imageView.layer.mask = ({
                    () -> CAShapeLayer in
                    class Anon: CAShapeLayer {
                        override func observeValue(forKeyPath keyPath: String?, of object: Any?, change: [NSKeyValueChangeKey : Any]?, context: UnsafeMutableRawPointer?) {
                            switch (keyPath, change?[.oldKey], change?[.newKey]) {
                                case ("superlayer", let old, _) where old != nil:
                                    (old as? NSObject)?.removeObserver(self, forKeyPath: "bounds")
                                case ("superlayer", _, let new) where new != nil:
                                    (new as? NSObject)?.addObserver(self, forKeyPath: "bounds", options: [.new], context: nil)
                                case ("bounds", _, _), ("superlayer", _, _):
                                    let r = self.superlayer!.bounds.height / 8.5
                                    self.frame = self.superlayer!.bounds
                                    self.path = UIBezierPath(roundedRect: self.superlayer!.bounds, byRoundingCorners: [.topRight, .bottomRight], cornerRadii: CGSize(width: r, height: r)).cgPath
                                default: break
                            }
                        }
                    }
                    return Anon()
                })()
                imageView.layer.mask?.observeValue(forKeyPath: "superlayer", of: nil, change: [.newKey: imageView.layer as Any], context: nil)