main.dart

import 'package:flutter/material.dart';

void main() async {
  runApp(
    MaterialApp(
      debugShowCheckedModeBanner: false,
      home: Scaffold(
        body: Container(
          color: Color(0xFF15202D),
          child: SizedBox.expand(
            child: ListVisited(),
          ),
        ),
      ),
    ),
  );
}

class ListVisited extends StatefulWidget {
  ListVisited() : super();

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

class _ListVisitedState extends State<ListVisited> {
  final List<int> list = Iterable.generate(100, (i) => i).toList();

  @override
  Widget build(BuildContext context) {
    return SingleChildScrollView(
      child: Column(
        children: [
          for (var index in list)
            DragTarget<int>(
              onWillAccept: (data) {
                return true;
              },
              onAccept: (data) {
                setState(() {
                  var newPos = list.indexOf(index);
                  var dragIndex = list.indexOf(data);
                  var tmp = index;
                  list[newPos] = data;
                  list[dragIndex] = tmp;
                });
              },
              builder: (context, List<int?> candidate, rejected) {
                return Draggable<int>(
                  data: index,
                  child: Container(
                    height: 200,
                    width: 300,
                    child: Center(child: Text(index.toString())),
                    color: Colors.blue,
                  ),
                  childWhenDragging: Container(
                    height: 200,
                    width: 300,
                    color: Colors.red,
                  ),
                  feedback: Container(
                    height: 200,
                    width: 300,
                    color: Colors.green,
                  ),
                );
              },
            ),
        ],
      ),
    );
  }
}