Dart generic type as class-member function's type

Question

I have a class which wokring with generics: Screen bloc

class ScreenWithUpdater<DataType> extends StatefulWidget {
  final Updater<DataType> updater;
  final Widget Function(BuildContext, DataType) bodyBuilder;
  final Duration updatePeriod;

  const ScreenWithUpdater({
    Key key,
    @required this.updater,
    @required this.bodyBuilder,
    @required this.updatePeriod,
  }) : super(key: key);
  @override
  _ScreenWithUpdaterState createState() => _ScreenWithUpdaterState();
}

class _ScreenWithUpdaterState<DataType>
    extends State<ScreenWithUpdater<DataType>> {
  UpdateBloc<DataType> _bloc;

  @override
  void initState() {
    super.initState();
  }

  @override
  void didChangeDependencies() {
    super.didChangeDependencies();
    _bloc = UpdateBloc<DataType>(
      updatePeriod: widget.updatePeriod,
      updater: widget.updater,
    );
  }

  @override
  void dispose() {
    _bloc.close();
    super.dispose();
  }

  @override
  Widget build(BuildContext context) {
    return BlocBuilder<UpdateBloc, UpdateState>(
      cubit: _bloc,
      builder: (
        context,
        state,
      ) {
        if (state is ShowLoader) {
          return LoaderWidget();
        } else if (state is UpdateMainState<DataType>) {
          return widget.bodyBuilder(context, state.data);
        } else {
          return Center(
            child: Text('Something went wrong'),
          );
        }
      },
    );
  }
}

And when i'm calling it in the way like this:

ScreenWithUpdater<List<OrderFromApi>>(
            updatePeriod: updatePeriod,
            updater: OrdersPageUpdater(),
            bodyBuilder: (context, orders) => OrdersPage(microOrders: orders),
          ),

It fails with an exception

type '(BuildContext, List) => OrdersPage' is not a subtype of type '(BuildContext, dynamic) => Widget'

---

But when i'm calling in the next way

ScreenWithUpdater(
            updatePeriod: updatePeriod,
            updater: OrdersPageUpdater(),
            bodyBuilder: (context, orders) => OrdersPage(microOrders: orders),
          ),

It works fine

Why? And what should i change to run my first version?

Solution

An issue was in next line:

 @override
  _ScreenWithUpdaterState createState() => _ScreenWithUpdaterState();
}

Then i've changed this line to

 @override
  _ScreenWithUpdaterState createState() => _ScreenWithUpdaterState<DataType>();
}

And everything is working now!