Argument labels '(_:, viewControllerAfterViewController:)' do not match any available overloads - ios

Question

Learning swift at the moment, any help would be appreciated - Updating code from swift 3 to swift 4 - getting the following error when building:
Argument labels '(_:, viewControllerAfterViewController:)' do not match any available overloads
I believe the code that is relevant to this error is in the goToNextVC func below:
func goToNextVC() {
let nextVC = pageViewController(self, viewControllerAfterViewController: viewControllers![0] )!
setViewControllers([nextVC], direction: .Forward, animated: true, completion: nil)
}
extension ViewController: UIPageViewControllerDataSource {
func pageViewController(_ pageViewController: UIPageViewController, viewControllerBefore viewController: UIViewController) -> UIViewController? {
// navigate right
switch viewController {
case cardsVC:
return profileVC
case profileVC:
return nil
case matchesVC:
return cardsVC
default:
return nil
}
}
full code:
// sets up page view controller
let pageController = ViewController(transitionStyle: .scroll, navigationOrientation: .horizontal, options: nil)
class ViewController: UIPageViewController {
// page view controllers here
let cardsVC: UIViewController = UIStoryboard(name: "Main", bundle: nil).instantiateViewController(withIdentifier: "CardsNavController")
let profileVC: UIViewController = UIStoryboard(name: "Main", bundle: nil).instantiateViewController(withIdentifier: "ProfileNavController")
let matchesVC: UIViewController! = UIStoryboard(name: "Main", bundle: nil).instantiateViewController(withIdentifier: "MatchesNavController")
override func viewDidLoad() {
super.viewDidLoad()
view.backgroundColor = UIColor.white
dataSource = self
// loads first view
setViewControllers([cardsVC], direction: .forward, animated: true, completion: nil)
}
// load left view
func goToNextVC() {
let nextVC = pageViewController(self, viewControllerAfterViewController: viewControllers![0] )!
setViewControllers([nextVC], direction: .Forward, animated: true, completion: nil)
}
// load right view
func goToPreviousVC() {
let previousVC = pageViewController(self, viewControllerBeforeViewController: viewControllers![0] )!
setViewControllers([previousVC], direction: .Reverse, animated: true, completion: nil)
}
override func didReceiveMemoryWarning() {
super.didReceiveMemoryWarning()
// Dispose of any resources that can be recreated.
}
}
// MARK - UIPageViewControllerDataSource
extension ViewController: UIPageViewControllerDataSource {
func pageViewController(_ pageViewController: UIPageViewController, viewControllerBefore viewController: UIViewController) -> UIViewController? {
// navigate right
switch viewController {
case cardsVC:
return profileVC
case profileVC:
return nil
case matchesVC:
return cardsVC
default:
return nil
}
}
// navigate left
func pageViewController(_ pageViewController: UIPageViewController, viewControllerAfter viewController: UIViewController) -> UIViewController? {
switch viewController {
case cardsVC:
return matchesVC
case profileVC:
return cardsVC
case matchesVC:
return nil
default:
return nil
}
}
}

Answer 1

This:
let nextVC = pageViewController(self, viewControllerAfterViewController: viewControllers![0] )!
needs to be:
let nextVC = pageViewController(self, viewControllerAfter: viewControllers![0] )!
And while you are at it, you need to get rid of the forced unwrap (the !). pageViewController(_:viewControllerAfter:) can return nil and the use of ! will cause your app to crash.
func goToNextVC() {
if let viewControllers = viewControllers {
if let nextVC = pageViewController(self, viewControllerAfter: viewControllers[0]) {
setViewControllers([nextVC], direction: .Forward, animated: true, completion: nil)
}
}
}