one<P> method
Puts on to listen eventName
event only once.
When the event is emitted, the callback
is called
and after removes eventName
event.
Implementation
void one<P>(Enum eventName, CallbackEvent<T, P> callback) {
void _oneCallback(inst, param) {
callback(inst, param);
off(eventName, callback);
off(eventName, _oneCallback);
}
on<P>(eventName, _oneCallback);
}