after the UIViewController is inherited into UIViewControllerExtension class, we can call all new function from any UIViewController

LoginVC.swift

//no need to import SJProgressHUD
class LoginVC:UIViewController{
  func viewDidLoad(){
  }
  
  func buttonTapped(){
    //magic here... wa can call it any where on each UIViewController
    showWaiting("please wait...")
  }
}