SharedPreferences调用零颤振

Question

我构建了一个函数，用于将令牌存储在共享首选项中，然后在第一次运行应用程序时从服务器获取数据。

下面的NoSuchMethodError被抛出构建AdminPage(脏，状态: AdminPageState#87bcd)：flutter:方法'[]‘在null上被调用。颤振:接收器:空颤振:尝试调用：

然后应用程序就可以正常工作了

我的代码是

Future<Map<String, dynamic>> getCards(String userid) async {
  BuildContext context;
  String jWTtoken = '';
  try {
    final SharedPreferences prefs = await SharedPreferences.getInstance();

    //  prefs = await SharedPreferences.getInstance();
    jWTtoken = prefs.getString('token');
    tokenfoo();
  } catch (e) {
    Navigator.pushReplacement(
      context,
      MaterialPageRoute(builder: (BuildContext context) => AuthPage()),
    );
  }

  final Map<String, dynamic> authData = {
    'Userid': '261',
    // 'Email':_formData['Email'],
    // 'Password':_formData['Password'],
  };

  final http.Response response = await http.post(
      'hurl',
      body: json.encode(authData),
      headers: {
        'Content-Type': 'application/json',
        'Authorization': 'Bearer ' + jWTtoken
      });

  final Map<String, dynamic> responseData = json.decode(response.body);

  if (responseData["StatusCode"] == 200) {

    null;
  
  } else if (responseData["StatusCode"] == 401) {
    print(responseData);
    Logout();
  } else {
    print(responseData);
    Logout();

    null;
  }
  return responseData;
}

有什么问题吗？

在调试模式中，错误出现在以下行中

 final SharedPreferences prefs = await SharedPreferences.getInstance();

错误：

小部件库╞═══════════════════════════════════════════════════════════flutter捕获的══╡异常:下面的NoSuchMethodError被抛出构建AdminPage(脏，状态: AdminPageState#7db9a)：══╡：方法'[]‘在null上被调用。颤振:接收者:空颤振:尝试调用:颤振:颤振:当异常被抛出时，这是堆栈:颤振： 0 Object.noSuchMethod (dart:core/运行时/libobject_patch.dart:50:5)颤振：#1 AdminPageState.build (软件包:idb/page/adminpage.dart:63:39)颤振： 2 (package:flutter/src/widgets/framework.dart:3809:27) StatefulElement.build颤振：#3 (package:flutter/src/widgets/framework.dart:3721:15) ComponentElement.performRebuild颤振：#4 (package:flutter/src/widgets/framework.dart:3547:5)颤振：#5 (package:flutter/src/widgets/framework.dart:3701:5)颤振：#6 (package:flutter/src/widgets/framework.dart:3848:11) StatefulElement._firstBuild颤振：#7 (package:flutter/src/widgets/framework.dart:3696:5) ComponentElement.mount颤振：#8 (package:flutter/src/widgets/framework.dart:2950:14) Element.inflateWidget颤振：#9 (package:flutter/src/widgets/framework.dart:2753:12) Element.updateChild颤振：#10 (package:flutter/src/widgets/framework.dart:3732:16) ComponentElement.performRebuild颤振：#11 (package:flutter/src/widgets/framework.dart:3547:5)颤振：#12 BuildOwner.buildScope (package:flutter/src/widgets/framework.dart:2286:33)颤振：#13 _WidgetsFlutterBinding&BindingBase&GestureBinding&ServicesBinding&SchedulerBinding&PaintingBinding&SemanticsBinding&RendererBinding&WidgetsBinding.drawFrame (package:flutter/src/widgets/binding.dart:676:20)颤振：#14 _WidgetsFlutterBinding&BindingBase&GestureBinding&ServicesBinding&SchedulerBinding&PaintingBinding&SemanticsBinding&RendererBinding._handlePersistentFrameCallback (package:flutter/src/rendering/binding.dart:219:5)颤振：#15 _WidgetsFlutterBinding&BindingBase&GestureBinding&ServicesBinding&SchedulerBinding._invokeFrameCallback (package:flutter/src/scheduler/binding.dart:990:15)颤振：#16 _WidgetsFlutterBinding&BindingBase&GestureBinding&ServicesBinding&SchedulerBinding.handleDrawFrame (package:flutter/src/scheduler/binding.dart:930:9)颤振：#17 _WidgetsFlutterBinding&BindingBase&GestureBinding&ServicesBinding&SchedulerBinding._handleDrawFrame (package:flutter/src/scheduler/binding.dart:842:5)颤振：#18 _invoke (飞镖:ui/Hooks.Dart:154:13)颤振：#19 _drawFrame (飞镖:ui/Hooks.Dart:143:3)

Answer 1

可能是因为初始化时jwToken是空的。尝试给jwToken一个字符串值，比如"test“。

Answer 2

创建要全局使用的全局类：

final SharedPreferences prefs = await SharedPreferences.getInstance();

static Future init() async {
    pref = await SharedPreferences.getInstance();
  }