title: How to get a percentage of how much the UIview is on screen
body: <p>I am building an IOS app that starts off with a UIPageViewController when you first start using the app. I have been looking for a way to fade in an imageView depending on how far the user has swiped to the next screen. ie: When they have swiped half the screen alpha = 0.5 and when the screen has been completely swiped alpha is = 1.0?</p>
  
  <p>I have tried using 'animateWithDuration' but that seems to wait till the view is fully on screen and so it shows a laggy fade in. I thought about using gestureRecognizers but pageViewController already has them and therefore I would have to override those.</p>