如何在集合视图单元格的内侧下边缘添加阴影？swift 3

import UIKit

class CustomCollectionViewCell: UICollectionViewCell {
    override init(frame: CGRect) {
        super.init(frame: frame)
        
        // 添加阴影视图
        let shadowView = UIView(frame: self.bounds)
        shadowView.backgroundColor = UIColor.clear
        shadowView.layer.shadowColor = UIColor.black.cgColor
        shadowView.layer.shadowOpacity = 0.5
        shadowView.layer.shadowOffset = CGSize(width: 0, height: 2)
        shadowView.layer.shadowRadius = 4
        self.contentView.addSubview(shadowView)
        
        // 添加其他内容视图
        // ...
    }
    
    required init?(coder aDecoder: NSCoder) {
        fatalError("init(coder:) has not been implemented")
    }
}

collectionView.register(CustomCollectionViewCell.self, forCellWithReuseIdentifier: "CustomCell")

func collectionView(_ collectionView: UICollectionView, cellForItemAt indexPath: IndexPath) -> UICollectionViewCell {
    let cell = collectionView.dequeueReusableCell(withReuseIdentifier: "CustomCell", for: indexPath) as! CustomCollectionViewCell
    
    // 配置单元格内容
    // ...
    
    return cell
}