Question

所以我尝试了共享首选项让用户在他们注销之前仍然登录，但是当我尝试重新启动应用程序时会出现这种情况

[VERBOSE-2:ui_dart_state.cc(199)] Unhandled Exception: Null check operator used on a null value
#0      MethodChannel.binaryMessenger (package:flutter/src/services/platform_channel.dart:142:86)
#1      MethodChannel._invokeMethod (package:flutter/src/services/platform_channel.dart:148:36)
#2      MethodChannel.invokeMethod (package:flutter/src/services/platform_channel.dart:331:12)
#3      MethodChannel.invokeMapMethod (package:flutter/src/services/platform_channel.dart:358:49)
#4      MethodChannelSharedPreferencesStore.getAll (package:shared_preferences_platform_interface/method_channel_shared_preferences.dart:44:25)
#5      SharedPreferences._getSharedPreferencesMap (package:shared_preferences/shared_preferences.dart:180:57)
#6      SharedPreferences.getInstance (package:shared_preferences/shared_preferences.dart:56:19)
#7      read_loggedIn (package:indonesia_bisa_2/API/LocalService.dart:11:41)
#8      main (package:indonesia_bisa_2/Main/main.dart:16:14)
#9      _runMainZoned.<anonymous closure>.<…>

这是检查代码

import 'package:shared_preferences/shared_preferences.dart';

Future<void> save_loggedIn() async {
  final prefs = await SharedPreferences.getInstance();
  prefs.setString("loggedIn", "true");
}

//this is for checking the saved variable to check if the user is already logged in or not
Future<String> read_loggedIn() async {
  final prefs = await SharedPreferences.getInstance();
  var value = prefs.getString("loggedIn");
  return value;
}

这是使用代码的时间

Future<void> main() async {
  runApp(new MaterialApp(
      debugShowCheckedModeBanner: false,
      home: (read_loggedIn() == "true") ? new SPAL (): new MyApp()));
}

Answer 1

问题是你调用了 read_loggedIn() ，它是一个未来，但你直接调用了它，虽然它没有返回值，但你得到了一个空值。所以你必须 await 直到你从 read_loggedIn() 得到结果，然后你才能使用它的值。

用下面的代码替换你的主要方法


Future<void> main() async {

    // you have to wait untill get the result
    var login = await read_loggedIn();
    
    runApp(new MaterialApp(
        debugShowCheckedModeBanner: false,
        // and you must check if your login value is null or not
        home: (login != null && login == "true") ? new SPAL(): new MyApp()));
}

Answer 2

您可以像这样更改 main 方法：

import 'package:shared_preferences/shared_preferences.dart';

Future<void> save_loggedIn() async {
  final prefs = await SharedPreferences.getInstance();

  prefs.setBool("loggedIn", true);
}

Future<Bool> read_loggedIn() async {
  final prefs = await SharedPreferences.getInstance();
  var value = prefs.getBool("loggedIn");

  return value;
}

Future<void> main() async {
  final bool login = await read_loggedIn();

  runApp(
    MaterialApp(
      debugShowCheckedModeBanner: false,
      home: (login ?? false) ? SPAL() : MyApp(),
    ),
  );
}

Answer 3

确保在WidgetsFlutterBinding.ensureInitialized();之前调用await SharedPreferences.getInstance();

例如在您的 main.dart 中：

void main() async {
  await runZonedGuarded(
    () async {
      WidgetsFlutterBinding.ensureInitialized();

      final prefs = await SharedPreferences.getInstance();
      runApp(MyApp(prefs));
    },
    (error, st) => print(error),
  );
}

?

Answer 4

当我用 iphone 模拟器测试我的应用程序时，我得到了完全相同的错误。（我有一个 m1 mac）。这可能是一个“时间问题”。我在 SharedPreferences.getInstance 之前添加了这一行 await Future.delayed(Duration(seconds: 2)); 并且错误消失了。

未处理的异常：在空值共享首选项上使用空检查运算符

4 个答案: