title: QLPreviewController detect when QLPreviewItem Changed
body: <p>I have application that use QLPreviewController to show document from MS. Office. I use this controller to multiple files. I need to log which files that already viewed. But I can't find event that triggered when PreviewItem changed.</p>
  
  <p>I try to trigger event when GetPreviewItem called, But this method just called 1 time each preview item. So I can't use this.</p>
  
  <p>Anyone can help me to find how to detect QLPreviewItem changed?</p>