How can I create round box trail animation

I hope you you can get Idea from this code. In this code you can drag Container anywhere in the Screen and set it. And also check this Gesture Detector Overview for Gesture Detector detail.

import 'package:flutter/cupertino.dart';
import 'package:flutter/gestures.dart';
import 'package:flutter/material.dart';

class GestureDetectorPage extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
  return Scaffold(
    body: Container(
      color: Colors.grey[100],
      width: double.infinity,
      height: double.infinity,
      child: MainContent(),
      ),
    );
  }
}

class MainContent extends StatefulWidget {
 @override
_MainContentState createState() => _MainContentState();
}

class _MainContentState extends State<MainContent> {
  GlobalKey key = GlobalKey();

  String dragDirection = '';
  String startDXPoint = '50';
  String startDYPoint = '50';
  String dXPoint;
  String dYPoint;
  String velocity;

  @override
  Widget build(BuildContext context) {
    return GestureDetector(
       onHorizontalDragStart: _onHorizontalDragStartHandler,
       onVerticalDragStart: _onVerticalDragStartHandler,
       onHorizontalDragUpdate: _onDragUpdateHandler,
       onVerticalDragUpdate: _onDragUpdateHandler,
       onHorizontalDragEnd: _onDragEnd,
       onVerticalDragEnd: _onDragEnd,
       dragStartBehavior: DragStartBehavior.start, // default
       behavior: HitTestBehavior.translucent,
       child: Stack(
         children: [
           Positioned(
             left: double.parse(this.startDXPoint),
             top: double.parse(this.startDYPoint),
             child: Container(
               decoration: BoxDecoration(
               color: Colors.green,
               borderRadius: BorderRadius.circular(100)
             ),
             child: Center(
               child: Padding(
                 padding: const EdgeInsets.all(20),
                   child: Text('Draggable', style: TextStyle(fontSize: 14, color: Colors.white),),
               ),
             ),
           )
         ),
       ],
     ),
   );
 }

void _onHorizontalDragStartHandler(DragStartDetails details) {
  setState(() {
    this.dragDirection = "HORIZONTAL";
    this.startDXPoint = '${details.globalPosition.dx.floorToDouble()}';
    this.startDYPoint = '${details.globalPosition.dy.floorToDouble()}';
  });
}

/// Track starting point of a vertical gesture
void _onVerticalDragStartHandler(DragStartDetails details) {
  setState(() {
    this.dragDirection = "VERTICAL";
    this.startDXPoint = '${details.globalPosition.dx.floorToDouble()}';
    this.startDYPoint = '${details.globalPosition.dy.floorToDouble()}';
 });
}

void _onDragUpdateHandler(DragUpdateDetails details) {
  setState(() {
    this.dragDirection = "UPDATING";
    this.startDXPoint = '${details.globalPosition.dx.floorToDouble()}';
    this.startDYPoint = '${details.globalPosition.dy.floorToDouble()}';
  });
}

/// Track current point of a gesture
void _onHorizontalDragUpdateHandler(DragUpdateDetails details) {
  setState(() {
    this.dragDirection = "HORIZONTAL UPDATING";
    this.dXPoint = '${details.globalPosition.dx.floorToDouble()}';
    this.dYPoint = '${details.globalPosition.dy.floorToDouble()}';
    this.velocity = '';
  });
}

/// Track current point of a gesture
void _onVerticalDragUpdateHandler(DragUpdateDetails details) {
  setState(() {
    this.dragDirection = "VERTICAL UPDATING";
    this.dXPoint = '${details.globalPosition.dx.floorToDouble()}';
    this.dYPoint = '${details.globalPosition.dy.floorToDouble()}';
    this.velocity = '';
  });
}

/// What should be done at the end of the gesture ?
void _onDragEnd(DragEndDetails details) {
  double result = details.velocity.pixelsPerSecond.dx.abs().floorToDouble();
  setState(() {
    this.velocity = '$result';
  });
 }

}

Output of this code is here