Repository realisation in dart

main.dart

/*
Repository realisation in dart
https://gist.github.com/PlugFox/bb2cf58f199886cc0971d75a3e337b6a
https://dartpad.dartlang.org/bb2cf58f199886cc0971d75a3e337b6a
*/
import 'dart:async';

// Repository controller
abstract class Repository {
  static final RepositoryProvider _repository = RepositoryProvider();
  
  static Future<Set<User>> fetchUsers() async =>
    await Future.wait([
      _repository.facebookProvider.fetchUsers(count: 15, offset: 100),
      _repository.vkontakteProvider.fetchUsers(count: 13, offset: -100),
    ]).then(
      (List<Set<User>> list) => list.fold<Set<User>>(<User>{}, (Set<User> a, Set<User> b) => a..addAll(b))
    ).then(
      (Set<User> users) => _repository.localProvider.storeUsers(users).then((_) => users)
    );
  
  factory Repository._() => null;
}

// Repository provider
class RepositoryProvider {
  final FacebookProvider facebookProvider = FacebookProvider();
  final VkontakteProvider vkontakteProvider = VkontakteProvider();
  final LocalProvider localProvider = LocalProvider();
  
  // SINGLETON +
  static final RepositoryProvider _repository = RepositoryProvider._internal();
  factory RepositoryProvider() => _repository;
  RepositoryProvider._internal() {
    print('Creating singleton RepositoryProvider');
  }
  // SINGLETON -
}

// PROVIDERS
class FacebookProvider {
  static const String _origin = 'Facebook';
  Future<Set<User>> fetchUsers({int count = 1, int offset = 0,}) async => 
    await List<User>.generate(count, (int id) => User(id + offset, socialNetwork: FacebookProvider._origin)).toSet();
}
class VkontakteProvider {  
  static const String _origin = 'Vkontakte';
  Future<Set<User>> fetchUsers({int count = 1, int offset = 0,}) async => 
    await List<User>.generate(count, (int id) => User(id + offset, socialNetwork: VkontakteProvider._origin)).toSet();
}
class LocalProvider {
  final Set<User> _users = <User>{};
  Future<void> storeUsers(Iterable<User> users) async => await this._users.addAll(users);
}

// Model
class User {
  final int _id;
  final String _origin;
  User(int id, {String socialNetwork = 'Unknown'})
    : assert(id is int && id != 0)
    , this._id = id
    , this._origin = socialNetwork;
  @override
  String toString() => '<${this._origin} user №${_id.toRadixString(16)}>';
  @override
  int get hashCode => this._id;
  @override
  bool operator ==(Object user) => user is User && this.hashCode == user.hashCode;
}


void main() async {
  print(await Repository.fetchUsers());
} // main()