ViewController.swift

// MARK: - UIPageViewControllerDataSource
    
func pageViewController(pageViewController: UIPageViewController, viewControllerBeforeViewController viewController: UIViewController) -> UIViewController? {
        
    let itemController = viewController as! PageItemController
        
    if itemController.itemIndex > 0 {
       return getItemController(itemController.itemIndex-1)
    }
        
    return nil
}
    
func pageViewController(pageViewController: UIPageViewController, viewControllerAfterViewController viewController: UIViewController) -> UIViewController? {
        
    let itemController = viewController as! PageItemController
        
    if itemController.itemIndex+1 < contentImages.count {
       return getItemController(itemController.itemIndex+1)
    }
        
    return nil
}
    
private func getItemController(itemIndex: Int) -> PageItemController? {
        
    if itemIndex < contentImages.count {
        let pageItemController = self.storyboard!.instantiateViewControllerWithIdentifier("ItemController") as! PageItemController
            pageItemController.itemIndex = itemIndex
            pageItemController.imageName = contentImages[itemIndex]
            return pageItemController
        }
        
    return nil
}