//

class PagesViewController: UIViewController {

    override func viewDidLoad() {
        super.viewDidLoad()

        // a UIView to hold the page view controller
        let pvcContainer = UIView()
        pvcContainer.backgroundColor = .gray

        [pvcContainer].forEach { v in
            v.translatesAutoresizingMaskIntoConstraints = false
            view.addSubview(v)
        }

        let g = view.safeAreaLayoutGuide
        NSLayoutConstraint.activate([

            pvcContainer.topAnchor.constraint(equalTo: g.topAnchor, constant: 100.0),
            pvcContainer.centerXAnchor.constraint(equalTo: g.centerXAnchor),
            pvcContainer.widthAnchor.constraint(equalToConstant: 200.0),
            pvcContainer.heightAnchor.constraint(equalTo: pvcContainer.widthAnchor, multiplier: 1.5),

        ])

        let pvc = MyPageViewController(transitionStyle: .scroll, navigationOrientation: .horizontal)
        addChild(pvc)

        pvc.view.translatesAutoresizingMaskIntoConstraints = false
        pvcContainer.addSubview(pvc.view)

        NSLayoutConstraint.activate([

            pvc.view.topAnchor.constraint(equalTo: pvcContainer.topAnchor),
            pvc.view.leadingAnchor.constraint(equalTo: pvcContainer.leadingAnchor),
            pvc.view.trailingAnchor.constraint(equalTo: pvcContainer.trailingAnchor),
            pvc.view.bottomAnchor.constraint(equalTo: pvcContainer.bottomAnchor),

        ])

        pvc.didMove(toParent: self)

    }

}

class MyPageViewController: UIPageViewController {

    // so we can get the index of the current page
    var currentIndex: Int {
        guard let vc = viewControllers?.first else { return 0 }
        return pageViewControllers.firstIndex(of: vc) ?? 0
    }

    let colors: [UIColor] = [
        .systemRed,
        .systemGreen,
        .systemBlue,
        .systemYellow,
        .green,
        .blue,
    ]

    var pageViewControllers: [UIViewController] = [UIViewController]()

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

    required init?(coder: NSCoder) {
        super.init(coder: coder)
    }

    override func viewDidLoad() {
        super.viewDidLoad()

        dataSource = self

        // instantiate all "pages"

        for i in 0..<colors.count {
            let vc = ExamplePageVC()
            vc.theLabel.text = "Page: \(i)"
            vc.view.backgroundColor = colors[i]
            pageViewControllers.append(vc)
        }

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

}

extension MyPageViewController: UIPageViewControllerDataSource {
    func pageViewController(_ pageViewController: UIPageViewController, viewControllerBefore viewController: UIViewController) -> UIViewController? {

        guard let viewControllerIndex = pageViewControllers.firstIndex(of: viewController) else { return nil }

        let previousIndex = viewControllerIndex - 1

        // if we are at the First controller,
        //  return nil
        guard previousIndex >= 0 else { return nil }

        return pageViewControllers[previousIndex]
    }

    func pageViewController(_ pageViewController: UIPageViewController, viewControllerAfter viewController: UIViewController) -> UIViewController? {

        guard let viewControllerIndex = pageViewControllers.firstIndex(of: viewController) else { return nil }

        let nextIndex = viewControllerIndex + 1

        // if we are at the Last controller,
        //  return nil
        guard nextIndex < pageViewControllers.count else { return nil }

        return pageViewControllers[nextIndex]
    }

}

class ExamplePageVC: UIViewController {

    let theLabel: UILabel = {
        let v = UILabel()
        v.translatesAutoresizingMaskIntoConstraints = false
        v.backgroundColor = UIColor(white: 0.9, alpha: 1.0)
        v.textAlignment = .center
        return v
    }()

    override func viewDidLoad() {
        super.viewDidLoad()
        view.backgroundColor = .blue

        view.addSubview(theLabel)
        NSLayoutConstraint.activate([
            theLabel.centerXAnchor.constraint(equalTo: view.centerXAnchor),
            theLabel.centerYAnchor.constraint(equalTo: view.centerYAnchor),
            theLabel.widthAnchor.constraint(equalTo: view.widthAnchor, multiplier: 0.8),
            theLabel.heightAnchor.constraint(equalTo: view.heightAnchor, multiplier: 0.8),
        ])

    }

}