android的Handler中sendEmptyMessage与sendMessage的区别？

看源码 根据参数的不同互相调用 public final boolean sendMessage(Message msg) { return sendMessageDelayed(msg, 0); } /** * 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); }