title: Reliable way to track Page Index in a UIPageViewController - Swift
body: <p>I have a UIPageViewController (custom one) inside a Container located in a regular UIViewController. i need to be able to call an event with each Page Change but ONLY if it really did change and not only half way or anything of that sort.</p>
  
  <p>using:</p>
  
  <pre><code>func pageViewController(pageViewController: UIPageViewController, viewControllerBeforeViewController viewController: UIViewController) -&gt; UIViewController?
  </code></pre>
  
  <p>is unreliable and not called each time for some reason.</p>
  
  <p>if your answer contains anything about willTransitionToViewControllers or didFinishAnimating please elaborate and not just mention them, since i already know they exist but dont understand the proper way to use them.</p>
  
  <p>Thank you</p>