main.dart

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

  class DemoApp extends StatelessWidget {
    final req = const RequestItem(
      url:  "https://jsonplaceholder.typicode.com/posts/10"
    );

    const DemoApp();

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

  }
  

item.dart

  class Item {
    final int id;
    final String title;
    Item({
      required this.id,
      required this.title
    });

    factory Item.fromJson(Map<String, dynamic> json) {
      return Item(
        id: json['id'] as int,
        title: json['title'] as String,
      );
    }
  }
  

base_request.dart

  abstract class HTTPRequest<T> {
    Future<T> execute();
  }
  

item_request.dart

  class RequestItem implements HTTPRequest<Item> {
    final String url;
    const RequestItem({ required this.url });

    Future<Item> execute() async {
      final response = await http.get(url);

      if (response.statusCode != 200) {
        throw http.ClientException("Oh darn!");
      }

      return _parseJson(response.body);
    }

    Item _parseJson(String response) =>
        Item.fromJson(jsonDecode(response));
  }
  

request_page.dart

  class HTTPWidget extends StatefulWidget {
    final HTTPRequest<Item> _request;
    const HTTPWidget(this._request);

    @override
    _HTTPWidgetState createState() => _HTTPWidgetState();
  }

  class _HTTPWidgetState extends State<HTTPWidget> {
    late final Future<Item> futureItems;

    @override
    void initState() {
      super.initState();
      futureItems = widget._request.execute();
    }

    @override
    Widget build(BuildContext context) {
      return FutureBuilder(
          future: futureItems,
          builder: (context, snapshot) {
            if (snapshot.hasError) {
              return const ErrorItemWidget();
            }

            if (snapshot.hasData) {
              return SuccessItemWidget(
                  snapshot.data
              );
            }

            return const Center(
                child: CircularProgressIndicator()
            );
          }
      );
    }
  }
  

success_widget.dart

  class SuccessItemWidget extends StatelessWidget {
    final Item? data;
    const SuccessItemWidget(this.data);

    @override
    Widget build(BuildContext context) {
      if (data != null) {
        return Text("${data!.id} - ${data!.title}");
      } else {
        return const Text("No data to show");
      }
    }
  }
  

error_widget.dart

  class ErrorItemWidget extends StatelessWidget {
    const ErrorItemWidget();

    @override
    Widget build(BuildContext context) {
      return const Icon(Icons.warning);
    }
  }