How to convert Response JSON to Object in Flutter?

Question

I am trying to convert success JSON string to Object. May be I am not doing in proper manner But I have couple of question related to converting JSON to Object.

Also I am getting this exeption in Log.

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. Should I need cast that data object in model too?
2. Which is the best and simple way to convert JSON to Object (SignUpResponse).
3. Its better to do with serialization ?

Response JSON :

{
    "data": {
        "_id": "5bd2a59f4c59de1ac517d7bf",
        "email": "[email protected]",
        "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
}

SignUpResponse which was created by online tool.

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;
    }
}

Calling 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);
    });
    }
});
}

Unable to understand what is happening here ? Why we have to do so much for convert our json to particular object ?

Answer 1

Don't need to use cast, you can parse directly to a Map

final Map parsed = json.decode(res); 

After you have a map you can use that data to convert into your Object.

final signUp = SignUpResponse.fromJson(parsed);

And if you want to parse an array of objects, you could do something like this:

//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();

Answer 2

The 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"
       },
    ]

The POJO Class :

    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 CALL :

    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();

Answer 3

Here is a basic answer for this if you want to access your information after the request with no extra steps.

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

the structure of the object from the request was like this:

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

Answer 4

Same answer but, what happens when you have nested JsonObjects? Easy:

Given a jsonResponse like this:

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

This is an example of how the code looks like:

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
    );
  }

Then, repeat the same strategy but for the inner class object:

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
    );
  }
}

Finally, you could cosume that, in another part of you project like this:

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