class ParentViewController: UIViewController {
  let button: UIButton()
  ...
  func onTapButton() {
    let popupVC = PopupViewController()
    popupVC.onDoneBlock = { [weak self] in
      self?.moveToNextView()
    }
  }
  
  func moveToNextView() {
    ...
  }
}

class PopupViewController: UIViewController {
  let closeButton: UIButton()
  
  var onDoneBlock: (() -> Void)? = nil
  
  func onTapButton() {
    self.dismiss(true, completion: self.onDoneBlock)
  }
}