Android消息处理机制Looper和Handler详解

Message：消息，其中包含了消息ID，消息处理对象以及处理的数据等，由MessageQueue统一列队，终由Handler处理。
Handler：处理者，负责Message的发送及处理。使用Handler时，需要实现handleMessage(Message msg)方法来对特定的Message进行处理，例如更新UI等。
MessageQueue：消息队列，用来存放Handler发送过来的消息，并按照FIFO规则执行。当然，存放Message并非实际意义的保存，而是将Message以链表的方式串联起来的，等待Looper的抽取。
Looper：消息泵，不断地从MessageQueue中抽取Message执行。因此，一个MessageQueue需要一个Looper。
Thread：线程，负责调度整个消息循环，即消息循环的执行场所。

 LooperThread Thread {
    Handler mHandler;

    run() {
     Looper.prepare();

     mHandler = Handler() {
        handleMessage(Message msg) {
         
       }
     };

     Looper.loop();
   }
 }

 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;
   }
 }
}

 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);
     }
   }
 ..........
 }

    Message msg = mHandler.obtainMessage();
    msg.what = what;
    msg.sendToTarget();

    MessageQueue queue = mQueue;
    if (queue != null) {
      msg.target = this;
      sent = queue.enqueueMessage(msg, uptimeMillis);
    }

    Looper me = myLooper();
    MessageQueue queue = me.mQueue;
    while (true) {
      Message msg = queue.next(); // might block
      if (msg != null) {
        if (msg.target == null) {
          // No target is a magic identifier for the quit message.
          return;
        }
        msg.target.dispatchMessage(msg);
        msg.recycle();
      }
    }

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

        class LooperThread extends Thread {
                public Handler mHandler;
                public void run() {
                        Looper.prepare();
                        mHandler = new Handler() {
                                public void handleMessage(Message msg) {
                                       // process incoming messages here
                                }
                        };
                        Looper.loop();
                }
        }