如何在不使用阴影路径和添加层的情况下仅将阴影添加到UIView的顶部

import UIKit

class ShadowView: UIView {
    override init(frame: CGRect) {
        super.init(frame: frame)
        self.backgroundColor = UIColor.clear
    }
    
    required init?(coder aDecoder: NSCoder) {
        super.init(coder: aDecoder)
        self.backgroundColor = UIColor.clear
    }
    
    override func draw(_ rect: CGRect) {
        let shadowPath = UIBezierPath(rect: self.bounds)
        self.layer.shadowPath = shadowPath.cgPath
        self.layer.shadowColor = UIColor.black.cgColor
        self.layer.shadowOffset = CGSize(width: 0, height: 2)
        self.layer.shadowOpacity = 0.5
        self.layer.shadowRadius = 4
    }
}

let targetView = UIView(frame: CGRect(x: 0, y: 0, width: 200, height: 200))
// 设置目标UIView的属性...

let shadowView = ShadowView(frame: targetView.bounds)
targetView.addSubview(shadowView)