ValueHolder - Dart / Flutter utility class to use with state management packages. It is used to model the loading of information in the background. In the example it is used with BloC, but it can be used with any other state management library.

example_widget.dart

Widget namesWidget() {
    return BlocBuilder<ExampleBloc, ExampleState>(
        buildWhen: (previous, current) =>
            previous.names != current.names,
        builder: (context, state) {
          if(state.names.loading) {
            return Text("Loading names...");
          } else if (state.names.error != null) {
            return Text("Error: ${state.names.error}", style: TextStyle(color: Colors.red, fontWeight: FontWeight.bold)); 
          } else {
            return Text("Names are: ${state.names.value.join(",")}");
          }
        });
  }

value_holder.dart

class ExampleBloc
    extends Bloc<ExampleEvent, ExampleState> {
 
  const ExampleBloc ({}) {
    on<InitState>(_onInitState);
  }
  
  FutureOr<void> _onInitState(event, emit) async {
    emit(ExampleState());

    emit(this.state.copyWith(
          names: ValueHolder(value: null, loading: true),
        ));
    
    // TODO Fetch async data from a repository/API...
    Future.delayed(Duration(milliseconds: 1000), ()=>["John", "Jane", "Juan", "Joan",])
      .then((names) => emit(this.state.copyWith(
        names: names, loading: false,
      )))
      .onError((error, stackTrace) {
        emit(this.state.copyWith(
          names: ValueHolder(value: null, loading: false, error: "$error")
      ));
    });
  }
}

class ExampleState extends Equatable {
  const ExampleState({
    this.names = const ValueHolder(value: null),
  });
  
  final ValueHolder<List<String>?> names;
  
  final copyWith({
    ValueHolder<List<String>?>? names,
  }) {
    return ExampleState(
      names: names ?? this.names,
    );
  }
  
  @override
  List<Object> get props => [
        names,
    ];
}

abstract class ExampleEvent extends Equatable {
  const ExampleEvent();
  
  @override
  List<Object?> get props => [];
}

class InitState extends ExampleEvent {}