refresh method
Triggers notifyListeners after a zero-duration delay.
Implementation
@override
Future<void> refresh({bool notifyImmediately = true}) async {
_sharedPreferences ??= await SharedPreferences.getInstance();
final v = _sharedPreferences!.get(key);
final newValue = fromValue(v as B?);
_set(newValue, notifyImmediately: notifyImmediately);
}