【Android消息处理机制】Handler中sendEmptyMessage与sendMessage的区别和sendMessageAtTime()与sendEmptyMessageDelay()区别

1、sendEmptyMessage与sendMessage的区别：

sendMessage()允许你处理Message对象(Message里可以包含数据,)。

sendEmptyMessage(int what)只能放数据。其中参数what作用：就类似于ID，处理消息的时候用于区分你send一个0和1，处理的时候就要判断了if(msg.what == 0){}else if(msg.what == 1){}。

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.

2、sendMessageAtTime()与sendEmptyMessageDelay()的区别：

函数原型： public boolean sendMessageAtTime (Message msg, long uptimeMillis) Message  //不用说 是待发送消息uptimeMillis   //sendMessageAtTime,即在确定的时间发送这个消息，这个时间通过这个参数指定这个时间由uptimeMillis()传递  这两句是等效的，都是延时1秒将消息加入列队msgHandle.sendMessageAtTime(msg, SystemClock.uptimeMillis()+1000);msgHandle.sendMessageDelayed(msg, 1000) sendMessageAtTime的uptimeMillis是相对系统开机时间的绝对时间，SystemClock.uptimeMillis()是当前开机时间。