draggable_rect.dart

import 'package:flutter/material.dart';

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


class MyApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) => MaterialApp(home: MyHomePage());
}

class MyHomePage extends StatefulWidget {
  @override
  _MyHomePageState createState() => _MyHomePageState();
}

class _MyHomePageState extends State<MyHomePage>
    with SingleTickerProviderStateMixin {
  Offset position = Offset(20, 20);
  Velocity velocity = Velocity.zero;

  bool _simulating = true;
  double _friction = 1000;
  double _time = 0;
  double _dt = 0;

  @override
  void initState() {
    this.createTicker((elapsed) {
      if (_simulating) {
        setState(
          () {
            //delta time calculation
            double _newTime = elapsed.inMicroseconds / 1e6;
            _dt = _newTime - _time;
            _time = _newTime;
            
            //euler integration
            if (velocity.pixelsPerSecond.distance < 10) return;
            velocity = Velocity(
              pixelsPerSecond: velocity.pixelsPerSecond -
                  velocity.pixelsPerSecond *
                      (_dt * _friction / velocity.pixelsPerSecond.distance),
            );
            position = position + velocity.pixelsPerSecond * _dt;
          },
        );
      }
    }).start();
    super.initState();
  }

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        title: Text('${(1 / _dt).toStringAsFixed(1)} fps'),
      ),
      body: LayoutBuilder(
        builder: (context, constraints) {
          return GestureDetector(
            onPanStart: (DragStartDetails details) {
              position = details.localPosition;
              velocity = Velocity.zero;
            },
            onPanUpdate: (DragUpdateDetails details) {
              position = details.localPosition;
              velocity = Velocity.zero;
            },
            onPanEnd: (DragEndDetails details) {
              velocity = details.velocity;
            },
            child: CustomPaint(
              painter: ShapePainter(position),
              child: Container(
                height: constraints.maxHeight,
                width: constraints.maxWidth,
              ),
            ), // This trailing comma makes auto-formatting nicer for build methods.
          );
        },
      ),
    );
  }
}

class ShapePainter extends CustomPainter {
  final Offset position;
  const ShapePainter(this.position);

  @override
  void paint(Canvas canvas, Size size) {
    canvas.drawRect(Rect.fromCircle(radius: 25, center: position),
        Paint()..color = Colors.pink);
  }

  @override
  bool shouldRepaint(CustomPainter oldDelegate) {
    return true;
  }
}