CollectionViewDidSelectItemAt.swift

// For one dimensional array
func collectionView(_ collectionView: UICollectionView, didSelectItemAt indexPath: IndexPath) {
	let item = mediaItems[indexPath.item]
	let pagedMediaViewController = PagedMediaController(mediaItem: item, currentIndex: indexPath.item)
	pagedMediaViewController.modalPresentationStyle = .overFullScreen
	present(pagedMediaViewController, animated: true)
}

//	In case you're dealing with a two dimensional array like I was dealing with since the collection view
//	had sections
func collectionView(_ collectionView: UICollectionView, didSelectItemAt indexPath: IndexPath) {
	let item = mediaItems[indexPath.section][indexPath.item]
	let joinedItems = mediaItems.flatMap { $0 }
	guard let currentIndexForPagedMediaVC = joinedItems.firstIndex(of: item) else { return }
	let pagedMediaViewController = PagedMediaController(mediaItem: item, currentIndex: currentIndexForPagedMediaVC)
	pagedMediaViewController.modalPresentationStyle = .overFullScreen
	present(pagedMediaViewController, animated: true)
}

mediaViewControllerAtIndex.swift

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)
	}
}

MediaViewControllerInit.swift

class MediaViewController: UIViewController {
	let mediaItem: MediaItem

	init(mediaItem: MediaItem) {
		self.mediaItem = mediaItem
	}

	required init(coder: NSCoder) {
		fatalError("Use init(mediaItem: MediaItem)")
	}

	// … More code
}

MediaViewControllerSubclasses.swift

class VideoViewController: MediaViewController {
	...
}

class PhotoViewController: MediaViewController {
	...
}

PagedMediaViewControllerDataSource.swift

extension PagedMediaViewController: UIPageViewControllerDataSource {

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


	pageViewController(_ pageViewController: UIPageViewController, viewControllerAfter viewController: UIViewController) -> UIViewController? {
		return mediaViewControllerAtIndex(currentIndex + 1)
	}
}

PagedMediaViewControllerDelegate.swift

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
	}
}

PagedMediaViewControllerInit.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)")
	}
}

PagedMediaViewControllerviewDidLoad.swift

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)
	}
}

TheWholeShabang.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? {
		return mediaViewControllerAtIndex(currentIndex - 1)
	}


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

UIPageViewControllerMediaItem.swift

struct MediaItem {
	let id: UUID
	let mediaType: MediaType
	let title: String?
	let mediaUrl: URL?
}

enum MediaType { case video, photo }