Squircle.swift

import UIKit

class Squircle: UIView {
  private var radius: CGFloat = 0
  private var smooth: CAShapeLayer?

  var smoothCorners: Bool = false {
    didSet {
      updateMaskIfNeeded()
    }
  }

  override func layoutSubviews() {
    super.layoutSubviews()

    if smoothCorners {
      updateMaskIfNeeded()
    }
  }

  func updateMaskIfNeeded() {
    if smoothCorners {
      if let maskBounds = smooth?.path?.boundingBoxOfPath {
        if !bounds.equalTo(maskBounds) || radius != layer.cornerRadius {
          updateMaskShape()
        }
      } else {
        let path = UIBezierPath(roundedRect: bounds, cornerRadius: layer.cornerRadius)
        let mask = CAShapeLayer()
        mask.path = path.cgPath
        layer.mask = mask
        
        smooth = mask
        radius = layer.cornerRadius

        layer.addObserver(self,
                          forKeyPath: NSStringFromSelector(#selector(getter: CALayer.cornerRadius)),
                          options: [],
                          context: nil)
      }
    } else if smooth != nil {
      layer.mask = nil
      smooth = nil

      layer.removeObserver(self, forKeyPath: NSStringFromSelector(#selector(getter: CALayer.cornerRadius)))
    }
  }

  func updateMaskShape() {
    smooth?.path = UIBezierPath(roundedRect: bounds, cornerRadius: layer.cornerRadius).cgPath
    radius = layer.cornerRadius
  }

  override func observeValue(forKeyPath keyPath: String?, of object: Any?, change: [NSKeyValueChangeKey : Any]?, context: UnsafeMutableRawPointer?) {
    if keyPath == NSStringFromSelector(#selector(getter: CALayer.cornerRadius)) {
      updateMaskIfNeeded()
    }
  }
}

I can't understand, what is the difference from cornerRadius?