PageViewController.swift

public final class PageViewController: UIViewController {

    // MARK: Initialization

    public required init(viewControllers: [UIViewController]) {
        self.viewControllers = viewControllers
        super.init(nibName: nil, bundle: nil)
    }

    public required init?(coder aDecoder: NSCoder) {
        fatalError("init(coder:) has not been implemented")
    }

    // MARK: Setting and Getting Attributes

    public var viewControllers: [UIViewController] {
        didSet {
            guard self.isViewLoaded else { return }
            self.updateViewControllersDisplay(animated: false)
            self.updateBackButtonDisplay(animated: false)
        }
    }

    /// The visible view controller.
    public var visibleViewController: UIViewController? {
        return self.pageViewController.viewControllers?.first
    }

    // MARK: View Life Cycle

    public override func viewDidLoad() {
        super.viewDidLoad()

        // PageViewController
        self.pageViewController.willMove(toParentViewController: self)
        self.addChildViewController(self.pageViewController)
        self.view.addSubview(self.pageViewController.view)
        self.pageViewController.view.translatesAutoresizingMaskIntoConstraints = false
        self.pageViewController.didMove(toParentViewController: self)

        // Subviews
        self.view.addSubview(self.backButton)

        // Display
        self.updateViewControllersDisplay(animated: false)
        self.updateBackButtonDisplay(animated: false)

        // Layout
        self.view.setNeedsUpdateConstraints()
    }

    // MARK: ChildViewControllers

    fileprivate lazy var pageViewController: UIPageViewController = {
        let controller = UIPageViewController(transitionStyle: .scroll, navigationOrientation: .vertical)
        controller.delegate = self
        controller.dataSource = self

        return controller
    }()

    // MARK: Subviews

    fileprivate lazy var backButton: UIButton = {
        let button = UIButton(type: .custom)
        button.translatesAutoresizingMaskIntoConstraints = false

        // Normal state

        // Action
        button.addTarget(self, action: #selector(backButtonDidTouchUpInside), for: .touchUpInside)

        return button
    }()

    // MARK: Layout

    private var didSetupConstraints = false

    public override func updateViewConstraints() {

        if !self.didSetupConstraints {

            NSLayoutConstraint.activate([
                self.pageViewController.view.leadingAnchor.constraint(equalTo: self.view.leadingAnchor),
                self.pageViewController.view.topAnchor.constraint(equalTo: self.view.topAnchor),
                self.pageViewController.view.trailingAnchor.constraint(equalTo: self.view.trailingAnchor),
                self.pageViewController.view.bottomAnchor.constraint(equalTo: self.view.bottomAnchor),

                self.backButton.trailingAnchor.constraint(equalTo: self.view.trailingAnchor, constant: -16.0),
                self.backButton.bottomAnchor.constraint(equalTo: self.bottomLayoutGuide.topAnchor, constant: -8.0)
                ])

            self.didSetupConstraints = true
        }

        super.updateViewConstraints()
    }

    // MARK: Action Selectors

    @objc private func backButtonDidTouchUpInside(sender: UIButton) {

        guard let visibleViewController = self.visibleViewController else {
            return
        }

        guard let viewController = self.pageViewController(self.pageViewController, viewControllerAfter: visibleViewController) else {
            return
        }

        self.pageViewController.setViewControllers([viewController], direction: .forward, animated: true, completion: nil)
        self.updateBackButtonDisplay(animated: false)
    }
}

// MARK: - Display

extension PageViewController {

    fileprivate func updateViewControllersDisplay(animated: Bool) {
        var viewControllers: [UIViewController]? = nil

        if let viewController = self.viewControllers.first {
            viewControllers = [viewController]
        }

        self.pageViewController.setViewControllers(viewControllers, direction: .reverse, animated: animated, completion: nil)
    }

    fileprivate func updateBackButtonDisplay(animated: Bool) {

        func setBackButtonAlpha(_ alpha: CGFloat) {
            guard animated else {
                self.backButton.alpha = alpha

                return
            }

            UIView.animate(withDuration: 0.3) {
                self.backButton.alpha = alpha
            }
        }

        guard let visibleViewController = self.visibleViewController else {
            setBackButtonAlpha(0.0)

            return
        }

        setBackButtonAlpha(self.pageViewController(self.pageViewController, viewControllerAfter: visibleViewController) == nil ? 0.0 : 1.0)
    }
}

// MARK: - UIPageViewControllerDelegate

extension PageViewController: UIPageViewControllerDelegate {

    public func pageViewController(_ pageViewController: UIPageViewController, didFinishAnimating finished: Bool, previousViewControllers: [UIViewController], transitionCompleted completed: Bool) {
        self.updateBackButtonDisplay(animated: true)
    }
}

// MARK: - UIPageViewControllerDataSource

extension PageViewController: UIPageViewControllerDataSource {

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

        guard let index = self.viewControllers.index(of: viewController) else {
            return nil
        }

        let indexAfter = self.viewControllers.index(after: index)

        guard (self.viewControllers.startIndex..<self.viewControllers.endIndex).contains(indexAfter) else {
            return nil
        }

        return self.viewControllers[indexAfter]
    }

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

        guard let index = self.viewControllers.index(of: viewController) else {
            return nil
        }

        let indexBefore = self.viewControllers.index(before: index)

        guard (self.viewControllers.startIndex..<self.viewControllers.endIndex).contains(indexBefore) else {
            return nil
        }

        return self.viewControllers[indexBefore]
    }
}