An @IBDesignable extension of the iOS class UIView to provide customization of borders within Interface Builder.

DesignableBorder.swift

protocol DesignableBorder {

    var cornerRadius: CGFloat { get set }
    var borderWidth: CGFloat { get set }
    var borderColor: UIColor? { get set }

}

@IBDesignable extension UIView: DesignableBorder {

    @IBInspectable var cornerRadius: CGFloat {
        get { return self.layer.cornerRadius }
        set { self.layer.cornerRadius = newValue }
    }

    @IBInspectable var borderWidth: CGFloat {
        get { return self.layer.borderWidth }
        set { self.layer.borderWidth = newValue }
    }

    @IBInspectable var borderColor: UIColor? {
        get {
            guard let cgColor = self.layer.borderColor else { return nil }
            return UIColor(cgColor: cgColor)
        }
        set { self.layer.borderColor = newValue?.cgColor }
    }

}