draggable_dartpad.dart

import 'package:flutter/material.dart';

void main() => runApp(const MaterialApp(home:Scaffold(body: Center(child:A()))));

class A extends StatelessWidget {
  const A({Key? key}) : super(key: key);

  @override
  Widget build(BuildContext context) {
    final from = [Object()];
    final to = [Object(), Object()];
    return Column(
      crossAxisAlignment: CrossAxisAlignment.center,
      children: [
        const SizedBox(height: 150),
        const Text('From'),
        const SizedBox(height: 15),
        DragTarget<int>(
          builder: (_, candidateData, __) {
            return B(items: from.map((item) => item.hashCode));
          },
        ),
        const SizedBox(height: 15),
        const Text('To'),
        const SizedBox(height: 15),
        DragTarget<int>(
          builder: (_, candidateData, __) {
            return B(items: to.map((item) => item.hashCode));
          },
        ),
      ],
    );
  }
}

class B extends StatelessWidget {
  final Iterable<int> items;

  const B({Key? key, required this.items}) : super(key: key);

  @override
  Widget build(BuildContext context) {
    return AnimatedCrossFade(
      duration: const Duration(milliseconds: 200),
      crossFadeState:
      items.isEmpty ? CrossFadeState.showFirst : CrossFadeState.showSecond,
      firstChild: const SizedBox(),
      secondChild: Wrap(
        children: [
          ...items.map((number) {
            final widget = Text(number.toString());
            return Draggable<int>(
              feedback: widget,
              childWhenDragging: Padding(
                padding: const EdgeInsets.only(left: 54, top: 15),
                child: Opacity(opacity: 0, child: Text(number.toString())),
              ),
              data: number,
              child: widget,
            );
          }),
        ],
      ),
    );
  }
}