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，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，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MessageQueue需要一個Looper。Thread：線程，負(fù)責(zé)調(diào)度整個消息循環(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(). //線程本地存儲功能的封裝，TLS，thread local storage,什么意思呢？因?yàn)榇鎯σ丛跅Ｉ?，例如函?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空間，也就是這個存儲空間是和線程相關(guān)的，一個線程內(nèi)有一個內(nèi)部存儲空間，這樣的話我把線程相關(guān)的東西就存儲到 //這個線程的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(); //消息隊列，MessageQueue，看名字就知道是個queue..   final MessageQueue mQueue;   volatile boolean mRun; //和本looper相關(guān)的那個線程，初始化為null   Thread mThread;   private Printer mLogging = null; //static變量，代表一個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Looper對象的，如果這個線程已經(jīng)設(shè)過了ｌｏｏｐｅｒ的話就會報錯 //這說明，一個線程只能設(shè)一個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，這個主消息循環(huán)是不會主動退出的 //     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)，整個程序就在這里while了。 //這個是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;//取消息隊列對象...     while (true) {       Message msg = queue.next(); // might block取消息隊列中的一個待處理消息..       //if (!me.mRun) {//是否需要退出？mRun是個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)的時候會打印相關(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)建一個新的looper對象，//內(nèi)部分配一個消息隊列，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if是干嘛用的暫時還不明白，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的消息隊列中去了     mQueue = mLooper.mQueue;     mCallback = null;   } //還有好幾種構(gòu)造函數(shù)，一個是帶callback的，一個是帶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handler可以設(shè)置一個callback。如果有callback的話， //凡是發(fā)到通過這個handler發(fā)送的消息，都有callback處理，相當(dāng)于一個總的集中處理 //待會看dispatchMessage的時候再分析 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時間是基于當(dāng)前調(diào)用時間的，所以需要獲得絕對時間傳遞給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，然后加入到消息隊列中 //對于隊列這種數(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中的那個消息循環(huán)處理嗎 //從消息隊列中得到一個消息后，會調(diào)用它的target的dispatchMesage函數(shù) //message的target已經(jīng)設(shè)置為handler了，所以 //最后會轉(zhuǎn)到handler的msg處理上來 //這里有個處理流程的問題 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();                }        }