Way to remove a callback using Watcher.attach + Cubit

[PackRuble]

This is generally the same story as in issue #9 with ChangeNotifier. However, a slightly more general syntax covering both cases at once appeared in my head:

mixin Detachability {
  List<VoidCallback>? _onDisposeCallbacks;

  void onDetach(void Function() cb) {
    _onDisposeCallbacks ??= [];
    _onDisposeCallbacks!.add(cb);
  }

  @protected
  void detach() {
    _onDisposeCallbacks?.forEach((cb) => cb.call());
    _onDisposeCallbacks = null;
  }
}

Which will make it possible to do things like this:

import 'package:bloc/bloc.dart';

class CubitImpl extends Cubit<int> with Detachability {
  CubitImpl() : super(0);
  
  void setValue(int value) => emit(value);

  @override
  Future<void> close() async {
    super.detach();
    return super.close();
  }
}

Future<void> main() async {
  // initialize...
  late CardotekaImpl cardoteka;
  late Card<int> counterCard;

  final cubit = CubitImpl();
  cardoteka.attach(
    counterCard,
    cubit.setValue,
    detacher: cubit.onDetach,
  );
}

We can't seem to do anything technically with those "parent" classes that we need to extends from. This also applies to Cubit and ValueNotifier and the like. And I don't like the fact that it's very easy to forget to override the close method when adding the “Detachability” mixin.

[PackRuble]

Yes, the important thing here is that I wouldn't want to force users to use a conditional CubitDetachability that is a standalone implementation of abstract MyCubit extends Cubit with Detachability because that's too tight an integration with other libraries, similar to how riverpod forces the use of ConsumerWidget instead of StatelessWidget (even though there is a Consumer in the package).