computed

class Computed<T> extends ChangeNotifier implements ValueNotifier<T> {
  final List<Listenable> values;
  final T Function() resolver;

  Listenable listenable;
  T _value;

  Computed(this.values, {this.resolver}) {
   listenable = Listenable.merge(values)..addListener(_listener);
  }

  void _listener(){
    _value = resolver();
    notifyListeners();
  }

  @override
  T get value => _value;

  @override
  set value(T newValue) {
   _value = newValue;
   notifyListeners();
  }

  @override
  void dispose(){
    super.dispose();
    listenable.removeListener(_listener);
  }

}