cpd_2022_hello/hello_world2/lib/main.dart

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

void main() {
  runApp(MyApp());
}

class MyApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      title: 'Flutter Demo',
      theme: ThemeData(
        primarySwatch: Colors.red,
        canvasColor: Colors.red.shade100,
        accentColor: Colors.blueAccent,
      ),
      home: MyHomePage(title: 'Hello World'),
    );
  }
}

class MyHomePage extends StatefulWidget {
  MyHomePage({Key? key, required this.title}) : super(key: key);

  final String title;

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

class _MyHomePageState extends State<MyHomePage> {
  int _counter = 0;

  @override
  void initState() {
    getCounter();
    super.initState();
  }

  void getCounter() async {
    SharedPreferences prefs = await SharedPreferences.getInstance();
    setState(() {
      _counter = prefs.getInt('counter') ?? 0;
    });
  }

  void _incrementCounter() async {
    int newCounter = _counter + 1;
    SharedPreferences prefs = await SharedPreferences.getInstance();
    await prefs.setInt('counter', newCounter);
    setState(() {
      _counter = newCounter;
    });
  }

  void reset() async {
    SharedPreferences prefs = await SharedPreferences.getInstance();
    await prefs.setInt('counter', 0);
    setState(() {
      _counter = 0;
    });
  }

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        title: Text(widget.title),
      ),
      body: Center(
        child: Column(
          mainAxisAlignment: MainAxisAlignment.center,
          children: <Widget>[
            Text(
              'You have pushed the button this many times:',
            ),
            Text(
              '$_counter',
              style: Theme.of(context).textTheme.headline4,
            ),
            ElevatedButton(
              onPressed: reset,
              child: Text('Reset'),
            ),
          ],
        ),
      ),
      floatingActionButton: FloatingActionButton(
        onPressed: _incrementCounter,
        tooltip: 'Increment',
        child: Icon(Icons.add),
      ),
    );
  }
}