main.dart

import 'package:flutter/material.dart';
import 'package:flutter/physics.dart';

void main() {
  runApp(const MaterialApp(home: AnimationPage()));
}

class AnimationPage extends StatefulWidget {
  const AnimationPage({super.key});

  @override
  State<AnimationPage> createState() => _AnimationPageState();
}

class _AnimationPageState extends State<AnimationPage>
    with TickerProviderStateMixin {
  late final AnimationController _controller;

  final _boxSize = 60.0;

  double _value = 20;

  @override
  void initState() {
    super.initState();
    _controller = AnimationController.unbounded(
      vsync: this,
      duration: Duration(seconds: 1),
    )..addListener(() {
      setState(() => _value = _controller.value);
      // y of the ground
      final ground = MediaQuery.of(context).size.height - 100 - _boxSize;

      if (_value > ground) {
        // box now touches the ground

        // retrieve the velocity of slightly weaker to opposite(upper) direction
        final velocity = _controller.velocity * -1 * 0.7;

        // stop previous animation
        _controller.stop();

        // start new animation
        _controller.animateWith(
          GravitySimulation(2000, ground - 1, 1000, velocity),
        );
      }
    });
  }

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      body: Stack(
        children: [
          CustomPaint(size: Size.infinite, painter: _GridPainter()),
          Positioned(
            left: 20,
            top: _value,
            child: Container(width: 60, height: 60, color: Colors.blue),
          ),
          Positioned(
            left: 0,
            right: 0,
            bottom: 0,
            child: Container(height: 100, color: Colors.brown),
          ),
        ],
      ),
      floatingActionButton: FloatingActionButton(
        onPressed: () {
          _controller.animateWith(GravitySimulation(2000, 20, 1000, 0));
        },
      ),
    );
  }
}

/// background grid
class _GridPainter extends CustomPainter {
  @override
  void paint(Canvas canvas, Size size) {
    final paint =
        Paint()
          ..color = Colors.grey.withOpacity(0.3)
          ..strokeWidth = 0.5;

    for (var i = 0.0; i < size.width; i += 10) {
      canvas.drawLine(Offset(i, 0), Offset(i, size.height), paint);
    }

    for (var i = 0.0; i < size.height; i += 10) {
      canvas.drawLine(Offset(0, i), Offset(size.width, i), paint);
    }
  }

  @override
  bool shouldRepaint(covariant CustomPainter oldDelegate) => false;
}