PageViewController doesnt work on Swift 2.0 - swift

Question

I have application that works well on ios8. However after updating everything to ios9 I am having problem with nill values. I cant figure out what value is nil. Here is the code:
func resetToChosenPage(index: Int!) {
/* Getting the page View controller */
pageViewController = self.storyboard?.instantiateViewControllerWithIdentifier("PageViewController") as! UIPageViewController
self.pageViewController.dataSource = self
pageContentViewController = self.viewControllerAtIndex(index)
if pageContentViewController != nil {
self.pageViewController.setViewControllers([pageContentViewController], direction: UIPageViewControllerNavigationDirection.Forward, animated: true, completion: nil)
}
/* We are substracting 30 because we have a start again button whose height is 30*/
self.pageViewController.view.frame = CGRectMake(0, self.navigationController!.navigationBar.frame.height , self.view.frame.width, self.view.frame.height - 30)
self.addChildViewController(pageViewController)
self.view.addSubview(pageViewController.view)
self.pageViewController.didMoveToParentViewController(self)
}
func viewControllerAtIndex(index : Int) -> UIViewController? {
if((dataArray.count == 0) || (index >= dataArray.count)) {
return nil
}
let pageContentViewController = self.storyboard?.instantiateViewControllerWithIdentifier("SingleArticleViewController") as! SingleArticleViewController
self.delegate = pageContentViewController
pageContentViewController.delegate = self
// pageContentViewController.delegateWebView = self
pageContentViewController.singleArticleIndex = index
pageContentViewController.categoryID = categoryID
pageContentViewController.dateOfPub = dataArray[index].pubDate
pageContentViewController.category = dataArray[index].category
pageContentViewController.newsTitle = dataArray[index].pageTitle
pageContentViewController.videoLink = dataArray[index].videoLink
return pageContentViewController
}
One line self.pageViewController.SetViewControllers([pageContentViewController]... I have error "fatal error: unexpectedly found nil while unwrapping an Optional value". The fact is this code works well on ios8. I checked and could not find nil values. Moreover, I checked all identifiers of storyboard but they are correct. Guys, what can be a problem if this code works on ios8?

Answer 1

I suggest try changing the if clause that wraps the setViewControllers: call so that it explicitly checks for both the pageContentViewController and the pageViewController:
if let pageVC = self.pageViewController, let contentVC = self.pageContentViewController {
pageVC.setViewControllers([contentVC], direction: UIPageViewControllerNavigationDirection.Forward, animated: true, completion: nil)
}
As a general thing, I would try to avoid using Implicitly Unwrapped Optionals (like that Int! in your resetToChosenPage: method. As they lead to these 'unexpectedly found nil' errors.
Also, since your resetToChosenPage: method doesn't check if the pageViewController has already been instantiated, calling it multiple times will result in many instances of the PageViewController being instantiated and piled on top of each other. Which is probably not what you want.