Lazy local storage, shared preferences

api.dart

/* ... */

mixin _LocalStorageAPI on _LocalStorageCache {
  Future<String?> getString(String key) => _eval<String?>((db) => db.getString(key));
  Future<void> setString(String key, String value) => _eval<void>((db) => db.setString(key, value));
}

initialization.dart

class LocalStorage with _LocalStorageCache, _LocalStorageAPI {
  LocalStorage(Future<SharedPreferences> Function() initialization) : _$initialization = initialization;

  @override
  final Future<SharedPreferences> Function() _$initialization;
}

mixin _LocalStorageCache {
  abstract final Future<SharedPreferences> Function() _$initialization;
  late final Future<SharedPreferences> _$db = _$initialization();
  Future<T> _eval<T>(T Function(SharedPreferences db) fn) => _$db.then<T>(fn);
}

singleton.dart

class LocalStorageSingleton with _LocalStorageCache, _LocalStorageAPI {
  LocalStorageSingleton._();
  static final LocalStorageSingleton _$instance = LocalStorageSingleton._();
  static LocalStorageSingleton get instance => _$instance;
}

mixin _LocalStorageCache {
  late final Future<SharedPreferences> _$db = SharedPreferences.getInstance();
  Future<T> _eval<T>(T Function(SharedPreferences db) fn) => _$db.then<T>(fn);
}