class ParentVC: UIViewController {
  override func viewDidLoad() {
    NotificationCenter.default.addObserver(self,
                                           selector: #selector(ParentVC.handleModalDismissed),
                                           name: NSNotification.Name(rawValue: "modalIsDimissed"),
                                           object: nil)
  }

  func handleModalDismissed() {
    // Do something
  }
}

class ModalVC: UIViewController {
  func someFunc() {
    dismiss(animated: true) {
      NotificationCenter.default.post(name: NSNotification.Name(rawValue: "modalIsDimissed"), object: nil)
    }
  }
}