如何在 flutter post 方法中修复 'Unhandled Exception: Null check operator used on a null value'

Question

我是 Flutter 新手。我正在尝试了解 flutter rest api。当我尝试使用 fullter api（使用 http dart 包）post 数据时，它显示了这个错误。

我尝试通过对我的代码进行一些更改来修复它，但我仍然无法修复它。这是我尝试执行的代码。

型号

 import 'dart:convert';

DataModel dataModelFromJSON(String str) => DataModel.fromJson(jsonDecode(str));
String dataModelToJson(DataModel data) => json.encode(data.toJson());

class DataModel {
 DataModel(
   {required this.name,
   required this.job,
   required this.id,
   required this.createdAt});

   String name;
   String job;
   String id;
   String createdAt;

factory DataModel.fromJson(Map<String, dynamic> json) => DataModel(
   name: json['name'],
   job: json['job'],
   id: json['id'],
   createdAt: json['createdAt']);

  Map<String, dynamic> toJson() =>
     {"name": name, "job": job, "id": id, "createdAt": createdAt};
}

main.dart

 import 'package:flutter/material.dart';
 import 'package:http/http.dart' as http;

 import 'DataModel.dart';

  void main() {
   runApp(
   MaterialApp(
   home: MyHomePage(),
 ),
 );
}

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

Future<DataModel?> submitData(String name, String job) async {
   var response = await http.post(
   Uri.https('reqres.in', 'api/users'),
   body: {"name": name, "job": job},
   );
   var data = response.body;
   print(data);

   if (response.statusCode == 201) {
    String responseString = response.body;
    dataModelFromJSON(responseString);
   } else
    return null;
   }

   class _MyHomePageState extends State<MyHomePage> {
      late DataModel _dataModel;
      TextEditingController nameController = TextEditingController();
      TextEditingController jobController = TextEditingController();

   @override
   Widget build(BuildContext context) {
     return Scaffold(
      appBar: AppBar(
      title: Center(
       child: Text('HTTP Post'),
       ),
     ),
     body: Container(
       padding: EdgeInsets.all(20.0),
       child: Column(
         children: [
           TextField(
             decoration: InputDecoration(
                 border: OutlineInputBorder(), hintText: 'Name'),
             controller: nameController,
           ),
           SizedBox(height: 20.0),
           TextField(
             decoration: InputDecoration(
                 border: OutlineInputBorder(), hintText: 'Job'),
             controller: jobController,
           ),
           ElevatedButton(
             onPressed: () async {
               String name = nameController.text;
               String job = jobController.text;
               DataModel? data = await submitData(name, job);
               setState(() {
                 _dataModel = data!;
               });
             },
             child: Text("Submit"),
           )
         ],
       ),
     ),
   );
 }
}

如果有人能帮我解决这个问题，我将不胜感激。

Answer 1

删除！从这一行 _dataModel = data!; 在你的数据模型中你有这个

    String responseString = response.body;
    dataModelFromJSON(responseString);
   } else
    return null;
   }

如果请求的状态代码不是 201，它将 return 为空，它正在使用 !进行空检查是导致错误的原因。

Answer 2

您没有在函数中 returning 数据在此处添加 return

 return dataModelFromJSON(responseString);

现在你的函数将是这样的

  Future<DataModel?> submitData(String name, String job) async {
       var response = await http.post(
       Uri.https('reqres.in', 'api/users'),
       body: {"name": name, "job": job},
       );
       var data = response.body;
       print(data);
    
       if (response.statusCode == 201) {
        String responseString = response.body;
       return dataModelFromJSON(responseString);
       } else
        return null;
       }

如何在 flutter post 方法中修复 'Unhandled Exception: Null check operator used on a null value'

How to fix 'Unhandled Exception: Null check operator used on a null value' in flutter post method

rest

mobile

flutter

flutter-dependencies