How to control shadow spread and blur?

Here’s how to apply all 6 Sketch shadow properties to a UIView’s layer with near perfect accuracy:

extension CALayer {
  func applySketchShadow(
    color: UIColor = .black,
    alpha: Float = 0.5,
    x: CGFloat = 0,
    y: CGFloat = 2,
    blur: CGFloat = 4,
    spread: CGFloat = 0)
  {
    masksToBounds = false
    shadowColor = color.cgColor
    shadowOpacity = alpha
    shadowOffset = CGSize(width: x, height: y)
    shadowRadius = blur / 2.0
    if spread == 0 {
      shadowPath = nil
    } else {
      let dx = -spread
      let rect = bounds.insetBy(dx: dx, dy: dx)
      shadowPath = UIBezierPath(rect: rect).cgPath
    }
  }
}

Say we want to represent the following:

You can easily do this via:

myView.layer.applySketchShadow(
  color: .black, 
  alpha: 0.5, 
  x: 0, 
  y: 0, 
  blur: 4, 
  spread: 0)

or more succinctly:

myView.layer.applySketchShadow(y: 0)

Example:

Left: iPhone 8 UIView screenshot; right: Sketch rectangle.

Note:

• When using a non-zero spread, it hardcodes a path based on the bounds of the CALayer. If the layer’s bounds ever change, you’d want to call the applySketchShadow() method again.