CollectionViewGradient.swift

class CustomCollectionViewCell: UICollectionViewCell
{
    //Gradient to add in the cell
    private lazy var gradient: CAGradientLayer = {
        let gradientLayer = CAGradientLayer()
        gradientLayer.colors = [UIColor.darkGray.cgColor, UIColor.orange.cgColor]
        gradientLayer.startPoint = CGPoint(x: 0, y: 0)
        gradientLayer.endPoint = CGPoint(x: 1, y: 1)
        gradientLayer.frame = self.bounds
        return gradientLayer
    }()

    //Handling adding and removing gradient as selection state changes
    override var isSelected: Bool{
        didSet{
            if self.isSelected
            {
                self.layer.insertSublayer(self.gradient, at: 0)
            }
            else
            {
                self.gradient.removeFromSuperlayer()
            }
        }
    }
}