PageViewController

PageViewController.swift

import UIKit
 
class PageViewController: UIPageViewController {
    private let firstItem: UIViewController
    private var pages: [UIViewController]
    
    private let pageChanged: (_ index: Int) -> Void

    let strongDelegate: UIPageViewControllerDelegate
    let strongDataSource: UIPageViewControllerDataSource
    
    init(_ firstItem: UIViewController, _ otherItems: [UIViewController], pageChanged: @escaping (_ index: Int) -> Void) {
        self.firstItem = firstItem
        self.pages = [firstItem] + otherItems
        self.pageChanged = pageChanged
        
        strongDelegate = PageViewControllerDelegate(pages: pages, pageChanged: pageChanged)
        strongDataSource = PageViewControllerDataSource(pages: pages)

        super.init(transitionStyle: .scroll, navigationOrientation: .horizontal)
        delegate = strongDelegate
        dataSource = strongDataSource
    }
    
    convenience init(_ firstItem: UIViewController, _ otherItems: UIViewController..., pageChanged: @escaping (_ index: Int) -> Void) {
        self.init(firstItem, otherItems, pageChanged: pageChanged)
    }
    
    required init?(coder: NSCoder) {
        fatalError("init(coder:) has not been implemented")
    }
    
    
    override func viewDidLoad() {
        super.viewDidLoad()
        setViewControllers([firstItem], direction: .forward, animated: false)
    }
    
    override func setViewControllers(_ viewControllers: [UIViewController]?, direction: UIPageViewController.NavigationDirection, animated: Bool, completion: ((Bool) -> Void)? = nil) {
        super.setViewControllers(viewControllers, direction: direction, animated: animated, completion: completion)
        guard let currentViewController = viewControllers?.first else { return }
        guard let index = pages.firstIndex(of: currentViewController) else { return }
        pageChanged(index)
    }
}

class PageViewControllerDataSource: NSObject, UIPageViewControllerDataSource {
    private let pages: [UIViewController]
    
    init(pages: [UIViewController]) {
        self.pages = pages
    }
    
    func pageViewController(_ pageViewController: UIPageViewController, viewControllerBefore viewController: UIViewController) -> UIViewController? {
        guard let viewControllerIndex = pages.firstIndex(of: viewController) else {
            return nil
        }
        let previousIndex = viewControllerIndex - 1
        guard previousIndex >= 0 else {
            return nil
        }
        guard pages.count > previousIndex else {
            return nil
        }
        
        return pages[previousIndex]
    }
    
    func pageViewController(_ pageViewController: UIPageViewController, viewControllerAfter viewController: UIViewController) -> UIViewController? {
        
        guard let viewControllerIndex = pages.firstIndex(of: viewController) else {
            return nil
        }
        let nextIndex = viewControllerIndex + 1
        let orderedViewControllersCount = pages.count
        guard orderedViewControllersCount != nextIndex else {
            return nil
        }
        guard orderedViewControllersCount > nextIndex else {
            return nil
        }
        
        return pages[nextIndex]
    }
}

class PageViewControllerDelegate: NSObject, UIPageViewControllerDelegate {
    private let pages: [UIViewController]
    private let pageChanged: (_ index: Int) -> Void
    
    init(pages: [UIViewController], pageChanged: @escaping (_ index: Int) -> Void) {
        self.pages = pages
        self.pageChanged = pageChanged
    }
    
    func pageViewController(_ pageViewController: UIPageViewController, didFinishAnimating finished: Bool, previousViewControllers: [UIViewController], transitionCompleted completed: Bool) {
        guard let currentViewController = pageViewController.viewControllers?.first else { return }
        guard let index = pages.firstIndex(of: currentViewController) else { return }
        pageChanged(index)
    }
}