阴影不会出现在collectionViewcell中

import UIKit

class ShadowCollectionViewCell: UICollectionViewCell {
    override init(frame: CGRect) {
        super.init(frame: frame)
        self.backgroundColor = .clear
    }
    
    required init?(coder aDecoder: NSCoder) {
        fatalError("init(coder:) has not been implemented")
    }
    
    override func layoutSubviews() {
        super.layoutSubviews()
        
        // 添加阴影效果
        self.layer.shadowColor = UIColor.black.cgColor
        self.layer.shadowOffset = CGSize(width: 0, height: 2)
        self.layer.shadowOpacity = 0.5
        self.layer.shadowRadius = 4
        self.layer.masksToBounds = false
    }
}

// 在UICollectionView的dataSource方法中注册和复用ShadowCollectionViewCell
collectionView.register(ShadowCollectionViewCell.self, forCellWithReuseIdentifier: "ShadowCell")

// 在UICollectionView的dataSource方法中返回ShadowCollectionViewCell作为cell
func collectionView(_ collectionView: UICollectionView, cellForItemAt indexPath: IndexPath) -> UICollectionViewCell {
    let cell = collectionView.dequeueReusableCell(withReuseIdentifier: "ShadowCell", for: indexPath) as! ShadowCollectionViewCell
    // 配置cell的内容
    return cell
}