThreadWithMessageHandle到去。
 //外线程获得这个handler的时候仍然要注意同步的问题，因为handler的创建是在run中的
  Handler threadHandler = new Handler(looper);
  threadHandler.sendMessage(...)
 }


 }



 ...

结束。