onArgs method
Listens to events broadcast under eventName, delivering the full
argument list exactly as the server sent it.
Use this for multi-parameter hub methods:
Clients.Group(r).SendAsync("UserTyping", userId, isTyping) yields
[userId, isTyping]. An event sent with no arguments yields [].
Implementation
@override
Stream<List<dynamic>> onArgs(String eventName) {
return _eventControllers.putIfAbsent(eventName, () => StreamController<List<dynamic>>.broadcast()).stream;
}