PageViewControllerArticleWholeShabang.swift

class PagedMediaViewController: UIPageViewController {

	// MARK: - Properties

	private var mediaItems: [MediaItem]
	private var currentIndex: Int


	// MARK: - Init

	init(mediaItems: [MediaItem], currentIndex: Int) {
		self.mediaItems = mediaItems
	  	self.currentIndex = currentIndex
		super.init(transitionStyle: .scroll, navigationOrientation: .horizontal, options: nil)
	}

	required init(coder: NSCoder) {
		fatalError("Use init(mediaItems: [MediaItem], currentIndex: Int)")
	}


	// MARK: - Lifecycle

	override func viewDidLoad() {
		super.viewDidLoad()
		dataSource = self
		delegate = self
	
		if let firstMediaVC = mediaViewControllerAtIndex(currentIndex) {
			let viewControllers = [firstMediaVC]
			setViewControllers(viewControllers, direction: .forward, animated: false, completion: nil)
		}
	}


	// MARK: - Helper Methods

	private func mediaViewControllerAtIndex(_ index: Int) -> MediaViewController? {
		// returns nil if the index is out of bounds, this way the app won't crash.
		// You can also put a print statement before you return.
		guard (0...mediaItems.count).contains(index) else { return nil }

		let item = mediaItems[index]

		switch item.mediaType {  
		case .video:  
			return VideoViewController(mediaItem: item)  
		case .photo:  
			return PhotoViewController(mediaItem: item)
		}
	}
}


// MARK: - DataSource Methods

extension PagedMediaViewController: UIPageViewControllerDataSource {

	pageViewController(_ pageViewController: UIPageViewController, viewControllerBefore viewController: UIViewController) -> UIViewController? {
		guard currentIndex > 0 else { return nil }
		return mediaViewControllerAtIndex(currentIndex - 1)
	}


	pageViewController(_ pageViewController: UIPageViewController, viewControllerAfter viewController: UIViewController) -> UIViewController? {
		guard currentIndex < mediaItems.count - 1 else { return nil }
		return mediaViewControllerAtIndex(currentIndex + 1)
	}
}


// MARK: - Delelgate Methods

extension PagedMediaViewController: UIPageViewControllerDelegate {
	
	func pageViewController(_ pageViewController: UIPageViewController, didFinishAnimating finished: Bool, previousViewControllers: [UIViewController], transitionCompleted completed: Bool) {
		guard
			let mediaVCs = pageViewController.viewControllers as? [MediaViewController],
			let currIndex = mediaItems.firstIndex(of: mediaVCs[0].mediaItem)
		else { return }

		currentIndex = currIndex
	}


	// If you want to specify the interface orientations you want to support, implement these as well:
	func pageViewControllerSupportedInterfaceOrientation(_ pageViewController: UIPageViewController) -> UIInterfaceOrientationMask {
		// example
		.allButUpsideDown
	}

	func pageViewControllerPreferredInterfaceOrientationForPresentation(_ pageViewController: UIPageViewController) -> UIInterfaceOrientationMask {
		// example
		.portrait
	}
}