[SWIFT 4.0] New variable in extension for existed class

ExtensionVariables.swift

class Testing {

}

private var key: Void?
extension Testing {
    var title: String {
        get {
            guard let value = objc_getAssociatedObject(self, &key) as? String else {
                return ""
            }

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