Flutter implementation Drag-Follower widgets using Stack widget

main.dart

import 'package:flutter/material.dart';

final Color darkBlue = Color.fromARGB(255, 255, 255, 255);

void main() {
  runApp(MyApp());
}

class MyApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      theme: ThemeData.dark().copyWith(scaffoldBackgroundColor: darkBlue),
      debugShowCheckedModeBanner: false,
      home: Scaffold(
        backgroundColor: Colors.white,
        body: Center(
          child: DragFollower(),
        ),
      ),
    );
  }
}

class DragFollower extends StatefulWidget {
  @override
  _DragFollowerState createState() => _DragFollowerState();
}

class _DragFollowerState extends State<DragFollower> {

  final double squareSize = 72;
  final double distanceFromSquare = 8;
  Offset currentPosition = Offset(0,0);

  @override
  Widget build(BuildContext context) {
    return SafeArea(
      child: Scaffold(
        body: Stack(
          children: [
            Positioned(
              top: currentPosition.dy,
              left: currentPosition.dx,
              child: GestureDetector(
                onPanUpdate: (DragUpdateDetails details) {
                  currentPosition = details.globalPosition;
                  print("" + currentPosition.dx.toString() + " : " + currentPosition.dy.toString());
                  setState((){});
                },
                child: Container(
                  width: squareSize,
                  padding: EdgeInsets.all(16),
                  color: Colors.red,
                  child: Text("Red"),
                ),
              ),
            ),
            Positioned(
              top: currentPosition.dy,
              left: currentPosition.dx + squareSize + distanceFromSquare,
              child: Container(
                padding: EdgeInsets.all(16),
                color: Colors.blue,
                child: Text("Blue"),
              ),
            ),
          ],
        ),
      ),
    );
  }
}