import 'package:flutter/widgets.dart';

void main() {
  runApp(CustomPaint(painter: _FooPainter()));
}


class _FooPainter extends CustomPainter {
  @override
  void paint(Canvas canvas, Size size) {
    final rect = Offset.zero & size;
    canvas.drawRect(rect, Paint()..color = const Color(0xffff0000));

    const opacity = 0.5;
    canvas.saveLayer(
      rect,
      Paint()
        ..blendMode = BlendMode.lighten
        ..color = Color.fromARGB((opacity * 255).round(), 255, 255, 255),
    );

    canvas.drawCircle(rect.center, 80, Paint()..color = const Color(0xff00ff00));

    canvas.restore();
  }

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