main.dart

import 'dart:async';

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> {
  double _value = 0.0;

  late Timer _timer;

  void _startAnimation() {
    _timer = Timer.periodic(const Duration(milliseconds: 10), (_) {
      setState(() => _value += 0.1);
      if (_value >= 1.0) {
        _timer.cancel();
      }
    });
  }

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      body: Stack(
        children: [
          CustomPaint(size: Size.infinite, painter: _GridPainter()),
          Positioned(
            left: 20,
            top: 20,
            child: Opacity(
              opacity: _value.clamp(0.0, 1.0),
              child: Container(width: 60, height: 60, color: Colors.blue),
            ),
          ),
        ],
      ),
      floatingActionButton: FloatingActionButton(onPressed: _startAnimation),
    );
  }
}

/// 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;
}