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

DesignableShadow.swift

protocol DesignableShadow {

    var shadowColor: UIColor? { get set }
    var shadowOffset: CGSize { get set }
    var shadowRadius: CGFloat { get set }
    var shadowOpacity: Float { get set }

}

@IBDesignable extension UIView: DesignableShadow {

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

    @IBInspectable var shadowOffset: CGSize {
        get { return self.layer.shadowOffset }
        set { self.layer.shadowOffset = newValue }
    }

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

    @IBInspectable var shadowOpacity: Float {
        get { return self.layer.shadowOpacity }
        set { self.layer.shadowOpacity = newValue }
    }

}