main.dart

import 'package:flutter/material.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;

  late final Animation<double> _sizeAnimation;
  late final Animation<Color?> _colorAnimation;

  double _size = 0.0;
  Color? _color;

  @override
  void initState() {
    super.initState();
    _controller = AnimationController(
      vsync: this,
      duration: Duration(seconds: 1),
    )..addListener(() {
      setState(() {
        // obtain current size value of [Tween]
        _size = _sizeAnimation.value;

        // obtain current color value of [ColorTween]
        _color = _colorAnimation.value;
      });
    });

    // Tween for [double] value
    _sizeAnimation = _controller.drive(Tween(begin: 50.0, end: 150.0));

    // Tween for [Color] value
    _colorAnimation = _controller.drive(
      ColorTween(begin: Colors.blue, end: Colors.red),
    );
  }

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      body: Stack(
        children: [
          CustomPaint(size: Size.infinite, painter: _GridPainter()),
          Positioned(
            left: 20,
            top: 20,
            child: Container(width: _size, height: _size, color: _color),
          ),
        ],
      ),
      floatingActionButton: FloatingActionButton(
        onPressed: _controller.forward,
      ),
    );
  }
}

/// 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;
}