on method
set a callback for events of this type.
Use the special event
name of 'all' to fire this callback on all events.
Implementation
EventObserver on(String event, void Function(Event<T> event) callback) {
if (!callbacks.containsKey(event)) callbacks[event] = [];
callbacks[event]!.add(callback);
return EventObserver(() => callbacks[event]?.remove(callback));
}