BrokenPageVC.swift

final class ViewController: UIPageViewController {

    class Inner: UIViewController {
        var color: UIColor!
        var name: String!

        override func viewDidLoad() {
            view.backgroundColor = color
        }

        override var description: String {
            return name
        }
    }

    private var controllers: [UIViewController] = []

    override init(transitionStyle style: UIPageViewController.TransitionStyle, navigationOrientation: UIPageViewController.NavigationOrientation, options: [UIPageViewController.OptionsKey : Any]? = nil) {
        super.init(
            transitionStyle: .scroll,
            navigationOrientation: .horizontal,
            options: options
        )
    }

    required init?(coder: NSCoder) {
        super.init(
            transitionStyle: .scroll,
            navigationOrientation: .horizontal,
            options: nil
        )
    }

    override func viewDidLoad() {
        super.viewDidLoad()

        let colors: [(UIColor, String)] = [(.red, "R"), (.green, "G"), (.blue, "B")]
        controllers = colors.map { color in
            let c = Inner()
            c.color = color.0
            c.name = color.1
            return c
        }

        setViewControllers([controllers[0]],
                           direction: .forward,
                           animated: false,
                           completion: nil)

        dataSource = self
        delegate = self
    }
}

extension ViewController: UIPageViewControllerDelegate {
    func pageViewController(_ pageViewController: UIPageViewController, didFinishAnimating finished: Bool, previousViewControllers: [UIViewController], transitionCompleted completed: Bool) {
        print(pageViewController.viewControllers!.first)
    }
}

extension ViewController: UIPageViewControllerDataSource {

    func pageViewController(_ pageViewController: UIPageViewController, viewControllerBefore viewController: UIViewController) -> UIViewController? {
        if let index = controllers.firstIndex(of: viewController),
            index > 0 {
            return controllers[index-1]
        } else {
            return nil
        }
    }

    func pageViewController(_ pageViewController: UIPageViewController, viewControllerAfter viewController: UIViewController) -> UIViewController? {
        if let index = controllers.firstIndex(of: viewController),
            index < controllers.count-1 {
            return controllers[index+1]
        } else {
            return nil
        }
    }
}