Swift 3 Extension for Adding Border on one of the sides (or for all sides).

CALayer+Border.swift

// Layer extension for adding border on one of the sides
extension CALayer {
  
  func addBorder(edge: UIRectEdge, color: UIColor, thickness: CGFloat) {
    
    let border = CALayer()
    
    switch edge {
    case UIRectEdge.top:
      border.frame = CGRect.init(x: 0, y: 0, width: frame.width, height: thickness)
      break
    case UIRectEdge.bottom:
      border.frame = CGRect.init(x: 0, y: frame.height - thickness, width: frame.width, height: thickness)
      break
    case UIRectEdge.left:
      border.frame = CGRect.init(x: 0, y: 0, width: thickness, height: frame.height)
      break
    case UIRectEdge.right:
      border.frame = CGRect.init(x: frame.width - thickness, y: 0, width: thickness, height: frame.height)
      break
    default:
      break
    }
    
    border.backgroundColor = color.cgColor
    self.addSublayer(border)
  }