a.swift

private func drawBox(overlayLayer: CALayer, normalisedRect: CGRect) {
  let x = normalisedRect.origin.x * overlayLayer.frame.size.width
  let y = normalisedRect.origin.y * overlayLayer.frame.size.height
  let width = normalisedRect.width * overlayLayer.frame.size.width
  let height = normalisedRect.height * overlayLayer.frame.size.height

  let outline = CALayer()
  outline.frame = CGRect(x: x, y: y, width: width, height: height).scaleUp(scaleUp: 0.1)
  outline.borderWidth = 2.0
  outline.borderColor = UIColor.red.cgColor

  overlayLayer.addSublayer(outline)
}