做一个手机助手查看器，碰到里面的Handler用sendEmptyMessage(int what)发消息，其实也可以用sendMessage（Message msg）的，但两者到底有啥区别？GOOGLE一下，没有看到什么好的答案，倒是看到一个大三的家伙有模有样的分析起来了安卓类的源代码，SHIT，此刻的我真是汗颜，不过老子说得好嘛---故师者，无长无优，闻道有先后而已。哦，记错了，是韩愈说的。收到了启发，也开始看起类的源代码起来。
其实两者没区别，请看下面Handler的源代码：
/**
* Sends a Message containing only the what value.
*
* @return Returns true if the message was successfully placed in to the
* message queue. Returns false on failure, usually because the
* looper processing the message queue is exiting.
*/
public final boolean sendEmptyMessage(int what)
{
return sendEmptyMessageDelayed(what, 0);
}
就是调用了sendEmptyMessageDelayed（）而已，下面看下这个方法：
/**
* Sends a Message containing only the what value, to be delivered
* after the specified amount of time elapses.
* @see #sendMessageDelayed(android.os.Message, long)
*
* @return Returns true if the message was successfully placed in to the
* message queue. Returns false on failure, usually because the
* looper processing the message queue is exiting.
*/
public final boolean sendEmptyMessageDelayed(int what, long delayMillis) {
Message msg = Message.obtain();
msg.what = what;
return sendMessageDelayed(msg, delayMillis);
}
而sendMessage（Message msg）的实现和上面一样，请看：
/**
* Pushes a message onto the end of the message queue after all pending messages
* before the current time. It will be received in {@link #handleMessage},
* in the thread attached to this handler.
*
* @return Returns true if the message was successfully placed in to the
* message queue. Returns false on failure, usually because the
* looper processing the message queue is exiting.
*/
public final boolean sendMessage(Message msg)
{
return sendMessageDelayed(msg, 0);
}
原来在sendEmptyMessageDelayed中就是构建了一个Message,然后把这个Message的what设置成sendEmptyMessage方法中的What参数即可。
一切恍然大悟！
然后，在主线程中，Looper类的loop（）通过 调用： msg.target.dispatchMessage(msg)，调用Hanler类的dispatchMessage（Message msg)方法，从而在主线程中处理了这个Message.
如果对Handler机制要深入了解的，请参考上面那位大三“老师”的博客文章：http://www.cnblogs.com/codingmyworld/archive/2011/09/14/2174255.html。