Handler handler =new Handler(){
    @Override
    public void handleMessage(Message msg) {
      super.handleMessage(msg);
      //這里進行一些UI操作等處理
    }

     new Thread(new Runnable() {
      @Override
      public void run() {
        Message message = Message.obtain();
        ........
        handler.sendMessage(message);
      }
    });
  };

public Handler() {
  this(null, false);
}
//他會調(diào)用本類中的如下構(gòu)造函數(shù)
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();
    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;
  }

public static void prepare() {
    prepare(true);
}
//將looper保存到ThreadLocal中，這里可以把ThreadLocal理解為一個以當(dāng)前線程為鍵的Map，所以一個線程中只會有一個looper
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));
}
//我們看到在new Looper(quitAllowed)中，創(chuàng)建了一個消息隊列MessageQueen
private Looper(boolean quitAllowed) {
  mQueue = new MessageQueue(quitAllowed);
  mThread = Thread.currentThread();
}

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 boolean enqueueMessage(MessageQueue queue, Message msg, long uptimeMillis) {
    msg.target = this;
    if (mAsynchronous) {
      msg.setAsynchronous(true);
    }
    return queue.enqueueMessage(msg, uptimeMillis);
  }

//MessageQueen的enqueueMessage方法
  boolean enqueueMessage(Message msg, long when) {
    if (msg.target == null) {
      throw new IllegalArgumentException("Message must have a target.");
    }
    if (msg.isInUse()) {
      throw new IllegalStateException(msg + " This message is already in use.");
    }

    synchronized (this) {
      if (mQuitting) {
        IllegalStateException e = new IllegalStateException(
            msg.target + " sending message to a Handler on a dead thread");
        Log.w(TAG, e.getMessage(), e);
        msg.recycle();
        return false;
      }

      msg.markInUse();
      msg.when = when;
      Message p = mMessages;
      boolean needWake;
      if (p == null || when == 0 || when < p.when) {
        // New head, wake up the event queue if blocked.
        msg.next = p;
        mMessages = msg;
        needWake = mBlocked;
      } else {
        // Inserted within the middle of the queue. Usually we don't have to wake
        // up the event queue unless there is a barrier at the head of the queue
        // and the message is the earliest asynchronous message in the queue.
        needWake = mBlocked && p.target == null && msg.isAsynchronous();
        Message prev;
        for (;;) {
          prev = p;
          p = p.next;
          if (p == null || when < p.when) {
            break;
          }
          if (needWake && p.isAsynchronous()) {
            needWake = false;
          }
        }
        msg.next = p; // invariant: p == prev.next
        prev.next = msg;
      }

      // We can assume mPtr != 0 because mQuitting is false.
      if (needWake) {
        nativeWake(mPtr);
      }
    }
    return true;
  }

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 (;;) {
      //不斷從消息隊列中取出msg
      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);
      }
      //將msg交由handler處理
      msg.target.dispatchMessage(msg);

      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();
    }
  }

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

//當(dāng)我們通過這種方式創(chuàng)建handler時，dispatchMessage中的mCallback就不為null
 public Handler(Callback callback) {
    this(callback, false);
 }
//Callback是一個接口，里面正好也有我們需要的handleMessage(Message msg)，dispatchMessage中的 if (mCallback != null) 語句內(nèi)的內(nèi)容，就是我們需要重寫的handleMessage(Message msg)方法
 public interface Callback {
   public boolean handleMessage(Message msg);
 }

//當(dāng)我們調(diào)用handler.post()方法執(zhí)行異步任務(wù)時
  public final boolean post(Runnable r)
  {
    return sendMessageDelayed(getPostMessage(r), 0);
  }
//getPostMessage(r)這個方法中我們看到給m.callback賦值了，就是我們傳入的runnable接口
  private static Message getPostMessage(Runnable r) {
    Message m = Message.obtain();
    m.callback = r;
    return m;
  }
//最后在handleCallback方法中我們執(zhí)行了它的run方法,這也就解釋了為什么在子線程中可以用handler.post(Runnable r)更新UI
  private static void handleCallback(Message message) {
    message.callback.run();
  }