title: where does UIPageViewController cache the previous and next controllers?
body: <p>When UIPageViewController calls the datasource with the viewControllerBeforeViewController and viewControllerAfterViewController methods, it is obtaining the view controller that will be displayed when the user swipes again. Is there a writable property in which it keeps this data until it needs to use it? The reason I ask is that I want the user to be able to swipe to go forward or backward, or simply tap to advance to the next slide. However, if I tap to advance from the first slide to the second (advancing using the setViewControllers method), I cannot swipe backwards; there is no back controller to go back to. This affects only the second slide. So I need to be able to set the previous controller programmatically. </p>
  
  <p>Any help would be greatly appreciated.</p>
  
  <p>Thanks.</p>