Flutter SharedPreferences 不可为 null 的实例字段

Question

当我想使用共享首选项来存储和显示值时遇到问题。在代码中，我需要创建公共(public)类供下面的函数使用。 但是它会返回以下错误:

错误:必须初始化不可为 null 的实例字段“sharedPreferences”。 ([login] lib/main.dart:32 处的 not_initialized_non_nullable_instance_field)

共享首选项共享首选项；//行:32

这是我的完整代码。

import 'package:flutter/material.dart';
import 'package:flutter/services.dart';
import 'package:http/http.dart' as http;
import 'package:login/login_page.dart';
import 'package:shared_preferences/shared_preferences.dart';

void main() => runApp(MyApp());

class MyApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      title: "Code Land",
      debugShowCheckedModeBanner: false,
      home: MyHomePage(),
      theme: ThemeData(
          accentColor: Colors.white70
      ),
    );
  }
}

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

class MyHomePageState extends State<MyHomePage> {

  SharedPreferences sharedPreferences;

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

  checkLoginStatus() async {
    sharedPreferences = await SharedPreferences.getInstance();
    if(sharedPreferences.getString("token") == null) {
      Navigator.of(context).pushAndRemoveUntil(MaterialPageRoute(builder: (BuildContext context) => LoginPage()), (Route<dynamic> route) => false);
    }
  }

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        title: Text("Code Land", style: TextStyle(color: Colors.white)),
        actions: <Widget>[
          FlatButton(
            onPressed: () {
              sharedPreferences.clear();
              sharedPreferences.commit();
              Navigator.of(context).pushAndRemoveUntil(MaterialPageRoute(builder: (BuildContext context) => LoginPage()), (Route<dynamic> route) => false);
            },
            child: Text("Log Out", style: TextStyle(color: Colors.white)),
          ),
        ],
      ),
      body: Center(child: Text("Main Page")),
      drawer: Drawer(),
    );
  }
}```

Answer 1

像这样更改该行，以便编译器知道您稍后将对其进行初始化:

late SharedPreferences sharedPreferences;