catalog_state.dart

@immutable
class CatalogState {
  CatalogState({
    @required this.repository,
    @required this.items,
  });
  //
  final CatalogRepository repository;
  final List<Item> items;

  //After getting items from the repository,
  //A new CatalogState is return with the
  Future<CatalogState> getItems() async {
    final items = await repository.fetchItems();
    return copyWith(
      items: items,
    );
  }

  static Stream<CatalogState> deleteItem(
    CatalogState currentState,
    Item item,
  ) async* {
    try {
      //yield the new state.
      //states_rebuilder will refresh the UI to display the new state
      yield currentState.copyWith(
        items: currentState.items.where((e) => e.id != item.id).toList(),
      );
      //remove the item from the backend
      await currentState.repository.removeItem(item);
    } catch (e) {
      //on error, yield the old state
      //states_rebuilder will refresh the UI to display the old state
      yield currentState;
      //rethrow the error; states_rebuilder will catch the error
      //display a SnackBar.
      rethrow;
    }
  }

  CatalogState copyWith({
    List<Item> items,
    CatalogRepository repository,
  }) {
    return CatalogState(
      items: items ?? this.items,
      repository: repository ?? this.repository,
    );
  }
}

//This is the initial state.
class CatalogInitialState extends CatalogState {
  CatalogInitialState(CatalogRepository repository)
      : super(
          items: [],
          repository: repository,
        );
}