drag_target

import 'package:flutter/material.dart';

void main() => runApp(new MyApp());

class MyApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      home: Scaffold(
        body: App(),
      ),
    );
  }
}

class App extends StatefulWidget {
  @override
  _AppState createState() => _AppState();
}

class _AppState extends State<App> {
  Color caughtColor = Colors.grey;
  @override
  Widget build(BuildContext context) {
    return Stack(
      children: <Widget>[
        DragBox(Offset(0.0, 0.0), Colors.green),
        Positioned(
            left: 100,
            bottom: 0.0,
            child: DragTarget(onAccept: (Color color) {
              caughtColor = color;
            }, builder: (
              BuildContext context,
              List<dynamic> accepted,
              List<dynamic> rejected,
            ) {
              return Container(
                width: 200,
                height: 200,
                color: accepted.isEmpty ? caughtColor : Colors.grey.shade200,
              );
            }))
      ],
    );
  }
}

class DragBox extends StatefulWidget {
  final Offset initPos;
  final Color itemColor;

  DragBox(this.initPos, this.itemColor);
  @override
  _DragBoxState createState() => _DragBoxState();
}

class _DragBoxState extends State<DragBox> {
  Offset position = Offset(0.0, 0.0);

  @override
  void initState() {
    super.initState();
    position = widget.initPos;
  }

  @override
  Widget build(BuildContext context) {
    return Positioned(
      left: position.dx,
      top: position.dy,
      child: Draggable(
        data: widget.itemColor,
        child: Container(
          width: 100,
          height: 100,
          color: widget.itemColor,
        ),
        onDraggableCanceled: (velocity, offset) {
          setState(() {
            position = offset;
          });
        },
        feedback: Container(
          width: 120,
          height: 120,
          color: widget.itemColor.withOpacity(0.5),
        ),
      ),
    );
  }
}