How can I access variables in Flutter Bloc without passing them as params to every single state?

Question

So I have a Flutter app with Bloc for state mangement. I want to keep a list of users in Bloc so I can access it from different places in the app. But as it is now I pass this list of users to every state. Can I somehow avoid this boilerplate code?

Here is an example of my Bloc State:

@immutable
abstract class UsersState {
  final List<User> users;
  final User? user;

  const UsersState({required this.users, required this.user});
}

class UsersInitial extends UsersState {
  UsersInitial() : super(users: [], user: null);
}

class UsersLoadedState extends UsersState {
  const UsersLoadedState({required List<User> users, required User? user})
      : super(users: users, user: user);
}

class UsersPendingState extends UsersState {
  const UsersPendingState({List<User> users = const [], User? user})
      : super(users: users, user: user);
}


class UsersErrorState extends UsersState {
  final String message;
  const UsersErrorState(
      {required List<User> users, required User? user, required this.message})
      : super(users: users, user: user);
}

class FriendAddedState extends UsersState {
  const FriendAddedState({required List<User> users, required User? user})
      : super(users: users, user: user);
}

In this example I don't want to have to pass list of users to FriendAddedState. And here is how it looks in Bloc:

 else if (event is AddFriend) {
        emit(FriendAddPending(users: users, user: user));
        final result = await usecases.addFriend(event.friendId);
        result.fold((error) {
          emit(UsersErrorState(
              users: users, user: user, message: error.message));
        }, (success) {
          user = success;
          emit(FriendAddedState(users: users, user: user));
        });
      } 

Answer 1

You don't have to define users and user in UserState if you don't need them in all the bloc states. You could just define them in UsersLoadedState FriendAddedState like this:

@immutable
abstract class UsersState {
  const UsersState();
}

class UsersInitial extends UsersState {
  const UsersInitial();
}

class UsersLoadedState extends UsersState {
  const UsersLoadedState({
    required this.users,
    required this.user,
  });

  final List<User> users;
  final User? user;
}

class UsersPendingState extends UsersState {
  const UsersPendingState();
}

class UsersErrorState extends UsersState {
  final String message;

  const UsersErrorState({required this.message});
}

class FriendAddedState extends UsersState {
  const FriendAddedState({
    required this.users,
    required this.user,
  });

  final List<User> users;
  final User? user;
}

[Recommended] if you want to access user in every state of bloc, you can use a single class for your bloc state and an enum to define its state:

@immutable
class UsersState {
  final List<User> users;
  final User? user;
  final String? message;
  final UserStatus status;

  const UsersState({
    required this.status,
    required this.users,
    this.user,
    this.message,
  });

  const UsersState.initial()
      : status = UserStatus.initial,
        users = const [],
        user = null,
        message = null;

  UsersState copyWith(
    List<User>? users,
    User? user,
    String? message,
    UserStatus? status,
  ) {
    return UsersState(
      status: status ?? this.status,
      users: users ?? this.users,
      message: message ?? this.message,
      user: user ?? this.user,
    );
  }
}

enum UserStatus {
  initial,
  loaded,
  pending,
  error,
  friendAdded,
}

then in your bloc class, anywhere you wanted to emit new state, use state.copyWith()!