CustomPageViewController.swift

import UIKit

class CustomPageViewController: UIPageViewController,UIPageViewControllerDataSource,UIPageViewControllerDelegate {
    
    fileprivate lazy var pages: [UIViewController] = {
        return [
            self.getViewController(withIdentifier: "firstVc"),
            self.getViewController(withIdentifier: "secondVc")
        ]
    }()
    
    fileprivate func getViewController(withIdentifier identifier: String) -> UIViewController
    {
        return UIStoryboard(name: "Main", bundle: nil).instantiateViewController(withIdentifier: identifier)
    }
    
    func pageViewController(_ pageViewController: UIPageViewController, viewControllerBefore viewController: UIViewController) -> UIViewController? {
        guard let viewControllerIndex = pages.index(of: viewController) else { return nil }
        let previousIndex = viewControllerIndex - 1
        guard previousIndex >= 0          else { return pages.last }
        guard pages.count > previousIndex else { return nil        }
        return pages[previousIndex]
    }
    
    func pageViewController(_ pageViewController: UIPageViewController, viewControllerAfter viewController: UIViewController) -> UIViewController? {
        guard let viewControllerIndex = pages.index(of: viewController) else { return nil }
        let nextIndex = viewControllerIndex + 1
        guard nextIndex < pages.count else { return pages.first }
        guard pages.count > nextIndex else { return nil         }
        return pages[nextIndex]
    }
    

    override func viewDidLoad() {
        super.viewDidLoad()

        self.dataSource = self
        self.delegate   = self
        if let firstVC = pages.first
        {
            setViewControllers([firstVC], direction: .forward, animated: true, completion: nil)
        }
        // Do any additional setup after loading the view.
    }

    
    override func viewDidLayoutSubviews() {
        //corrects scrollview frame to allow for full-screen view controller pages
        for subView in self.view.subviews {
            if subView is UIScrollView {
                subView.frame = self.view.bounds
            }
        }
        super.viewDidLayoutSubviews()
    }

    override func didReceiveMemoryWarning() {
        super.didReceiveMemoryWarning()
        // Dispose of any resources that can be recreated.
    }
    

    /*
    // MARK: - Navigation

    // In a storyboard-based application, you will often want to do a little preparation before navigation
    override func prepare(for segue: UIStoryboardSegue, sender: Any?) {
        // Get the new view controller using segue.destinationViewController.
        // Pass the selected object to the new view controller.
    }
    */

    func presentationCount(for pageViewController: UIPageViewController) -> Int {
        return pages.count
    }
    
    func presentationIndex(for pageViewController: UIPageViewController) -> Int {
        return 0
    }
    
}