Extends blocs with an additional stream which can serve as a way of indicating single-time events (so-called "presentation events").

flutter pub add bloc_presentation

First, create an event which will be emitted:

class FailedToUpvote implements BlocPresentationEvent {
  const FailedToUpvote(this.reason);

  final String reason;
}

Next, extend your Bloc/Cubit with the presentation mixin which will give you access to the emitPresentation method:

class CommentCubit extends Cubit<CommentState> with BlocPresentationMixin {
  // body
}

Now in your methods instead of emitting new state, you can emit a single-time presentation event without overwriting your bloc/cubit state:

void upvote() {
	// upvoting logic

	if (!success) {
		// we can emit it and forget about cleaning it from the state
		emitPresentation(const FailedToUpvote('bad connection'));
	} else {
		emit(/* new state */);
	}
}

In this case above, we do not want to lose our bloc/cubit state after a non-fatal failure. Instead, we want to communicate this failure and not emit any new states. Then, in the UI code one can react to such events using BlocPresentationListener or useBlocPresentationListener:

BlocPresentationListener<CommentCubit>(
	listener: (context, event) {
		if (event is FailedToUpvote) {
			ScaffoldMessenger.of(context)
				..hideCurrentSnackBar()
				..showSnackBar(SnackBar(content: Text(event.reason)));
		}
	},
	child: MyWidget(),
)

By default, CommentCubit will be looked up using package:provider in the widget tree. However, a bloc can be provided directly using the BlocPresentationListener.bloc parameter (analogous to how package:bloc listeners work).

See example