prefer_immutable_bloc_state

v0.4.0 Warning Bloc / Riverpod

This rule flags Bloc and Cubit state classes that are missing the @immutable annotation. It detects state classes both by type parameter analysis (classes used as the state type in Bloc<Event, State> or Cubit<State>) and by naming convention (classes ending with State), including their subclasses.

Why use this rule

Mutable state objects are a common source of subtle bugs in the Bloc pattern. If you mutate a state object in place instead of creating a new one, emit won’t trigger a rebuild because Bloc compares state by reference equality. Marking state classes as @immutable signals this intent and enables analyzer warnings when you accidentally add mutable fields.

See also: Bloc state management

Don’t

import 'package:bloc/bloc.dart';
import 'package:meta/meta.dart';

abstract class CounterEvent {}

// Missing @immutable annotation
sealed class CounterState {}

class CounterInitial extends CounterState {}

class CounterLoaded extends CounterState {
  final int count;
  CounterLoaded(this.count);
}

class CounterBloc extends Bloc<CounterEvent, CounterState> {
  CounterBloc() : super(CounterInitial());
}

Do

import 'package:bloc/bloc.dart';
import 'package:meta/meta.dart';

abstract class CounterEvent {}

@immutable
sealed class CounterState {}

@immutable
class CounterInitial extends CounterState {}

@immutable
class CounterLoaded extends CounterState {
  final int count;
  CounterLoaded(this.count);
}

class CounterBloc extends Bloc<CounterEvent, CounterState> {
  CounterBloc() : super(CounterInitial());
}

Configuration

To disable this rule:

plugins:
  many_lints:
    diagnostics:
      prefer_immutable_bloc_state: false