如何在flutter应用中从本地存储返回json数据

dependencies:
  flutter:
    sdk: flutter
  shared_preferences: ^2.0.8

import 'package:shared_preferences/shared_preferences.dart';

SharedPreferences prefs = await SharedPreferences.getInstance();

String jsonData = '{"name": "John", "age": 30}';
await prefs.setString('myJsonData', jsonData);

String jsonData = prefs.getString('myJsonData');

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 Local Storage',
      theme: ThemeData(
        primarySwatch: Colors.blue,
      ),
      home: MyHomePage(),
    );
  }
}

class MyHomePage extends StatefulWidget {
  @override
  _MyHomePageState createState() => _MyHomePageState();
}

class _MyHomePageState extends State<MyHomePage> {
  SharedPreferences prefs;
  String jsonData;

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

  Future<void> initPrefs() async {
    prefs = await SharedPreferences.getInstance();
    setState(() {
      jsonData = prefs.getString('myJsonData');
    });
  }

  Future<void> saveJsonData() async {
    String jsonData = '{"name": "John", "age": 30}';
    await prefs.setString('myJsonData', jsonData);
    setState(() {
      this.jsonData = jsonData;
    });
  }

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        title: Text('Flutter Local Storage'),
      ),
      body: Center(
        child: Column(
          mainAxisAlignment: MainAxisAlignment.center,
          children: <Widget>[
            Text(
              'JSON Data:',
            ),
            Text(
              jsonData ?? 'No data',
              style: Theme.of(context).textTheme.headline6,
            ),
            RaisedButton(
              onPressed: saveJsonData,
              child: Text('Save JSON Data'),
            ),
          ],
        ),
      ),
    );
  }
}