title: How to detect when a UIPageViewController's view has changed
body: <p>I have an app with a PageViewController and I want to make my own custom PageViewController indicator. Everything is all set up. The only thing I need to do now is to figure out how to tell when the view controller's view has changed and what view it is currently on.</p>
  <p>I have linked to <a href="https://vimeo.com/258716090" rel="nofollow noreferrer">this demo</a>. In the first part I am clicking on the buttons at the top to change the page and the slider indicating what the current page is working, however after that I swipe my finger to change the controller, and I want to make the slider indicator to move with the page.</p>