shared_manager.dart

class LocaleSharedManager {
  static final LocaleSharedManager _instance = LocaleSharedManager._init();
  static LocaleSharedManager get instance => _instance;
  late final SharedPreferences _preferences;

  LocaleSharedManager._init() {
    SharedPreferences.getInstance().then((value) {
      _preferences = value;
    });
  }

  /// Force Initiliaze with [Future] at [SharedPreferences]
  /// !MUST be use in main dart with main function
  /// Example:
  ///  [await LocaleSharedManager.instance.prefencesInit()]
  Future<void> prefencesInit() async {
    _preferences = await SharedPreferences.getInstance();
  }

  Future<void> setStringValue(PreferenceKeys key, String value) async {
    await _preferences.setString(key.rawValue, value);
  }

  String? getStringValue(PreferenceKeys key) => _preferences.getString(key.rawValue);
}

enum PreferenceKeys { THEME, FIRST_LOGIN_APP, NAME }

extension on PreferenceKeys {
  String get rawValue {
    switch (this) {
      case PreferenceKeys.FIRST_LOGIN_APP:
        return 'firstApp/$index';
      case PreferenceKeys.THEME:
        return 'theme/$index';
      default:
        return '$this/$index';
    }
  }
}

class SampleView extends StatefulWidget {
  @override
  _SampleViewState createState() => _SampleViewState();
}

class _SampleViewState extends State<SampleView> {
  final localeManager = LocaleSharedManager.instance;
  @override
  void initState() {
    super.initState();
    initPrefences();
  }

  Future<void> initPrefences() async {
    //call it in main dart
    await LocaleSharedManager.instance.prefencesInit();
    setState(() {});
  }

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      floatingActionButton: ElevatedButton(
          onPressed: () {
            localeManager.setStringValue(PreferenceKeys.NAME, 'vb10');
          },
          child: Text('Save @vb10')),
      body: Text(localeManager.getStringValue(PreferenceKeys.NAME) ?? ''),
    );
  }
}