如何只填充自定义UIView矩形的一部分？

import UIKit

class CustomView: UIView {
    override func draw(_ rect: CGRect) {
        // 设置背景颜色
        backgroundColor = UIColor.white
        
        // 创建CAShapeLayer对象
        let shapeLayer = CAShapeLayer()
        layer.addSublayer(shapeLayer)
        
        // 创建路径
        let path = UIBezierPath(rect: CGRect(x: 20, y: 20, width: 200, height: 200))
        
        // 添加圆弧
        path.addArc(withCenter: CGPoint(x: 100, y: 100), radius: 50, startAngle: 0, endAngle: CGFloat.pi, clockwise: true)
        
        // 设置CAShapeLayer的path和fillColor
        shapeLayer.path = path.cgPath
        shapeLayer.fillColor = UIColor.red.cgColor
    }
}