[proposal] feat(bloc): cancelable async operations

**Description** As a developer, I want to be able to await asynchronous operations within a bloc/cubit which are automatically canceled if the instance is closed while the async operation is pending. An example use-case is when using a cubit to fetch some data asynchronously from a screen in which a user can go back. ```dart static Route<MyScreen> route() { return MaterialPageRoute( builder: (context) => BlocProvider( create: (context) => MyCubit()..load(), child: const MyScreen(), ), ); } enum MyState { idle, loading } class MyCubit extends Cubit<MyState> { MyCubit() : super(MyState.idle); Future<void> load() async { emit(MyState.loading); await Future<void>.delayed(const Duration(seconds: 3)); emit(MyState.idle); } } ``` In this scenario, as soon as `MyScreen` is pushed onto the navigation stack, the `load()` method is called on a newly created instance of `MyCubit`. It's possible that the user might get tired of waiting and press the back button before `load()` has completed. In this case, the Future will still complete after the `MyCubit` has been closed and the subsequent `emit(MyState.idle)` will be evaluated which will result in a `StateError`: `Unhandled Exception: Bad state: Cannot emit new states after calling close` **Desired Solution** It would be nice if we had a `cancelable` (open to naming suggestions) API which allowed developers to await asynchronous operations which would automatically be canceled if the bl

Posted by felangel on Dec 16, 2021, 4:23 AM

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