ViewController.m

#pragma mark UIPageViewControllerDataSource

- (UIViewController *)pageViewController:(UIPageViewController *)pageViewController viewControllerBeforeViewController:(UIViewController *)viewController
{
    PageItemController *itemController = (PageItemController *)viewController;
    
    if (itemController.itemIndex > 0)
    {
        return [self itemControllerForIndex:itemController.itemIndex-1];
    }
    
    return nil;
}

- (UIViewController *)pageViewController:(UIPageViewController *)pageViewController viewControllerAfterViewController:(UIViewController *)viewController
{
    PageItemController *itemController = (PageItemController *)viewController;
    
    if (itemController.itemIndex+1 < [_contentImages count])
    {
        return [self itemControllerForIndex:itemController.itemIndex+1];
    }
    
    return nil;
}

- (PageItemController *)itemControllerForIndex:(NSUInteger)itemIndex
{
    if (itemIndex < [_contentImages count])
    {
        PageItemController *pageItemController = [self.storyboard instantiateViewControllerWithIdentifier:@"ItemController"];
        pageItemController.itemIndex = itemIndex;
        pageItemController.imageName = _contentImages[itemIndex];
        return pageItemController;
    }
    
    return nil;
}