CustomPainter corner As decoration

cornerRectPaint.dart

class CustomDecorationPainter extends CustomPainter {
  final double padding;
  final double frameSFactor;

  CustomDecorationPainter({
    required this.padding,
    required this.frameSFactor,
  });
  @override
  void paint(Canvas canvas, Size size) {
    final frameHWidth = size.width * frameSFactor;

    Paint paint = Paint()
      ..color = Colors.redAccent
      ..strokeCap = StrokeCap.round
      ..style = PaintingStyle.fill
      ..strokeWidth = 4;

    /// background
    canvas.drawRRect(
        RRect.fromRectAndRadius(
          Rect.fromLTRB(0, 0, size.width, size.height),
          Radius.circular(18),
        ),
        paint);

    /// top left
    canvas.drawLine(
      Offset(0 + padding, padding),
      Offset(
        padding + frameHWidth,
        padding,
      ),
      paint..color = Colors.black,
    );

    canvas.drawLine(
      Offset(0 + padding, padding),
      Offset(
        padding,
        padding + frameHWidth,
      ),
      paint..color = Colors.black,
    );

    /// top Right
    canvas.drawLine(
      Offset(size.width - padding, padding),
      Offset(size.width - padding - frameHWidth, padding),
      paint..color = Colors.black,
    );
    canvas.drawLine(
      Offset(size.width - padding, padding),
      Offset(size.width - padding, padding + frameHWidth),
      paint..color = Colors.black,
    );

    /// Bottom Right
    canvas.drawLine(
      Offset(size.width - padding, size.height - padding),
      Offset(size.width - padding - frameHWidth, size.height - padding),
      paint..color = Colors.black,
    );
    canvas.drawLine(
      Offset(size.width - padding, size.height - padding),
      Offset(size.width - padding, size.height - padding - frameHWidth),
      paint..color = Colors.black,
    );

    /// Bottom Left
    canvas.drawLine(
      Offset(0 + padding, size.height - padding),
      Offset(0 + padding + frameHWidth, size.height - padding),
      paint..color = Colors.black,
    );
    canvas.drawLine(
      Offset(0 + padding, size.height - padding),
      Offset(0 + padding, size.height - padding - frameHWidth),
      paint..color = Colors.black,
    );
  }

  @override
  bool shouldRepaint(covariant CustomPainter oldDelegate) {
    return false;
  }
}