parsing - Flutter - How to parsed nested json to a class with generics?

Question

I'm wondering how can I parse a nested json to a class with generic types. My intention is to wrap responses from the backend (like loginRespose that contains a token) with a code and a message

I have

class BaseResponse<T>{
  int code;
  String message;
  T responseObject;

  BaseResponse.fromJson(Map<String, dynamic> parsedJson)
    : code = parsedJson['Code'],
      message = parsedJson['Message'],
      responseObject = T.fromJson(parsedJson['ResponseObject']); //This is what I'd like to do
}

Obviously the last line throws an error because T doesn't has a named constructor "fromJson". I tried adding some restrictions to the Type but I didn't find any solutions. Do you have any idea on how to pull this off?

Answer 1

You can't do such thing, at least not in flutter. As dart:mirror is disabled and there's no interface for classes constructors.

You'll have to take a different route.

I'll recommend using POO instead. You would here give up on deserializing responseObject from your BaseResponse. And then have subclass of BaseResponse handles this deserialization

Typically you'd have one subclass per type:

class IntResponse extends BaseResponse<int> {
  IntResponse.fromJson(Map<String, dynamic> json) : super._fromJson(json) {
    this.responseObject = int.parse(json["Hello"]);
  }
}

You can then hide this mess away by adding a custom factory constructor on BaseResponse to make it more convenient to use.

class BaseResponse<T> {
  int code;
  String message;
  T responseObject;

  BaseResponse._fromJson(Map<String, dynamic> parsedJson)
      : code = parsedJson['Code'],
        message = parsedJson['Message'];

  factory BaseResponse.fromJson(Map<String, dynamic> json) {
    if (T == int) {
      return IntResponse.fromJson(json) as BaseResponse<T>;
    }
    throw UnimplementedError();
  }
}

Then either instantiate the wanted type directly, or use the factory constructor :

final BaseResponse foo = BaseResponse.fromJson<int>({"Hello": "42", "Code": 42, "Message": "World"});