How to draw an Arc in the middle of CustomPainter shape - flutter

Question

I am trying to draw the following shape using CustomPainter, I am looking for a way
to make this shape as one shape (union), so for example using Stack with Container and half circle shape is not desired.
I have managed to paint the base rectangular shape as the following image shows, is it possible to build on top of this code to draw a half a circle in the middle ?
Container(
height: 85,
color: Colors.blueGrey,
child: Stack(
children: [
CustomPaint(
size: Size(size.width, 85),
painter: MyCustomPainter(),
)
],
),
),
class MyCustomPainter extends CustomPainter {
#override
void paint(Canvas canvas, Size size) {
var paint = Paint()
..color = Colors.red
..style = PaintingStyle.fill;
var path = Path()..moveTo(0, 20);
path.quadraticBezierTo(0, 0, 0, size.height * 0.5);
path.quadraticBezierTo(0, 0, size.width * 0.2, 0);
path.quadraticBezierTo(size.width, 0, size.width * 0.8, 0);
path.quadraticBezierTo(size.width, 0, size.width, size.height * 0.5);
path.lineTo(size.width, size.height);
path.lineTo(0, size.height);
canvas.drawPath(path, paint);
}
#override
bool shouldRepaint(CustomPainter oldDelegate) {
return false;
}
}

Answer 1

While the question about using union operation, I am focusing on
canvas.drawPath(
Path.combine(PathOperation.union, path1, circlePath), paint);
The current paint takes full height and doesnt maintain the ration based on your desire result. I am using Rect on Path to demonstrating the union operation. Change the size/value according to your needs.
class MyCustomPainter extends CustomPainter {
#override
void paint(Canvas canvas, Size size) {
var paint = Paint()
..color = Colors.red
..style = PaintingStyle.fill;
final double base = size.height * .4;
const corner = Radius.circular(16);
final path1 = Path()
..addRRect(
RRect.fromLTRBAndCorners(0, base, size.width, size.height,
topLeft: corner, topRight: corner),
);
final center = Offset(size.width / 2, size.height / 2);
final radius = size.height / 2;
final circlePath = Path()
..addOval(Rect.fromCircle(center: center, radius: radius));
canvas.drawPath(
Path.combine(PathOperation.union, path1, circlePath), paint);
}
#override
bool shouldRepaint(CustomPainter oldDelegate) => false;
}
More about CustomPaint