shared_prefs.dart

/// Use SharedPreferences in Flutter effortlessly
/// https://medium.com/@simonpham/use-sharedpreferences-in-flutter-effortlessly-835bba8f7418

// utils/shared_prefs.dart
class SharedPrefs {
  static SharedPreferences _sharedPrefs;

  factory SharedPrefs() => SharedPrefs._internal();

  SharedPrefs._internal();

  Future<void> init() async {
    _sharedPrefs ??= await SharedPreferences.getInstance();
  }

  String get username => _sharedPrefs.getString(keyUsername) ?? "";

  set username(String value) {
    _sharedPrefs.setString(keyUsername, value);
  }
}

// constants/strings.dart
const String keyUsername = "key_username";

// main.dart
Future<void> main() async {
  WidgetsFlutterBinding.ensureInitialized();
  await SharedPrefs().init();
  runApp(
    MyApp(),
  );
}

class MyApp extends StatelessWidget {

  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      home: Scaffold(
        body: Text("Hi ${SharedPrefs().username}"),
      ),
    );
  }
}