title: viewControllerAfterViewController: does not get called with UIPageViewController
body: <p>I ran into a quite strange behavior of UIPageViewController.</p>
  
  <p>I'm using UIPageViewController to display a set of ViewControllers, so I conform to UIPageViewControllerDataSource protocol, and implemented the following method:</p>
  
  <pre><code>- (UIViewController *)pageViewController:(UIPageViewController *)pageViewController viewControllerAfterViewController:(NHBufferRoomViewController *)viewController
  {
      NSLog(@"Got here!!");
      NSInteger newIndex = [_imageManager currentImageIdx] + 1;
      if (newIndex &gt;= [_imageManager count]) return nil;
  
      NSLog(@"%d", newIndex);
      NHBufferRoomViewController *controller = [self.storyboard instantiateViewControllerWithIdentifier:@"BufferRoom"];
      [controller updateImageAtIndex:newIndex];
      [[NSNotificationCenter defaultCenter] postNotificationName:kCurrentImageIndex object:[NSNumber numberWithInteger:newIndex]];
      return controller;
  }
  </code></pre>
  
  <p>But when I swipe the page view controller, sometimes, the view controller did move to the next page, but the <code>NSLog(@"Got here!!");</code> does not get called until I swipe again, this is quite strange. As far as I can understand, the page view controller should call this method when it needs to display a new page, right? But why the method is not called?</p>
  
  <p>I'm setting the transition type to scroll by the way. Thanks in advance!</p>