title: how to show view controller from UIPreviewAction in swift
body: <p>I do support 3D Touch, I did so that when using strong pulled the cell appeared controller and then the user picked it up, he was shown the action, there will be a button to show I want when the user clicks on it, the controller fully opens. How can I show this controller?</p>
  
  <pre><code> @available(iOS 9.0, *)
  override func previewActionItems() -&gt; [UIPreviewActionItem] {
      let showAction = UIPreviewAction(title: "Show", style: .Default) { [weak self] (action: UIPreviewAction, vc: UIViewController) -&gt; Void in
          guard let weakSelf = self else {
              return;
          }
  
          if let _popAction = weakSelf.popAction {
              _popAction()
          }
          self?.showViewController(vc, sender: nil)
          print("show city controller")
      }
  
      return [showAction]
  }
  </code></pre>