sendEmptymessage 和sendMessage的区别

无意中一次碰到Handler用sendEmptyMessage(int what)发消息，后面有一次面试也被问到，那么和sendMessage（）两者有啥区别？追根溯源来看看handler类的源代码。

其实两者没区别，请看下面Handler的源代码，先看sendEmptyMessage：

/**
* 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。
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.
看到这里明白了，其实这两个方法是一样一样的，一个传Message类型的msg，一个传int类型的what，传what的，最终会转为msg。