mypainter.dart

class MyPainter extends CustomPainter {
  final Paint paint;

  MyPainter(this.paint);

  @override
  void paint(Canvas canvas, Size size) {
    canvas.drawRect(
      Rect.fromLTRB(0, 0, size.width, size.height),
      paint,
    );
  }

  @override
  bool shouldRepaint(MyPainter oldDelegate) {
    return oldDelegate.paint != paint;
  }
}