Extension property in Swift

NSObject+ExtensionProps.swift

extension NSObject {
    private struct AssociatedKeys {
        static var extensionProps: UInt8 = 0
    }
    private var extensionProps: NSObject? {
        get {
            return objc_getAssociatedObject(self, &AssociatedKeys.extensionProps) as? NSObject
        }
        set {
            objc_setAssociatedObject(self, &AssociatedKeys.extensionProps, newValue, objc_AssociationPolicy.OBJC_ASSOCIATION_RETAIN_NONATOMIC)
        }
    }
}