Shared preferences must be initialized error-CodePudding

In flutter I have faced with my below error about non-nullable variable. I am using shared preferences package for storing user info in device. I added the error in detail below:

Error: The non-nullable variable 'localStorage' must be initialized. Try adding an initializer expression.

shared_preferences: ^2.0.9

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

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

**SharedPreferences localStorage;**

TextEditingController emailController = TextEditingController();
TextEditingController passwordController = TextEditingController();

class MyApp extends StatelessWidget {
  static Future init() async {
    localStorage = await SharedPreferences.getInstance();
  }

  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      home: MyHome(),
    );
  }
}

class MyHome extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return Scaffold(
        body: Padding(
      padding: const EdgeInsets.all(30.0),
      child: Center(
        child: Column(
          children: [
            Padding(
              padding: const EdgeInsets.only(top: 200),
            ),
            Container(
              margin: EdgeInsets.symmetric(vertical: 10),
              child: Column(
                crossAxisAlignment: CrossAxisAlignment.start,
                children: <Widget>[
                  Text(
                    'E-mail Id',
                    style: TextStyle(fontWeight: FontWeight.bold, fontSize: 15),
                  ),
                  SizedBox(
                    height: 10,
                  ),
                  TextField(
                      controller: emailController,
                      obscureText: false,
                      decoration: InputDecoration(
                        border: InputBorder.none,
                        fillColor: Colors.grey,
                        filled: true,
                      )),
                ],
              ),
            ),
            Container(
              margin: EdgeInsets.symmetric(vertical: 10),
              child: Column(
                crossAxisAlignment: CrossAxisAlignment.start,
                children: <Widget>[
                  Text(
                    'Password :',
                    style: TextStyle(fontWeight: FontWeight.bold, fontSize: 15),
                  ),
                  SizedBox(
                    height: 10,
                  ),
                  TextField(
                      controller: passwordController,
                      obscureText: false,
                      decoration: InputDecoration(
                        border: InputBorder.none,
                        fillColor: Colors.grey,
                        filled: true,
                      )),
                ],
              ),
            ),
            ElevatedButton(
              onPressed: save,
              child: Text('Login'),
            ),
            Padding(
              padding: EdgeInsets.only(top: 50),
            ),
            if (localStorage != null)
              Padding(
                padding: const EdgeInsets.all(15.0),
                child: Text(
                  "User Logged in!!! ->  Email Id: ${localStorage.get('email')}  Password: ${localStorage.get('password')}",
                  style: TextStyle(fontSize: 20),
                ),
              ),
          ],
        ),
      ),
    ));
  }
}

save() async {
  await MyApp.init();
  localStorage.setString('email', emailController.text.toString());
  localStorage.setString('password', passwordController.text.toString());
}

CodePudding user response：

You have to initialize SharedPreferences at the exact state to use it or try to pass the value from MyApp to MyHome.

CodePudding user response：

In Flutter with Dart null-safety (Dart versions 2.12 ) variables are required to always have a value, unless you specifically declare them as perhaps being null.

For your use case above, you could declare your global localStorage using the late keyword:

late SharedPreferences localStorage;

This tells Flutter that "I will assign this variable a non-null value prior to use, I promise."

You can fulfill this promise in main() (which I've made async, btw):

Future<void> main() async {
  WidgetsFlutterBinding.ensureInitialized();
  // ↑ Req'd for using SharedPrefs (MethodChannel operations) prior to runApp
  localStorage = await SharedPreferences.getInstance();
  runApp(const MyApp());
}

Then you can use your global variable localStorage as expected.