counter.dart

enum CounterEvent {
  increment,
  decrement,
}

class Counter extends Store<int, CounterEvent> {
  Counter() : super(initialState: 0);

  @override
  int reducer(int previousState, CounterEvent action) {
    switch (action) {
      case CounterEvent.increment:
        return previousState + 1;
      case CounterEvent.decrement:
        return previousState - 1;
    }
  }
}

store.dart

abstract class Store<T, A> implements Listenable {
  Store({T initialState}) : _state = ValueNotifier<T>(initialState);

  final ValueNotifier<T> _state;

  void dispatch(A action) {
    _state.value = reducer(_state.value, action);
  }

  T reducer(T previousState, A action);

  @override
  void addListener(VoidCallback listener) => _state.addListener(listener);

  @override
  void removeListener(VoidCallback listener) => _state.removeListener(listener);

  void dispose() {
    _state.dispose();
  }
}