Find the rootViewController to present alert globally

ShowAlert.swift

if var topController = UIApplication.shared.keyWindow?.rootViewController {
    while let presentedViewController = topController.presentedViewController {
        topController = presentedViewController
    }
    // topController should now be your topmost view controller
    // Dismissing an UIAlertController which is being displayed and setting the topController down one level
    if topController is UIAlertController {
        if let vc = topController.presentingViewController {
            topController = vc
            vc.dismiss(animated: true, completion: nil)
        }

    }

    // Now present the alert controller
    topController.present(self, animated: true, completion: nil)
}