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("Drag and Drop"),
          ),
          body: const Center(
            child: DragWidget(),
          )
        ),
      );
    }
  }
  

game_score.dart

  class GameScore with ChangeNotifier {
    int _score = 0;
    int _turn = 0;
    int _currentValue = _random();

    int get score => _score;
    int get turn => _turn;
    int get currentValue => _currentValue;

    static int _random() => Random().nextInt(100) + 1;

    void addPoints(int pts) {
      _score += pts;
      _turn++;
      _currentValue = _random();

      notifyListeners();
    }

  }
  

drag_widget.dart

  class DragWidget extends StatelessWidget {
    const DragWidget();

    @override
    Widget build(BuildContext context) {
      return Center(
        child: Column(
          mainAxisAlignment: MainAxisAlignment.center,
          children: <Widget>[
            Padding(
              padding: const EdgeInsets.only(bottom: 30),
              child: Consumer<GameScore>(
                builder: (context, game, _) {
                  return Text("Total: ${game.score}",
                    style: const  TextStyle(
                      color: Colors.blueAccent,
                      fontSize: 18
                    ),
                  );
                },
              ),
            ),

            Row(
              mainAxisAlignment: MainAxisAlignment.spaceAround,
              children: const <Widget>[
                EvenContainer(),
                NumberContainer(),
                OddContainer(),
              ],
            ),
          ],
        ),
      );
    }

  }
  

number_container.dart

  class NumberContainer extends StatelessWidget {
    const NumberContainer();

    @override
    Widget build(BuildContext context) {
      return Consumer<GameScore>(
        builder: (context, game, _) {
          return Draggable(
            data: game.currentValue,
            feedback: Container(
              width: 60,
              height: 60,
              decoration: BoxDecoration(
                borderRadius: BorderRadius.circular(60),
                color: Colors.black26,
              ),
              child: Center(
                child: Text("${game.currentValue}",
                  style: const TextStyle(
                      color: Colors.white,
                      decoration: TextDecoration.none,
                      fontSize: 15
                  ),
                ),
              ),
            ),
            child: Container(
              width: 60,
              height: 60,
              child: Center(
                child: Text("${game.currentValue}"),
              ),
            ),
          );
        },
      );
    }
  }
  

snack_message.dart

  mixin  SnackMessage {

    void showMessage(BuildContext context, String text) {
      Scaffold.of(context).showSnackBar(
        SnackBar(
          content: Text(text),
          duration: const Duration(milliseconds: 600),
        )
      );
    }

  }
  

odd_number.dart

  class OddContainer extends StatelessWidget with SnackMessage {
    const OddContainer();

    @override
    Widget build(BuildContext context) {

      return Container(
          width: 60,
          height: 60,
          decoration: BoxDecoration(
            borderRadius: BorderRadius.circular(10),
            color: Colors.lightGreen,
          ),
          child: DragTarget<int>(
            onAccept: (data) => _onAccept(context, data),
            onWillAccept: _willAccept,
            builder: (context, _, __) {
              return const Rectangle(text: "Odd");
            },
          )
      );
    }

    void _onAccept(BuildContext context, int data) {
      context.read<GameScore>().addPoints(data);
      showMessage(context, "Points: + $data");
    }

    bool _willAccept(int? data) => data != null && data % 2 != 0;

  }
  

even_number.dart

  class EvenContainer extends StatelessWidget with SnackMessage {
    const EvenContainer();

    @override
    Widget build(BuildContext context) {

      return Container(
          width: 60,
          height: 60,
          decoration: BoxDecoration(
            borderRadius: BorderRadius.circular(10),
            color: Colors.blueAccent,
          ),
          child: DragTarget<int>(
            onAccept: (data) => _onAccept(context, data),
            onWillAccept: _willAccept,
            builder: (context, _, __) {
              return const Rectangle(text: "Even");
            },
          )
      );
    }

    void _onAccept(BuildContext context, int data) {
      context.read<GameScore>().addPoints(data);
      showMessage(context, "Points: + $data");
    }

    bool _willAccept(int? data) => data != null && data % 2 == 0;

  }
  

number_box.dart

  class Rectangle extends StatelessWidget {
    final String text;
    const Rectangle({
      required this.text
    });

    @override
    Widget build(BuildContext context) {
      return Center(
        child: Text(text,
          style: TextStyle(
            color: Colors.white
          ),
        ),
      );
    }
  }