在swift 4中如何在UIView中创建三个边(右，左，下)的阴影？

import UIKit

class ShadowView: UIView {
    
    override func layoutSubviews() {
        super.layoutSubviews()
        
        // 设置阴影颜色
        self.layer.shadowColor = UIColor.black.cgColor
        
        // 设置阴影偏移量和透明度
        self.layer.shadowOffset = CGSize(width: 0, height: 2)
        self.layer.shadowOpacity = 0.5
        
        // 创建阴影路径
        let shadowPath = UIBezierPath()
        
        // 添加右边的阴影路径
        shadowPath.move(to: CGPoint(x: self.bounds.maxX, y: self.bounds.minY))
        shadowPath.addLine(to: CGPoint(x: self.bounds.maxX, y: self.bounds.maxY))
        
        // 添加左边的阴影路径
        shadowPath.move(to: CGPoint(x: self.bounds.minX, y: self.bounds.minY))
        shadowPath.addLine(to: CGPoint(x: self.bounds.minX, y: self.bounds.maxY))
        
        // 添加底部的阴影路径
        shadowPath.move(to: CGPoint(x: self.bounds.minX, y: self.bounds.maxY))
        shadowPath.addLine(to: CGPoint(x: self.bounds.maxX, y: self.bounds.maxY))
        
        // 设置阴影路径
        self.layer.shadowPath = shadowPath.cgPath
    }
}

let myView = ShadowView(frame: CGRect(x: 0, y: 0, width: 200, height: 200))