addListener method
Implementation
@mustCallSuper
void addListener(String eventName, Function callback, {bool once = false}) {
assert(callback != null);
assert(eventName != null);
assert(eventName.isNotEmpty);
if (!_events.containsKey(eventName)) {
_events[eventName] = <_KuzzleListener>[];
}
if (!_exist(eventName, callback)) {
_events[eventName]!.add(_KuzzleListener(callback, once: once));
}
}