做一個手機助手查看器，碰到裏面的Handler用sendEmptyMessage(int what)發消息，其實也可以用sendMessage（Message msg）的，但兩者到底有啥區別？GOOGLE一下，沒有看到什麼好的答案，倒是看到一個大三的傢伙有模有樣的分析起來了安卓類的源代碼，SHIT，此刻的我真是汗顏，不過老子說得好嘛---故師者，無長無優，聞道有先後而已。哦，記錯了，是韓愈說的。收到了啓發，也開始看起類的源代碼起來。
其實兩者沒區別，請看下面Handler的源代碼：
/**
* 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);
}
就是調用了sendEmptyMessageDelayed（）而已，下面看下這個方法：
/**
* 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）的實現和上面一樣，請看：
/**
* 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);
}
原來在sendEmptyMessageDelayed中就是構建了一個Message,然後把這個Message的what設置成sendEmptyMessage方法中的What參數即可。
一切恍然大悟！
然後，在主線程中，Looper類的loop（）通過 調用： msg.target.dispatchMessage(msg)，調用Hanler類的dispatchMessage（Message msg)方法，從而在主線程中處理了這個Message.
如果對Handler機制要深入瞭解的，請參考上面那位大三“老師”的博客文章：http://www.cnblogs.com/codingmyworld/archive/2011/09/14/2174255.html。