How to call a function from another class

In my rootviewcontroller i have three tabs below the navigation menu and each tab represents a list of products , in navigation menu i have a cart icon with a number on it to show the number of icons in the cart. each tab is a seprate viewcontroller and they all refrenced in rootviewcontroller like this :

override func viewControllers(for pagerTabStripController: PagerTabStripViewController) -> [UIViewController] {
    let layout = UICollectionViewFlowLayout()
    let firstTab = FirstTabController(collectionViewLayout: layout)
    let seccondTab = SeccondTabController(collectionViewLayout: layout)
    let thirdTab = ThirdTabController(collectionViewLayout: layout)

    return [firstTab ,seccondTab ,thirdTab ]
}

I have created a function in defaultviewcontroller to reset the cart number in navigtion menu but i don't know how to fire this function from firsttabController

This is my function in firsttabcontroller

func AddToCart(sender:UIButton) {
    let activityIndicatorView = NVActivityIndicatorView(frame: self.view.frame, type: .circleStrokeSpin, color: .red, padding: 170)
    self.view.addSubview(activityIndicatorView)
    activityIndicatorView.startAnimating()

    let parameters : [String: Any] = [
        "productid": sender.tag ,
        "quantityid": 1
    ]

    ApiServiceCart.sharedInstance.addProductToCartCatalog(parameters: parameters) { (success) in
        activityIndicatorView.removeFromSuperview()
        if success == true {
            // here i want to fire DefaultController().setupNavigationMenu()
            self.displayMessage(Title: MessageTitle.Successfull.rawValue, Message: Message.AddToCart_Successfull.rawValue)
        }
        else {
            self.displayMessage(Title: MessageTitle.Error.rawValue, Message: Message.AddToCart_Error.rawValue)
        }
    }
}

I tried protocol delegate but it didn't work

github.com/kavehnaseri/Protino/blob/master/Protino . the function i want to call from default controller is (setupNavBarButtonsWithCartOnLeft) wichi is in helper/extension+UIViewController

Finally found the solution ,I should have used NotificationCenter

So , i had a function called setupBarButtonMenu created in UIViewController extension and what i wanted to do was just to call it in the ParentViewController from ChildViewController . i added an observer in ParentViewController like this :

override func viewDidLoad() {
    super.viewDidLoad()
    NotificationCenter.default.addObserver(self, selector: #selector(self.setupNavBarButtonsWithCartOnLeft), name: NSNotification.Name(rawValue: "CartChanged"), object: nil)
}

and in another function from everywhere i can fire the funtion like this :

NotificationCenter.default.post(name: NSNotification.Name(rawValue: "CartChanged"), object: nil)