Cannot use parent class of provided class

[Jannik-dev]

Description
Hi, I have raised this issue in the bloc package and was delegated to provider (felangel/bloc#4077). I am encountering the following issue, when I provide a certain class and try to use subclasses in a bloc builder an ProviderNotFoundException is thrown.

Consider the following example:

import 'package:flutter/material.dart';
import 'package:flutter_bloc/flutter_bloc.dart';

class AbstractCubit<T> extends Cubit<T> {
  AbstractCubit(super.initialState);
}

class IntCubit extends AbstractCubit<int> {
  IntCubit() : super(0);

  void doStuff() {
    emit(1);
  }
}

void main() {
  runApp(const MyApp());
}

class MyApp extends StatelessWidget {
  const MyApp({super.key});

  @override
  Widget build(BuildContext context) {
    return BlocProvider<IntCubit>(
      create: (context) => IntCubit(),
      child: const MaterialApp(
        title: 'Flutter Demo',
        home: MyHomePage(),
      ),
    );
  }
}

class MyHomePage extends StatefulWidget {
  const MyHomePage({super.key});

  @override
  State<MyHomePage> createState() => _MyHomePageState();
}

class _MyHomePageState extends State<MyHomePage> {
  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        backgroundColor: Theme.of(context).colorScheme.inversePrimary,
      ),
      body: const BlocWidget<int>(),
    );
  }
}

class BlocWidget<T> extends StatelessWidget {
  const BlocWidget({
    super.key,
  });

  @override
  Widget build(BuildContext context) {
    return BlocBuilder<AbstractCubit<T>, T>(
      builder: (BuildContext context, T state) {
        return const Text('test');
      },
    );
  }
}

Expected Behavior
The BlocBuilder<AbstractCubit, T> uses the provided IntCubit (since IntCubit is an AbstractCubit).

[rrousselGit]

Correct. This is unsupported.

If you wish for your object to be used with AbstractCubit instead of IntCubit, have your provider use AbstractCubit:

BlocProvider<AbstractCubit<int>>(

[Jannik-dev]

Thanks for the quick reply, but simply providing AbstractCubit instead of IntCubit does not work, since I need the IntCubit functionality in another part of the app (in this example the doStuff method). But I don't want to use IntCubit in BlocWidget, because it does not need it.

My only idea right now is to provide IntCubit as AbstractCubit and as IntCubit but I dislike this approach too, because there could be more classes in the hierarchy.

[rrousselGit]

What you're asking is not possible in any case. You'll always have to write extra logic to support both context.watch<AbstractCubit<int>> and context.watch<IntCubit>

[Jannik-dev]

Why cannot context.watch<AbstractCubit<int>> use the IntCubit? I mean, the IntCubit is an AbstractCubit<int>. Is there no simple way how I can use abstractions?

[rrousselGit]

That's not how InheritedWidgets work. They use exact type matching. There's no such thing as specifying a type, but getting a subclass instead. You have to manually expose your provider with said type. No way around it.

[Jannik-dev]

Thank you very much for your explanation