main.dart

  void main() => runApp(DemoApp());

  class DemoApp extends StatelessWidget {
    const DemoApp();

    @override
    Widget build(BuildContext context) {
      return MaterialApp(
          home: Scaffold(
              appBar: AppBar(
                title: const Text("Download Demo"),
              ),
              body: Center(
                child: const FetchDataPage(),
              )
          )
      );
    }
  }
  

api/json_models/post.dart

  part 'post.g.dart';

  @JsonSerializable()
  class Post {
    final int userId;
    final int id;
    final String title;
    final String body;
    const Post(this.userId, this.id, this.title, this.body);

    factory Post.fromJson(Map<String, dynamic> json) => _$PostFromJson(json);
    Map<String, dynamic> toJson() => _$PostToJson(this);
  }
  

api/json_models/todos.dart

  part "todo.g.dart";

  @JsonSerializable(explicitToJson: true)
  class TodoObject {
    final List<Todo> data;
    const TodoObject(this.data);

    factory TodoObject.fromJson(Map<String, dynamic> json) =>
        _$TodoObjectFromJson(json);

    Map<String, dynamic> toJson() => _$TodoObjectToJson(this);
  }

  @JsonSerializable()
  class Todo {
    final int userId;
    final int id;
    final String title;
    final bool completed;

    const Todo(this.userId, this.id, this.title, this.completed);

    factory Todo.fromJson(Map<String, dynamic> json) =>
        _$TodoFromJson(json);

    Map<String, dynamic> toJson() => _$TodoToJson(this);
  }
  

api/json_parsers/json_parsers.dart

  library json_parser;

  export 'post_parser.dart';
  export 'todo_parser.dart';
  export 'object_decoder.dart';

  abstract class JsonParser<T> {
    const JsonParser();

    Future<T> parseFromJson(String json);
  }
  

api/json_parsers/object_decoder.dart

  mixin ObjectDecoder<T> on JsonParser<T> {
    Map<String, dynamic> decodeJsonObject(String json) =>
        jsonDecode(json) as Map<String, dynamic>;
  }

  mixin ListDecoder<T> on JsonParser<T> {
    List<dynamic> decodeJsonList(String json) =>
        jsonDecode(json) as List<dynamic>;
  }
  

api/json_parsers/post_parser.dart

  class PostParser extends JsonParser<Post> with ObjectDecoder<Post> {
    const PostParser();

    @override
    Future<Post> parseFromJson(String json) async {
      final decoded = decodeJsonObject(json);
      return Post.fromJson(decoded);
    }
  }
  

api/json_parsers/todo_parser.dart

  class TodoParser extends JsonParser<List<Todo>> with ListDecoder<List<Todo>> {
    const TodoParser();

    @override
    Future<List<Todo>> parseFromJson(String json) async {
      return decodeJsonList(json)
          .map((value) => Todo.fromJson(value as Map<String, dynamic>))
          .toList();
    }
  }
  

api/http_client.dart

  class RequestREST {
    final String endpoint;
    final Map<String, String> data;
    const RequestREST({
      this.endpoint,
      this.data = const {},
    });

    /// HTTP dio client
    static final _client = Dio(
        BaseOptions(
          baseUrl: "https://jsonplaceholder.typicode.com/",
          connectTimeout: 3000, // 3 seconds
          receiveTimeout: 3000, // 3 seconds
          headers: <String, String>{"api-key": "add_one_if_needed", },
        )
    );

    Future<T> executeGet<T>(JsonParser<T> parser) async {
      final response = await _client.get<String>(endpoint);
      return parser.parseFromJson(response.data);
    }

    Future<T> executePost<T>(JsonParser<T> parser) async {
      final formData = FormData.fromMap(data);
      final response = await _client.post<String>(
        endpoint,
        data: formData,
      );

      return parser.parseFromJson(response.data);
    }
  }