Warning in UIView due to transform-only layer

I want to draw shadow around MyCustomView which is created in this way:

func tableView(_ tableView: UITableView, viewForHeaderInSection section: Int) -> UIView? { let header = MyCustomView() header.contentView.dropShadow() // tried also header.dropShadow() return header } 

And here are definitions:

class MyCustomView: UIView { //... var contentView = UIView() init() { super.init(frame: CGRect( x: CGFloat(0), y: CGFloat(0), width: CGFloat(200), height: CGFloat(200))) //... let marginGuide = self.layoutMarginsGuide contentView.addSubview(someLabel) contentView.addSubview(anotherLabel) // Background colors self.backgroundColor = UIColor.init(hex: "EBEBF1") contentView.backgroundColor = .white // translatesAutoresizingMaskIntoConstraints contentView.translatesAutoresizingMaskIntoConstraints = false // contentView constraints contentView.topAnchor.constraint( equalTo: marginGuide.topAnchor, constant: CGFloat(0)).isActive = true contentView.bottomAnchor.constraint( equalTo: marginGuide.bottomAnchor, constant: marginSize).isActive = true contentView.trailingAnchor.constraint( equalTo: marginGuide.trailingAnchor, constant: -marginSize).isActive = true contentView.leadingAnchor.constraint( equalTo: marginGuide.leadingAnchor, constant: CGFloat(7)).isActive = true } } extension UIView { func dropShadow(scale: Bool = true) { self.layer.masksToBounds = false self.layer.shadowColor = UIColor.darkGray.cgColor self.layer.shadowOpacity = 0.5 self.layer.shadowOffset = CGSize(width: 0, height: -1) self.layer.shadowRadius = 3 self.layer.shadowPath = UIBezierPath(rect: self.bounds).cgPath self.layer.shouldRasterize = true self.layer.rasterizationScale = scale ? UIScreen.main.scale : 1 } } 

Instead of shadow I get warnings:

<CATransformLayer: 0x60400023e780> - changing property shadowColor in transform-only layer, will have no effect <CATransformLayer: 0x60400023e780> - changing property shadowOpacity in transform-only layer, will have no effect <CATransformLayer: 0x60400023e780> - changing property shadowOffset in transform-only layer, will have no effect <CATransformLayer: 0x60400023e780> - changing property shadowPath in transform-only layer, will have no effect <CATransformLayer: 0x60400023e780> - changing property shouldRasterize in transform-only layer, will have no effect <CATransformLayer: 0x60400023e780> - changing property rasterizationScale in transform-only layer, will have no effect <CATransformLayer: 0x60400023e780> - changing property shadowPath in transform-only layer, will have no effect

Do you have any idea why is this happening?