stored_properties_in_swift_extention_method2.swift

    struct AssociatedKeys {
        static var placeholderColor: String = "placeholderColor"
    }
    
    var  placeholderColor: UIColor? {
        set {
            objc_setAssociatedObject(self, &AssociatedKeys.placeholderColor, newValue, objc_AssociationPolicy.OBJC_ASSOCIATION_RETAIN)
        }
        
        get {
            guard let value = objc_getAssociatedObject(self, &AssociatedKeys.placeholderColor) as? UIColor else { return nil }
            return value
        }
    }