UICollectionViewCellDropShadow.swift

func collectionView(_ collectionView: UICollectionView, cellForItemAt indexPath: IndexPath) -> UICollectionViewCell {
        
            let cell: CustomCollectionViewCell = collectionView.dequeueReusableCell(forIndexPath: indexPath)
            ...
            //add shadow
            cell.applyShadow()
            ....
            return cell
        
}

extension UICollectionViewCell{
    func applyShadow(cornerRadius: CGFloat = 8, shadowColor: UIColor = UIColor.black.withAlphaComponent(0.6 ), shadowOffset: CGSize = .zero, shadowRadius: CGFloat = 4){
        let cell = self
        cell.contentView.layer.masksToBounds = true//mask contentview layer
        cell.layer.cornerRadius = cornerRadius
        cell.layer.borderWidth = 0.0
        cell.layer.shadowColor = shadowColor.cgColor
        cell.layer.shadowOffset = shadowOffset
        cell.layer.shadowRadius = shadowRadius
        cell.layer.shadowOpacity = 1
        cell.layer.masksToBounds = false //main thing is to unmask cell layer
    }
}