A declaration which used when user wants to put stored property in extension

PropertyStoring.swift

protocol PropertyStoring {
    func getAssociatedObject<T>(_ key: UnsafeRawPointer, defaultValue: T) -> T
}
extension PropertyStoring {
    func getAssociatedObject<T>(_ key: UnsafeRawPointer, defaultValue: T) -> T {
        guard let value = objc_getAssociatedObject(self, key) as? T else {
            return defaultValue
        }
        return value
    }
}

// Example
extension UIButton: PropertyStoring {
    fileprivate CustomProperty {
        static var tapCount = 0
    }
    var tapCount: Int {
        get {
            return getAssociatedObject(&CustomProperty.tapCount, defaultValue: CustomProperty.tapCount)
        }
        set {
            return objc_setAssociatedObject(self, &CustomProperty.tapCount, newValue, .OBJC_ASSOCIATION_RETAIN )
        }
    }
    
}