In Memory Store (Article Example)

main.dart

import 'dart:async';

void main() {
  final authRepo = AuthenticationRepository();

  // listen to cache updates
  authRepo.user.listen((event) => print(event));

  const user = User(firstName: 'John', lastName: 'Doe', age: 29);
  // Create user
  authRepo.createUser(user);
  
  Future.delayed(const Duration(milliseconds: 100), () {
    // Value is cached, nothing to print.
    authRepo.updateUser(user.copyWith(firstName: 'John'));
  });

  Future.delayed(const Duration(milliseconds: 150), () {
    authRepo.updateUser(user.copyWith(firstName: 'Jane'));
  });
}

class AuthenticationRepository {
  AuthenticationRepository({CacheClient? cache})
      : _cache = cache ?? CacheClient();

  final CacheClient _cache;

  static const _cacheKey = 'unique_key';

  Stream<User> get user {
    return _cache.cacheUpdates
        .where((cache) => cache.containsKey(_cacheKey))
        .map((cache) => cache[_cacheKey] as User);
  }

  void createUser(User user) {
    _cache.write<User>(key: _cacheKey, value: user);
  }

  void updateUser(User user) {
    _cache.write<User>(key: _cacheKey, value: user);
  }
}

class CacheClient {
  CacheClient() : _cache = <String, Object>{};

  final Map<String, Object> _cache;

  final _cacheUpdatesController =
      StreamController<Map<String, Object>>.broadcast();

  Stream<Map<String, Object>> get cacheUpdates =>
      _cacheUpdatesController.stream;

  void write<T extends Object>({required String key, required T value}) {
    if (_cache[key] == value)  {
      return;
     }
    
    _cache[key] = value;
    _cacheUpdatesController.add(_cache);
  }

  T? read<T extends Object>({required String key}) {
    final value = _cache[key];
    if (value is T) return value;

    return null;
  }

  void dispose() {
    _cacheUpdatesController.close();
  }
}

class User {
  const User({required this.firstName, required this.lastName, this.age});
  final String firstName;
  final String lastName;
  final int? age;

  User copyWith({
    String? firstName,
    String? lastName,
    int? age,
  }) {
    return User(
      firstName: firstName ?? this.firstName,
      lastName: lastName ?? this.lastName,
      age: age ?? this.age,
    );
  }

  @override
  bool operator ==(Object other) {
    if (identical(this, other)) return true;

    return other is User &&
        other.firstName == firstName &&
        other.lastName == lastName &&
        other.age == age;
  }

  @override
  int get hashCode => firstName.hashCode ^ lastName.hashCode ^ age.hashCode;

  @override
  String toString() => 'User: $firstName $lastName, $age';
}