Full source code of the Network Service

network_service.dart

import 'package:freezed_annotation/freezed_annotation.dart';
import 'dart:io';

part 'NetworkRequestBody.freezed.dart';
part 'NetworkResponse.freezed.dart';

class AccessTokenResponse {
  String? accessToken;
  AccessTokenResponse.fromJson(Map<String, dynamic> json) {
    accessToken = json['access_token'];
  }
}

@freezed
class NetworkRequestBody with _$NetworkRequestBody {
  const factory NetworkRequestBody.empty() = Empty;
  const factory NetworkRequestBody.json(Map<String, dynamic> data) = Json;
  const factory NetworkRequestBody.text(String data) = Text;
}

enum NetworkRequestType { GET, POST, PUT, PATCH, DELETE }

class NetworkRequest {
  const NetworkRequest({
    required this.type,
    required this.path,
    required this.data,
    this.queryParams,
    this.headers,
  });
  
  final NetworkRequestType type;
  final String path;
  final NetworkRequestBody data;
  final Map<String, dynamic>? queryParams;
  final Map<String, String>? headers;
}

@freezed
class NetworkResponse<Model> with _$NetworkResponse {
  const factory NetworkResponse.ok(Model data) = Ok;
  const factory NetworkResponse.invalidParameters(String message) = InvalidParameters;
  const factory NetworkResponse.noAuth(String message) = NoAuth; //401
  const factory NetworkResponse.noAccess(String message) = NoAccess; //403
  const factory NetworkResponse.badRequest(String message) = BadRequest; //400
  const factory NetworkResponse.notFound(String message) = NotFound; //404
  const factory NetworkResponse.conflict(String message) = Conflict; //409
  const factory NetworkResponse.noData(String message) = NoData; //500
}

class _PreparedNetworkRequest<Model> {
  const _PreparedNetworkRequest(
    this.request,
    this.parser,
    this.dio,
    this.headers,
    this.onSendProgress,
    this.onReceiveProgress,
  );
  final NetworkRequest request;
  final Model Function(Map<String, dynamic>) parser;
  final Dio dio;
  final Map<String, dynamic> headers;
  final ProgressCallback? onSendProgress;
  final ProgressCallback? onReceiveProgress;
}

Future<NetworkResponse<Model>> executeRequest<Model>(
  _PreparedNetworkRequest request,
) async {
  try {
    dynamic body = request.request.data.whenOrNull(
      json: (data) => data,
      text: (data) => data,
    );
    final response = await request.dio.request(
      request.request.path,
      data: body,
      queryParameters: request.request.queryParams,
      options: Dio.Options(
        method: request.request.type.name,
        headers: request.headers,
      ),
      onSendProgress: request.onSendProgress,
      onReceiveProgress: request.onReceiveProgress,
    );
    return NetworkResponse.ok(request.parser(response.data));
  } on Dio.DioError catch (error) {
    final errorText = error.toString();
    if (error.requestOptions.cancelToken!.isCancelled) {
      return NetworkResponse.noData(errorText);
    }
    switch (error.response?.statusCode) {
      case 400:
        return NetworkResponse.badRequest(errorText);
      case 401:
        return NetworkResponse.noAuth(errorText);
      case 403:
        return NetworkResponse.noAccess(errorText);
      case 404:
        return NetworkResponse.notFound(errorText);
      case 409:
        return NetworkResponse.conflict(errorText);
      default:
        return NetworkResponse.noData(errorText);
    }
  } catch (error) {
    return NetworkResponse.noData(error.toString());
  }
}

class NetworkService {
  NetworkService({
    required this.baseUrl,
    dioClient,
    httpHeaders,
  })  : this._dio = dioClient,
        this._headers = httpHeaders ?? {};
  Dio? _dio;
  final String baseUrl;
  Map<String, String> _headers;
  Future<Dio> _getDefaultDioClient() async {
    _headers['content-type'] = 'application/json; charset=utf-8';
    final dio = Dio()
      ..options.baseUrl = baseUrl
      ..options.headers = _headers
      ..options.connectTimeout = 5000 // 5 seconds
      ..options.receiveTimeout = 3000; // 3 seconds
      
    }
    return dio;
  }
  void addBasicAuth(String accessToken) {
    _headers['Authorization'] = 'Bearer $accessToken';
  }
  Future<NetworkResponse<Model>> execute<Model>(
    NetworkRequest request,
    Model Function(Map<String, dynamic>) parser, {
    ProgressCallback? onSendProgress = null,
    ProgressCallback? onReceiveProgress = null,
  }) async {
    if (_dio == null) {
      _dio = await _getDefaultDioClient();
    }
    final req = _PreparedNetworkRequest<Model>(
      request,
      parser,
      _dio!,
      {..._headers, ...(request.headers ?? {})},
      onSendProgress,
      onReceiveProgress,
    );
    final result = await compute(
      executeRequest<Model>,
      req,
    );
    return result;
  }
}

line 14 till line 19, what's happening there? also I'm getting an error after each "="
like : "The name 'Json' isn't a type and can't be used in a redirected constructor"

line 14 till line 19, what's happening there? also I'm getting an error after each "=" like : "The name 'Json' isn't a type and can't be used in a redirected constructor"

It's about freezed package which generate factory constructors initializers I think.
Same for NetworkResponse.

@AliJ91 yes, it's Freezed package annotation. Basically it creates states for your component. You can define them as classes as well.