title: Get scroll position of UIPageViewController
body: <p>I am using a <code>UIPageViewController</code>, and I need to get the scroll position of the ViewController as the users swipe so I can partially fade some assets while the view is transitioning to the next <code>UIViewController</code>.</p>
  
  <p>The delegate and datasource methods of <code>UIPageViewController</code> don't seem to provide any access to this, and internally I'm assuming that the <code>UIPageViewController</code> must be using a scroll view somewhere, but it doesn't seem to directly subclass it so I'm not able to call </p>
  
  <pre><code>func scrollViewDidScroll(scrollView: UIScrollView) {
  
  }
  </code></pre>
  
  <p>I've seen some other posts suggestion to grab a reference to the <code>pageViewController!.view.subviews</code> and then the first index is a scrollView, but this seems <em>very</em> hacky. I'm wondering if there is a more standard way to handle this.</p>