final class ViewController: UIPageViewController {
    
    private var controllers: [UIViewController] = []
    
    override func viewDidLoad() {
        super.viewDidLoad()
        
        let colors: [UIColor] = [.red, .orange, .green, .cyan]
        controllers = colors.map { color in
            let c = UIViewController()
            c.view.backgroundColor = color
            return c
        }
        
        setViewControllers([controllers[0]],
                           direction: .forward,
                           animated: false,
                           completion: nil)
        
        dataSource = self
    }
}

extension ViewController: UIPageViewControllerDataSource {
    
    /// 右にスワイプ（戻る）した場合のメソッド
    func pageViewController(_ pageViewController: UIPageViewController, viewControllerBefore viewController: UIViewController) -> UIViewController? {
        if let index = controllers.firstIndex(of: viewController),
            index > 0 {
            return controllers[index-1]
        } else {
            return nil
        }
    }
    
    /// 左にスワイプ（進む）した場合のメソッド
    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
        }
    }
    
    func presentationCount(for pageViewController: UIPageViewController) -> Int {
        return controllers.count
    }
}

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