说明：  实现多人聊天室，其中默认状态下为广播式聊天，即一个发消息，其它人都能看到，且当用户退出聊天室时，也会通知其他人及服务器端
(To achieve more than chat rooms, which by default to broadcast interactive chat, that is, an impending arrival others can see, and when the user out of chat rooms, it will also inform the other person and server-side)