Protocol extension in Swift 3 [duplicate]

Answer 1

As Martin R said you can't add stored properties to a class through class extensions. But you can use the objective C associated objects to do it via an extension

private var key: Void?

extension UIImageView {
    public var isFlipped: Bool? {
        get {
            return objc_getAssociatedObject(self, &key) as? Bool
        }

        set {
            objc_setAssociatedObject(self,
                                     &key, newValue,
                                     .OBJC_ASSOCIATION_RETAIN_NONATOMIC)
        }
    }
}