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();                }        }