How to create objects of generic types in flutter/dart?

Question

How can we create object of generic types in dart?

For my use case, each of my api responses are wrapped as ApiResponse class. For login api response, I get a json object such as

{
    "data": {
        "email": "[email protected]",
        "name": "A"
    },
    "message": "Successful",
    "status": true
}

So to parse these responses, I created classes as below, but they throw compile time error stating that The method 'fromJson' isn't defined for the class 'Type'. :

class ApiResponse<T extends BaseResponse> {
  bool status;
  String message;
  T data;

  ApiResponse.fromJson(Map<String, dynamic> json) {
    status = json['status'];
    message = json['message'];
    data = T.fromJson(json['data']);  // <<<------- here is the compile time error
  }
}

abstract class BaseResponse {
  BaseResponse.fromJson(Map<String, dynamic> json);
}

class Login extends BaseResponse {
  String name, email;

  Login.fromJson(Map<String, dynamic> json) : super.fromJson(json) {
    name = json['name'];
    email = json['email'];
  }
}

// and I want to use it like below
usage() {
  ApiResponse<Login> a = ApiResponse.fromJson({});
  String name = a.data.name;
}

Can anyone help me fix the error?

Umm, usually you don't call methods on a type right? I wonder if your intent is to do data = data.fromJson(json['data']) — Stefano Saitta
– Stefano Saitta, Commented Jan 10, 2020 at 8:34
We get this error if we use data = data.fromJson(json['data']) :: main.dart:9:17: Error: The method 'fromJson' isn't defined for the class 'BaseResponse'. - 'BaseResponse' is from 'main.dart'. data = data.fromJson(json['data']); ^^^^^^^^ Error: Compilation failed. — Jainam Jhaveri
– Jainam Jhaveri, Commented Apr 8, 2020 at 10:13
@JainamJhaveri did you solve this? please help me by answering the question — akshay bhange
– akshay bhange, Commented Oct 21, 2020 at 15:23
You can't inherit constructors in Dart. You can only inherit instance members. — Firat
– Firat, Commented Jun 14, 2021 at 19:18

Hosam Hasan · Accepted Answer · 2021-07-08 14:42:35Z

ok let's add a little hack to manage your use case:

let say that you have two responses Login and Error both of Type BaseResponse

abstract class BaseResponse {}

class Login extends BaseResponse {
  Login.fromJson(Map<String, dynamic> json) {}
}

class Error extends BaseResponse {
  Error.fromJson(Map<String, dynamic> json) {}
}

the trick is that you will decide the type of BaseResponse at ApiResponse constructor like this

class ApiResponse<T extends BaseResponse> {
  T data;

  ApiResponse.fromJson(Map<String, dynamic> json) {
    data = _getProperDataValue<T>(json['data']);
  }

  static E _getProperDataValue<E extends BaseResponse>(
      Map<String, dynamic> jsonData) {
    switch (E) {
      case Login:
        return Login.fromJson(jsonData) as E;
      case Error:
        return Error.fromJson(jsonData) as E;
      default:
        throw UnsupportedError('Not Supported Type');
    }
  }
}

both responses will work

void main(List<String> arguments) {
  final loginRes = ApiResponse<Login>.fromJson(<String, dynamic>{
    'data': <String, dynamic>{},
  });
 final errorRes = ApiResponse<Error>.fromJson(<String, dynamic>{
    'data': <String, dynamic>{},
  });
}

Augustin R · Accepted Answer · 2020-01-10 08:28:03Z

0

You need to cast T as BaseResponse :

data = (T as BaseResponse).fromJson(json['data']);

answered Jan 10, 2020 at 8:28

Augustin R

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4 Comments

Jainam Jhaveri Over a year ago

We get following error:: ``` main.dart:9:32: Error: The method 'fromJson' isn't defined for the class 'BaseResponse'. - 'BaseResponse' is from 'main.dart'. data = (T as BaseResponse).fromJson(json['data']); ^^^^^^^^ Error: Compilation failed. ```

Augustin R Over a year ago

Can you try to use factory keyword before BaseResponse.fromJson(Map<String, dynamic> json);?

Rahul sharma Over a year ago

@AugustinR using factory will also not work

Thang Duong Aug 28 at 8:40

Use pub.dev/packages/json_factory_generator

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