What I did (to present loading state example) is:

@freezed
abstract class CounterState with _$CounterState {
  const factory CounterState.initial([@Default(0) int value]) = _Initial;
  const factory CounterState.loading(int value) = _Loading;
  const factory CounterState.current(int value) = _Current;
}

class CounterStateNotifier extends StateNotifier<CounterState>
    with LocatorMixin {
  CounterStateNotifier() : super(CounterState.initial());

  void increment() {
    state = CounterState.loading(state.value);
    Timer(
      Duration(seconds: 1),
      () => state = CounterState.current(state.value + 1),
    );
  }
}

StateNotifierBuilder(
              stateNotifier: context.watch<CounterStateNotifier>(),
              builder: (context, CounterState state, _) => state.when(
                initial: (value) => Text(
                  '$value',
                  style: Theme.of(context).textTheme.headline4,
                ),
                loading: (_) => CircularProgressIndicator(),
                current: (value) => Text(
                  '$value',
                  style: Theme.of(context).textTheme.headline4,
                ),
              ),
            )

But that is obviously 'hack'. Any tip? Remi Rousselet / Ryan Edge