off method
Implementation
void off(String eventName, Function? fn) {
if (fn == null) {
events[eventName] = [];
} else if (events.containsKey(eventName)) {
events[eventName] =
events[eventName]!.where((callback) => callback != fn).toList();
if (events[eventName]!.isEmpty) {
events.remove(eventName);
}
}
}