json - 如何在 Flutter 中将响应 JSON 转换为对象？

Question

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>>()**

1. 我是否也需要在模型中转换该数据对象？
2. 将 JSON 转换为对象 (SignUpResponse) 的最佳且简单的方法是什么。
3. 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？为什么我们必须为将 json 转换为特定对象做这么多工作？

Answer 1

不需要使用cast，可以直接解析成Map

final Map parsed = json.decode(res); 

拥有 map 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();