如何在Flutter中保存页面认证中的用户登录？

import 'package:shared_preferences/shared_preferences.dart';

// 保存用户登录信息
void saveUserLoginInfo(String username, String password) async {
  SharedPreferences prefs = await SharedPreferences.getInstance();
  await prefs.setString('username', username);
  await prefs.setString('password', password);
}

// 获取用户登录信息
Future<Map<String, String>> getUserLoginInfo() async {
  SharedPreferences prefs = await SharedPreferences.getInstance();
  String username = prefs.getString('username');
  String password = prefs.getString('password');
  return {'username': username, 'password': password};
}

import 'package:flutter/material.dart';

class LoginPage extends StatefulWidget {
  @override
  _LoginPageState createState() => _LoginPageState();
}

class _LoginPageState extends State<LoginPage> {
  TextEditingController _usernameController = TextEditingController();
  TextEditingController _passwordController = TextEditingController();

  @override
  void initState() {
    super.initState();
    getUserLoginInfo().then((userInfo) {
      if (userInfo['username'] != null && userInfo['password'] != null) {
        _usernameController.text = userInfo['username'];
        _passwordController.text = userInfo['password'];
      }
    });
  }

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        title: Text('Login'),
      ),
      body: Padding(
        padding: EdgeInsets.all(16.0),
        child: Column(
          children: [
            TextField(
              controller: _usernameController,
              decoration: InputDecoration(
                labelText: 'Username',
              ),
            ),
            TextField(
              controller: _passwordController,
              decoration: InputDecoration(
                labelText: 'Password',
              ),
              obscureText: true,
            ),
            RaisedButton(
              child: Text('Login'),
              onPressed: () {
                String username = _usernameController.text;
                String password = _passwordController.text;
                saveUserLoginInfo(username, password);
                // 进行登录操作
              },
            ),
          ],
        ),
      ),
    );
  }
}