Message：消息，其中包含了消息ID，消息處理對象以及處理的數(shù)據(jù)等，由MessageQueue統(tǒng)一列隊(duì)，終由Handler處理。
Handler：處理者，負(fù)責(zé)Message的發(fā)送及處理。使用Handler時(shí)，需要實(shí)現(xiàn)handleMessage(Message msg)方法來對特定的Message進(jìn)行處理，例如更新UI等。
MessageQueue：消息隊(duì)列，用來存放Handler發(fā)送過來的消息，并按照FIFO規(guī)則執(zhí)行。當(dāng)然，存放Message并非實(shí)際意義的保存，而是將Message以鏈表的方式串聯(lián)起來的，等待Looper的抽取。
Looper：消息泵，不斷地從MessageQueue中抽取Message執(zhí)行。因此，一個(gè)MessageQueue需要一個(gè)Looper。
Thread：線程，負(fù)責(zé)調(diào)度整個(gè)消息循環(huán)，即消息循環(huán)的執(zhí)行場所。

 LooperThread Thread {
    Handler mHandler;

    run() {
     Looper.prepare();

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

     Looper.loop();
   }
 }

 public class Looper {
  //static變量，判斷是否打印調(diào)試信息。
   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().
 //線程本地存儲(chǔ)功能的封裝，TLS，thread local storage,什么意思呢？因?yàn)榇鎯?chǔ)要么在棧上，例如函數(shù)內(nèi)定義的內(nèi)部變量。要么在堆上，例如new或者malloc出來的東西
 //但是現(xiàn)在的系統(tǒng)比如Linux和windows都提供了線程本地存儲(chǔ)空間，也就是這個(gè)存儲(chǔ)空間是和線程相關(guān)的，一個(gè)線程內(nèi)有一個(gè)內(nèi)部存儲(chǔ)空間，這樣的話我把線程相關(guān)的東西就存儲(chǔ)到
 //這個(gè)線程的TLS中，就不用放在堆上而進(jìn)行同步操作了。
   private static final ThreadLocal sThreadLocal = new ThreadLocal();
 //消息隊(duì)列，MessageQueue，看名字就知道是個(gè)queue..
   final MessageQueue mQueue;
   volatile boolean mRun;
 //和本looper相關(guān)的那個(gè)線程，初始化為null
   Thread mThread;
   private Printer mLogging = null;
 //static變量，代表一個(gè)UI Process（也可能是service吧，這里默認(rèn)就是UI）的主線程
   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中設(shè)上這個(gè)Looper對象的，如果這個(gè)線程已經(jīng)設(shè)過了ｌｏｏｐｅｒ的話就會(huì)報(bào)錯(cuò)
 //這說明，一個(gè)線程只能設(shè)一個(gè)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設(shè)置的UI程序的主消息循環(huán)，注意，這個(gè)主消息循環(huán)是不會(huì)主動(dòng)退出的
 //  
   public static final void prepareMainLooper() {
     prepare();
     setMainLooper(myLooper());
 //判斷主消息循環(huán)是否能退出....
 //通過quit函數(shù)向looper發(fā)出退出申請
     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.
   */
 //消息循環(huán)，整個(gè)程序就在這里while了。
 //這個(gè)是static函數(shù)喔！
   public static final void loop() {
     Looper me = myLooper();//從該線程中取出對應(yīng)的looper對象
     MessageQueue queue = me.mQueue;//取消息隊(duì)列對象...
     while (true) {
       Message msg = queue.next(); // might block取消息隊(duì)列中的一個(gè)待處理消息..
       //if (!me.mRun) {//是否需要退出？mRun是個(gè)volatile變量，跨線程同步的，應(yīng)該是有地方設(shè)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.
  */
//返回和線程相關(guān)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.
  */
//設(shè)置調(diào)試輸出對象，looper循環(huán)的時(shí)候會(huì)打印相關(guān)信息，用來調(diào)試用最好了。
 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;
 }
//創(chuàng)建一個(gè)新的looper對象，
//內(nèi)部分配一個(gè)消息隊(duì)列，設(shè)置mRun為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;
 }
 //后面就簡單了，打印，異常定義等。
 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默認(rèn)構(gòu)造函數(shù)
 public Handler() {
 //這個(gè)if是干嘛用的暫時(shí)還不明白，涉及到j(luò)ava的深層次的內(nèi)容了應(yīng)該
     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());
       }
     }
 //獲取本線程的looper對象
 //如果本線程還沒有設(shè)置looper，這回拋異常
     mLooper = Looper.myLooper();
     if (mLooper == null) {
       throw new RuntimeException(
         "Can't create handler inside thread that has not called Looper.prepare()");
     }
 //無恥啊，直接把looper的queue和自己的queue搞成一個(gè)了
 //這樣的話，我通過handler的封裝機(jī)制加消息的話，就相當(dāng)于直接加到了looper的消息隊(duì)列中去了
     mQueue = mLooper.mQueue;
     mCallback = null;
   }
 //還有好幾種構(gòu)造函數(shù)，一個(gè)是帶callback的，一個(gè)是帶looper的
 //由外部設(shè)置looper
   public Handler(Looper looper) {
     mLooper = looper;
     mQueue = looper.mQueue;
     mCallback = null;
   }
 // 帶callback的，一個(gè)handler可以設(shè)置一個(gè)callback。如果有callback的話，
 //凡是發(fā)到通過這個(gè)handler發(fā)送的消息，都有callback處理，相當(dāng)于一個(gè)總的集中處理
 //待會(huì)看dispatchMessage的時(shí)候再分析
 public Handler(Looper looper, Callback callback) {
     mLooper = looper;
     mQueue = looper.mQueue;
     mCallback = callback;
   }
 //
 //通過handler發(fā)送消息
 //調(diào)用了內(nèi)部的一個(gè)sendMessageDelayed
 public final boolean sendMessage(Message msg)
   {
     return sendMessageDelayed(msg, 0);
   }
 //FT，又封裝了一層，這回是調(diào)用sendMessageAtTime了
 //因?yàn)檠訒r(shí)時(shí)間是基于當(dāng)前調(diào)用時(shí)間的，所以需要獲得絕對時(shí)間傳遞給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設(shè)置為自己，然后加入到消息隊(duì)列中
 //對于隊(duì)列這種數(shù)據(jù)結(jié)構(gòu)來說，操作比較簡單了
       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;
   }
 //還記得looper中的那個(gè)消息循環(huán)處理嗎
 //從消息隊(duì)列中得到一個(gè)消息后，會(huì)調(diào)用它的target的dispatchMesage函數(shù)
 //message的target已經(jīng)設(shè)置為handler了，所以
 //最后會(huì)轉(zhuǎn)到handler的msg處理上來
 //這里有個(gè)處理流程的問題
 public void dispatchMessage(Message msg) {
 //如果msg本身設(shè)置了callback，則直接交給這個(gè)callback處理了
     if (msg.callback != null) {
       handleCallback(msg);
     } else {
 //如果該handler的callback有的話，則交給這個(gè)callback處理了---相當(dāng)于集中處理
      if (mCallback != null) {
         if (mCallback.handleMessage(msg)) {
           return;
         }
      }
 //否則交給派生處理,基類默認(rèn)處理是什么都不干
       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();
                }
        }