Receive messages through a channel event handler
Members can receive each other's messages through the
MessageReceived() method of a channel event handler. A
SBDBaseMessage object for each received message is one of the following three message types.
A text message sent by a user
A binary file message sent by a user
A text message sent by an admin through the Chat API.
UNIQUE_HANDLER_ID is a unique identifier to register multiple concurrent handlers.
If the UI isn't valid anymore, remove the channel event handler.