未为 Dart 中的类型定义 getter 'fromJson'

Question

我正在学习 flutter，尝试使用 Getx 的 GetConnect Custom configuration，

当尝试为默认请求设置 defaultDecoder 时，它抱怨：

The getter 'fromJson' isn't defined for the type 'Resp'.
Try importing the library that defines 'fromJson', correcting the name to the name of an existing getter, or defining a getter or field named 'fromJson'.dart"

如何修复这个错误？

这是我的代码：

import 'package:get/get.dart';

class AuthService extends GetConnect {
  @override
  void onInit() {
    // All request will pass to jsonEncode so CasesModel.fromJson()

    httpClient.defaultDecoder = Resp.fromJson;
    httpClient.baseUrl = 'http://localhost/app_api/auth';

   
  }

  Future<Response<Map<String, dynamic>>> getCaptcha(String mobile) =>
      get("get_captcha");
}

class Resp {
  final int Errcode;
  final String Errmsg;
  final Map<String, dynamic> Data;

  Resp(this.Errcode, this.Errmsg, this.Data);

  factory Resp.fromJson(Map<String, dynamic> json) {
    return Resp(json["errcode"], json["errmsg"], json["data"]);
  }
}