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();
      }
  }

// sThreadLocal.get() will return null unless you've called prepare().
static final ThreadLocal<Looper> sThreadLocal = new ThreadLocal<Looper>();
private static Looper sMainLooper;  // guarded by Looper.class

final MessageQueue mQueue;
final Thread mThread;

private Printer mLogging;

/** 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()}.
  */
public static void prepare() {
    prepare(true);
}

private static void prepare(boolean quitAllowed) {
    if (sThreadLocal.get() != null) {
        throw new RuntimeException("Only one Looper may be created per thread");
    }
    sThreadLocal.set(new Looper(quitAllowed));
}

private Looper(boolean quitAllowed) {
    mQueue = new MessageQueue(quitAllowed);
    mThread = Thread.currentThread();
}

/**
 * Run the message queue in this thread. Be sure to call
 * {@link #quit()} to end the loop.
 */
public static void loop() {
    final Looper me = myLooper();
    if (me == null) {
        throw new RuntimeException("No Looper; Looper.prepare() wasn't called on this thread.");
    }
    final MessageQueue queue = me.mQueue;

    // Make sure the identity of this thread is that of the local process,
    // and keep track of what that identity token actually is.
    Binder.clearCallingIdentity();
    final long ident = Binder.clearCallingIdentity();

    for (;;) {
        Message msg = queue.next(); // might block
        if (msg == null) {
            // No message indicates that the message queue is quitting.
            return;
        }

        // This must be in a local variable, in case a UI event sets the logger
        Printer logging = me.mLogging;
        if (logging != null) {
            logging.println(">>>>> Dispatching to " + msg.target + " " +
                    msg.callback + ": " + msg.what);
        }

        msg.target.dispatchMessage(msg);//查看Message的源码可以看到，它持有一个handler引用，
        //Handler对象的dispatchMessage方法可以将消息最终传递到我们经常看到的handlerMessage的回调方法中。

        if (logging != null) {
            logging.println("<<<<< Finished to " + msg.target + " " + msg.callback);
        }

        // Make sure that during the course of dispatching the
        // identity of the thread wasn't corrupted.
        final long newIdent = Binder.clearCallingIdentity();
        if (ident != newIdent) {
            Log.wtf(TAG, "Thread identity changed from 0x"
                    + Long.toHexString(ident) + " to 0x"
                    + Long.toHexString(newIdent) + " while dispatching to "
                    + msg.target.getClass().getName() + " "
                    + msg.callback + " what=" + msg.what);
        }

        msg.recycleUnchecked();
    }
}

/**
 * Return the Looper object associated with the current thread.  Returns
 * null if the calling thread is not associated with a Looper.
 */
public static @Nullable Looper myLooper() {
    return sThreadLocal.get();
}

final MessageQueue mQueue;
final Looper mLooper;
final Callback mCallback;
final boolean mAsynchronous;
IMessenger mMessenger;

public void dispatchMessage(Message msg) {
    if (msg.callback != null) { //1，Message对象中有一个callback对象，它是一个Runnable类型
        handleCallback(msg);
    } else {
        if (mCallback != null) { //2，Handler中可以有一个处理消息的回调接口
            if (mCallback.handleMessage(msg)) { //
                return;
            }
        }
        handleMessage(msg);  //3，如果1，2中都为空，则将消息丢给我们经常使用的handlerMessage()方法
    }
}

private static void handleCallback(Message message) {
        message.callback.run();
}

//Handler处理消息的回调接口
public interface Callback {   
        public boolean handleMessage(Message msg);
}

//这个局部变量可以通过Handler构造函数传递进来
final Callback mCallback;

 public final boolean post(Runnable r){
   return  sendMessageDelayed(getPostMessage(r), 0);
 }


public final boolean postDelayed(Runnable r, long delayMillis){
    return sendMessageDelayed(getPostMessage(r), delayMillis);
}


public final boolean postAtTime(Runnable r, long uptimeMillis){
    return sendMessageAtTime(getPostMessage(r), uptimeMillis);
}


public final boolean sendMessage(Message msg){
    return sendMessageDelayed(msg, 0);
}


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) {
    MessageQueue queue = mQueue;
    if (queue == null) {
        RuntimeException e = new RuntimeException(
                this + " sendMessageAtTime() called with no mQueue");
        Log.w("Looper", e.getMessage(), e);
        return false;
    }
    return enqueueMessage(queue, msg, uptimeMillis);
}

private static Message getPostMessage(Runnable r) {
    Message m = Message.obtain();
    m.callback = r;
    return m;
}

public int what; // 用于区别不同Message
public int arg1; // arg1和arg2用于传递简单的整型变量
public int arg2; // arg1和arg2用于传递简单的整型变量
public Object obj; // 用于传递复杂数据
/*package*/ Bundle data; // 可以用于传递复杂数据，需要借助getter和setter方法
/*package*/ Handler target; // 指向发送/处理当前Message的Handler
/*package*/ Runnable callback; // 当使用post方式发送消息时，该参数将被赋值

this.handler = new Handler() {
    @Override
    public void handleMessage(Message msg) {
        super.handleMessage(msg);
        switch (msg.what) {
            case LOAD_SUCCESS:
                //处理成功的情况
                break;
            case LOAD_FAIL:
                //处理失败的情况
                break;
        }

    }
};

  public Handler() {
      this(null, false);
  }

  /**此处还有若干构造方法，此处略去**/

  public Handler(Callback callback, boolean async) {
      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());
          }
      }

      mLooper = Looper.myLooper();   // 在此处获取Looper对象
      if (mLooper == null) {
          throw new RuntimeException(
              "Can't create handler inside thread that has not called Looper.prepare()");
      }
      mQueue = mLooper.mQueue;
      mCallback = callback;
      mAsynchronous = async;
  }

/**
     * Return the Looper object associated with the current thread.  Returns
     * null if the calling thread is not associated with a Looper.
     */
    public static @Nullable Looper myLooper() {
        return sThreadLocal.get();
    }

/**
     * 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.  See also: {@link #prepare()}
     */
    public static void prepareMainLooper() {
        prepare(false);//此处调用了prepare方法
        synchronized (Looper.class) {
            if (sMainLooper != null) {
                throw new IllegalStateException("The main Looper has already been prepared.");
            }
            sMainLooper = myLooper();
        }
    }

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

public static void main(String[] args) {
        //此处省去其他无关代码

        Looper.prepareMainLooper(); //prepare()方法在这里被调用

        ActivityThread thread = new ActivityThread();
        thread.attach(false);

        if (sMainThreadHandler == null) {
            sMainThreadHandler = thread.getHandler();
        }

        if (false) {
            Looper.myLooper().setMessageLogging(new
                    LogPrinter(Log.DEBUG, "ActivityThread"));
        }

        // End of event ActivityThreadMain.
        Trace.traceEnd(Trace.TRACE_TAG_ACTIVITY_MANAGER);
        Looper.loop();  //loop()方法在这里被调用，终于找到你了

        throw new RuntimeException("Main thread loop unexpectedly exited");
    }

 if (Looper.myLooper() != Looper.ge tMainLooper()) {
        // If we finish marking off of the main thread, we need to
        // actually do it on the main thread to ensure correct ordering.
        Handler mainThread = new Handler(Looper.getMainLooper());
        mainThread.post(new Runnable() {
            @Override
            public void run() {
                mEventLog.add(tag, threadId);
                mEventLog.finish(this.toString());
            }
        });
        return;
  }

    /**
     * Creates the worker pool. Processing will not begin until {@link #start()} is called.
     *
     * @param cache A Cache to use for persisting responses to disk
     * @param network A Network interface for performing HTTP requests
     * @param threadPoolSize Number of network dispatcher threads to create
     */
    public RequestQueue(Cache cache, Network network, int threadPoolSize) {
        this(cache, network, threadPoolSize,
                new ExecutorDelivery(new Handler(Looper.getMainLooper())));
    }