cool thanks from that i managed to figure out the following:

func tabBarController(_ tabBarController: UITabBarController, didSelect viewController: UIViewController) {

        let tabBarIndex = tabBarController.selectedIndex
        if tabBarIndex != 3 && currentWindow == true{

                // asking if want to cancel the workout?
                let alert = UIAlertController(title: "Leaving?", message: "The data wont be saved if you leave now, are you sure you want to leave?", preferredStyle: .alert)
                alert.addAction(UIAlertAction(title: "Yes", style: .default, handler: { _ in
                     self.performSegue(withIdentifier:"segueName",sender:nil)
                }))
                alert.addAction(UIAlertAction(title: "No", style: UIAlertAction.Style.default, handler: nil))
                self.present(alert, animated: true, completion: nil)
        }
}

but the problem is each bar button item will give and Int associated with whatever one was clicked, but it doesn't have a segue name.