shared_preference_service.dart

import 'package:flutter/material.dart';
import 'package:get_it/get_it.dart';
import 'package:shared_preferences/shared_preferences.dart';

GetIt sl = GetIt.I;

Future<void> setupLocators() async {
  final SharedPreferences pref = await SharedPreferences.getInstance();

  sl.registerSingleton<SharedPreferencesService>(
      SharedPreferencesService(pref));
}

Future<void> main() async {
  WidgetsFlutterBinding.ensureInitialized();

  await setupLocators();

  runApp(
    MaterialApp(
      home: TestScreen(),
    ),
  );
}

class SharedPreferencesService {
  final SharedPreferences _pref;

  final String kUsername = 'username';

  SharedPreferencesService(this._pref);

  Future<void> setUsername(String username) async {
    await _pref.setString(kUsername, username);
  }

  String getUsername() {
    return _pref.getString(kUsername) ?? '';
  }
}

class TestScreen extends StatefulWidget {
  TestScreen({Key key}) : super(key: key);

  @override
  _TestScreenState createState() => _TestScreenState();
}

class _TestScreenState extends State<TestScreen> {
  @override
  void initState() {
    super.initState();
  }

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      body: _buildBody(),
      floatingActionButton: FloatingActionButton(
        child: Text('+'),
        onPressed: () {
          sl.get<SharedPreferencesService>().setUsername('Nexus');
          setState(() {});
        },
      ),
    );
  }

  Widget _buildBody() {
    return Container(
      child: Center(
        child: Text(
          'Username: ${sl.get<SharedPreferencesService>().getUsername()}',
        ),
      ),
    );
  }

  @override
  void dispose() {
    super.dispose();
  }
}