Migrating my bloc app to null safety. error: 'AssignmentBloc' doesn't conform to the bound 'BlocBase<AssignmentState>' of the type parameter 'B'

Question

我正在将我的 flutter 应用程序迁移到 null-safety，但我在 BlocProvider 和 BlocBuilder 上遇到了这个错误。

'AssignmentBloc' doesn't conform to the bound 'BlocBase<AssignmentState>' of the type parameter 'B'.
Try using a type that is or is a subclass of 'BlocBase<AssignmentState>'.

我已经检查了类似问题的解决方案，在我看来我已经按照他们的建议做了。不过我可能遗漏了一些东西，我需要帮助。

小部件。

class _AssignmentScreenWidgetState extends State<_AssignmentScreenWidget> {
  AssignmentBloc? _bloc;
  bool assignmentAdd = false;

  @override
  void initState() {
    super.initState();
    _bloc = BlocProvider.of<AssignmentBloc>(context)
      ..add(FetchEvent(widget.courseId, widget.assignmentId));
  }

  final GlobalKey<AssignmentDraftWidgetState> assignmentDraftWidgetState =
      GlobalKey<AssignmentDraftWidgetState>();

  @override
  Widget build(BuildContext context) {
    return BlocListener<AssignmentBloc, AssignmentState>( // where the errors are
      bloc: _bloc,
      listener: (BuildContext context, AssignmentState state) {
        if (state is CacheWarningAssignmentState) {
          showDialog(
              context: context, builder: (context) => WarningLessonDialog());
        }
      },
      child: BlocBuilder<AssignmentBloc, AssignmentState>( // where the errors are
          builder: (context, state) {
        return Scaffold(
        ...

集团。

class AssignmentBloc extends Bloc<AssignmentEvent, AssignmentState?> {
  final AssignmentRepository _assignmentRepository;
  final CacheManager cacheManager;

  AssignmentBloc(this._assignmentRepository, this.cacheManager) : super(null);

  @override
  AssignmentState get initialState => InitialAssignmentState();
  ...

州。

@immutable
abstract class AssignmentState {}

class InitialAssignmentState extends AssignmentState {}

class LoadedAssignmentState extends AssignmentState {
    final AssignmentResponse assignmentResponse;

    LoadedAssignmentState(this.assignmentResponse);
}

class ErrorAssignmentState extends AssignmentState {}
class CacheWarningAssignmentState extends AssignmentState {}

非常感谢我能得到的所有帮助。

Answer 1

您的 _bloc 变量实际上不可为空。它只是在构造函数中不可用。

所以让它成为 late 而不是可空的：

late AssignmentBloc _bloc;

这应该可以解决你以后的问题，因为现在你的 bloc 的类型参数不再是 AssignmentBloc? 而是一个真正的 AssignmentBloc.

Migrating my bloc app to null safety. error: 'AssignmentBloc' doesn't conform to the bound 'BlocBase<AssignmentState>' of the type parameter 'B'

Migrating my bloc app to null safety. error: 'AssignmentBloc' doesn't conform to the bound 'BlocBase<AssignmentState>' of the type parameter 'B'

state-management

dart

flutter

bloc

flutter-bloc