[Dart] Multi-value notifier & consumer

1_generic_notifier.dart

import 'package:flutter/widgets.dart';
import 'package:provider/provider.dart';

class MultiValueNotifier<E extends Enum, T> extends ChangeNotifier {
  // Map of name to value.
  final values = <E, T>{};

  add(E name, T value) {
    values[name] = value;
    notifyListeners();
  }

  remove(E name) {
    if (values.remove(name) != null) {
      notifyListeners();
    }
  }

  get(E name) => values[name];
}

class ValueConsumer<E extends Enum, T>
    extends Selector<MultiValueNotifier<E, T>, T> {
  ValueConsumer({
    Key? key,
    required E name,
    required ValueWidgetBuilder<T> builder,
    ShouldRebuild<T>? shouldRebuild,
    Widget? child,
  }) : super(
          key: key,
          shouldRebuild: shouldRebuild,
          builder: builder,
          selector: (_, notifier) => notifier.get(name),
          child: child,
        );
}

2_provider.dart

// Just an example, define enum for your use case.
enum Values {
  one,
  two;
}

// This is the class we want to have multiple instances and provide them in Providers.
class Foo {}

class MyWidget extends StatelessWidget {
  final notifier = MultiValueNotifier<Values, Foo>();
  
  @override
  Widget build(BuildContext context) {
    return MultiProvider(
      providers[
        ChangeNotifierProvider<MultiValueNotifier<Values, Foo>>.value(value: notifier),
      ];
      child: // ...
    );
  }
  
  valueOneReady(Foo foo) {
    notifier.add(Values.one, foo);
  }
  
  valueTwoReady(Foo foo) {
    notifier.add(Values.two, foo);
  }
}

3_consumer.dart

class WidgetOne extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return ValueConsumer(
      name: Values.one,
      builder: (context, value, child) {
        // value is 'foo' from valueOneReady
        // ...
      }
    );
  }
}

class WidgetTwo extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return ValueConsumer(
      name: Values.two,
      builder: (context, value, child) {
        // value is 'foo' from valueTwoReady
        // ...
      }
    );
  }
}