如何在 Flutter 中将响应 JSON 转换为对象？答案

【问题标题】：How to convert Response JSON to Object in Flutter?如何在 Flutter 中将响应 JSON 转换为对象？
【发布时间】：2019-03-30 19:23:07
【问题描述】：

我正在尝试将成功的 JSON 字符串转换为 Object。可能是我没有以正确的方式做但是我有几个与将 JSON 转换为 Object 相关的问题。

我也在 Log 中得到了这个异常。

NoSuchMethodError: Class '**_InternalLinkedHashMap<String, dynamic>' has no instance method 'cast' with matching arguments**.<br>
Receiver: _LinkedHashMap len:3
**Tried calling: cast<Map<String, dynamic>>()**

我是否也需要在模型中转换该数据对象？
将 JSON 转换为对象 (SignUpResponse) 的最佳且简单的方法是什么。
最好用序列化？

响应 JSON：

{
    "data": {
        "_id": "5bd2a59f4c59de1ac517d7bf",
        "email": "fjhsd@gmail.com",
        "phoneNumber": "2417874147",
        "isAddressApproved": 0,
        "unitNumber": "144",
        "streetAddress": "sdfsddsf",
        "area": "asd",
        "zipCode": "112233",
        "totalOrder": 0,
        "todayOrder": 0,
        "isPauseDelivery": false,
        "vacationStartDt": null,
        "vacationEndDt": null,
        "defaultLiter": 1
    },
    "message": "New User Created",
    "error": false
}

由在线工具创建的SignUpResponse。

class SignUpResponse {
    Data data;
    String message;
    bool error;

    SignUpResponse({this.data, this.message, this.error});

    SignUpResponse.fromJson(Map<String, dynamic> json) {
    data = json['data'] != null ? new Data.fromJson(json['data']) : null;
    message = json['message'];
    error = json['error'];
    }

    Map<String, dynamic> toJson() {
    final Map<String, dynamic> data = new Map<String, dynamic>();
    if (this.data != null) {
        data['data'] = this.data.toJson();
    }
    data['message'] = this.message;
    data['error'] = this.error;
    return data;
    }
}

class Data {
    String sId;
    String email;
    String phoneNumber;
    int isAddressApproved;
    String unitNumber;
    String streetAddress;
    String area;
    String zipCode;
    int totalOrder;
    int todayOrder;
    bool isPauseDelivery;
    Null vacationStartDt;
    Null vacationEndDt;
    int defaultLiter;

    Data(
        {this.sId,
        this.email,
        this.phoneNumber,
        this.isAddressApproved,
        this.unitNumber,
        this.streetAddress,
        this.area,
        this.zipCode,
        this.totalOrder,
        this.todayOrder,
        this.isPauseDelivery,
        this.vacationStartDt,
        this.vacationEndDt,
        this.defaultLiter});

    Data.fromJson(Map<String, dynamic> json) {
    sId = json['_id'];
    email = json['email'];
    phoneNumber = json['phoneNumber'];
    isAddressApproved = json['isAddressApproved'];
    unitNumber = json['unitNumber'];
    streetAddress = json['streetAddress'];
    area = json['area'];
    zipCode = json['zipCode'];
    totalOrder = json['totalOrder'];
    todayOrder = json['todayOrder'];
    isPauseDelivery = json['isPauseDelivery'];
    vacationStartDt = json['vacationStartDt'];
    vacationEndDt = json['vacationEndDt'];
    defaultLiter = json['defaultLiter'];
    }

    Map<String, dynamic> toJson() {
    final Map<String, dynamic> data = new Map<String, dynamic>();
    data['_id'] = this.sId;
    data['email'] = this.email;
    data['phoneNumber'] = this.phoneNumber;
    data['isAddressApproved'] = this.isAddressApproved;
    data['unitNumber'] = this.unitNumber;
    data['streetAddress'] = this.streetAddress;
    data['area'] = this.area;
    data['zipCode'] = this.zipCode;
    data['totalOrder'] = this.totalOrder;
    data['todayOrder'] = this.todayOrder;
    data['isPauseDelivery'] = this.isPauseDelivery;
    data['vacationStartDt'] = this.vacationStartDt;
    data['vacationEndDt'] = this.vacationEndDt;
    data['defaultLiter'] = this.defaultLiter;
    return data;
    }
}

调用 REST POST API

void postCallSignUp(BuildContext context, var body) {
// This Function will check Internet is available or not.
Utils.checkConnection().then((connectionResult) {
    if (connectionResult) {
    http.post(Constants.BASE_URL + Constants.CREATE_USER,
        body: body,
        headers: {
            "Accept": "application/json",
            "content-type": "application/json"
        }).then((http.Response response) {
        final String res = response.body;
        final int statusCode = response.statusCode;

        if (statusCode < 200 || statusCode > 400 || json == null) {
        _onFailureResponse(new Exception("Error while fetching data"));
        } else {
        //Parsing json response to particular Object.
        print(json.decode(res));

        // Unable to cast json here...
        final parsed = json.decode(res).cast<Map<String, dynamic>>();
        parsed.map<SignUpResponse>((json) => SignUpResponse.fromJson(json));
        SignUpResponse signUpResponse = parsed.map<SignUpResponse>((json) => SignUpResponse.fromJson(json));
        _onSuccessResponse(signUpResponse);
        }
    });
    } else {
    _onNoInternetConnection();
    Utils.showAlert(context, "Alert", "Internet is not connected.", () {
        Navigator.pop(context);
    });
    }
});
}

无法理解这里发生了什么？为什么我们必须做这么多来将我们的 json 转换为特定的对象？

【问题讨论】：

使用这个； final Map 已解析 = json.decode(res);
所以我们不需要投射 ?
其实你只需要：final Map parsed = json.decode(res);
以及如何将解析后的 json 转换为 SignUpResponse？
我在下面添加了我的答案

标签： json http dart flutter

【解决方案1】：

不需要使用cast，可以直接解析成Map

final Map parsed = json.decode(res);

拥有地图后，您可以使用该数据转换为您的对象。

final signUp = SignUpResponse.fromJson(parsed);

如果你想解析一个对象数组，你可以这样做：

//assuming this json returns an array of signupresponse objects
final List parsedList = json.decode(res); 

List<SignUpResponse> list = parsedList.map((val) =>  SignUpResponse.fromJson(val)).toList();

【讨论】：

这很奇怪，而 list = json.decode(res).map((val) => SignUpResponse.fromJson(val)).toList(); 似乎失败了。
也许你可以帮助解决这个问题。 stackoverflow.com/questions/61977195/…

【解决方案2】：

JSON：

    [
     {
       "id":1,
       "name":"Afghanistan",
       "iso3":"AFG",
       "iso2":"AF",
       "phone_code":"93",
       "capital":"Kabul",
       "currency":"AFN"
     },
     {
       "id":2,
       "name":"Aland Islands",
       "iso3":"ALA",
       "iso2":"AX",
       "phone_code":"+358-18",
       "capital":"Mariehamn",
       "currency":"EUR"
       },
    ]

POJO 类：

    class country{
       String id;
       String name;
       String currency;

       country({this.id,this.name,this.currency});

       factory country.fromJson(Map<String, dynamic> parsedJson){
         return country(
         id: parsedJson['id'].toString(),
         name : parsedJson['name'].toString(),
         currency: parsedJson['currency'].toString() 
         );
       }
    }

API 调用：

    await http.post("http://calikidsmap.com/test.php").then((response){

    var ddd=jsonDecode(response.body);

    Country_object_list = (ddd as List)
      .map((data) => new country.fromJson(data))
      .toList();

【讨论】：

【解决方案3】：

将此添加到pubspec.yaml

dependencies:
  # Your other regular dependencies here
  json_annotation: <latest_version>

dev_dependencies:
  # Your other dev_dependencies here
  build_runner: <latest_version>
  json_serializable: <latest_version>

对象类。

import 'package:json_annotation/json_annotation.dart';


part 'user.g.dart'; //"user" is your dart file name

/// JSON serialization logic to be generated.
@JsonSerializable()

class User {
  User(this.name, this.email);

  String name;
  String email;

  factory User.fromJson(Map<String, dynamic> json) => _$UserFromJson(json);

  Map<String, dynamic> toJson() => _$UserToJson(this);
}

并在 Android Studio 的终端中运行。

flutter pub run build_runner build

现在像这样转换：

Map results =  json.decode(response.body);
User user = User.fromJson(results);

来源：https://flutter.dev/docs/development/data-and-backend/json

【讨论】：

工作得非常好。谢谢！

【解决方案4】：

同样的答案，但是，当您嵌套了 JsonObjects 时会发生什么？简单：

给定一个像这样的 jsonResponse：

{
  "id": 1,
  "type": 15,
  "innerClass": {
    "id": 1,
    "type": "testing"
  }
}

这是代码外观的示例：

class MainClass {
  int id = 0;
  int version = 0;
  InnerClass innerClass;

  MainClass (
      this.id,
      this.version,
      this.innerClass
      );
  
//Create the same factory converter, but using another inner factory 'fromJson' converter
//for the inner class object (InnerClass.fromJson(json['inner_class']))

  factory MainClass.fromJson(dynamic json) {
    return MainClass(
        json['id'] as int, // << put your json response keys instead.
        json['version'] as int,
        InnerClass.fromJson(json['innerClass']) // << this one
    );
  }

然后，对内部类对象重复相同的策略：

class InnerClass {
  int id = 0;
  String type = 'testing_type';
  InnerClass (
      this.id,
      this.type
      );
  factory InnerClass.fromJson(dynamic json) {
    return InnerClass(
        json['id'] as int,
        json['type'] as String
    );
  }
}

最后，你可以假设，在你项目的另一部分，像这样：

try {
          mainClassDartObject = MainClass.fromJson(response.body);
     } catch (e) {
          print(e);
     }

【讨论】：

【解决方案5】：

如果您想在请求后访问您的信息而无需额外步骤，这里是一个基本答案。

 Map<String, dynamic> data = jsonDecode(response.body);
 String token = tokenData["data"]["access_token"];

请求中的对象结构如下：

{
  "success":1,
  "error":[],
  "data": {
     "access_token":"13d2ec9d1094903b1e81de8af059233e9f36ec4d",
     "expires_in":2628000,
     "token_type":"Bearer"
  }
}

【讨论】：