func createPhotoGallery() -> RIGImageGalleryViewController {

    let urls: [URL] = [
          "https://placehold.it/1920x1080",
          "https://placehold.it/1080x1920",
          "https://placehold.it/350x150",
          "https://placehold.it/150x350",
        ].flatMap(URL.init(string:))

    let rigItems: [RIGImageGalleryItem] = urls.map { _ in
        RIGImageGalleryItem(placeholderImage: UIImage(named: "placeholder") ?? UIImage(),
                            isLoading: true)
    }

    let rigController = RIGImageGalleryViewController(images: rigItems)

    for (index, URL) in urls.enumerated() {
        let request = imageSession.dataTask(with: URLRequest(url: URL)) { [weak rigController] data, _, error in
            if let image = data.flatMap(UIImage.init), error == nil {
                rigController?.images[index].image = image
                rigController?.images[index].isLoading = false
            }
        }
        request.resume()
    }

    return rigController
}

@objc func showGallery(_ sender: UIButton) {
    let photoViewController = createPhotoGallery()
    photoViewController.dismissHandler = dismissPhotoViewer
    photoViewController.actionButtonHandler = actionButtonHandler
    photoViewController.actionButton = UIBarButtonItem(barButtonSystemItem: .action, target: nil, action: nil)
    photoViewController.traitCollectionChangeHandler = traitCollectionChangeHandler
    photoViewController.countUpdateHandler = updateCount
    let navigationController = UINavigationController(rootViewController: photoViewController)
    present(navigationController, animated: true, completion: nil)
}

func dismissPhotoViewer(_ :RIGImageGalleryViewController) {
    dismiss(animated: true, completion: nil)
}

func actionButtonHandler(_: RIGImageGalleryViewController, galleryItem: RIGImageGalleryItem) {
}

func updateCount(_ gallery: RIGImageGalleryViewController, position: Int, total: Int) {
    gallery.countLabel.text = "\(position + 1) of \(total)"
}

func traitCollectionChangeHandler(_ photoView: RIGImageGalleryViewController) {
    let isPhone = UITraitCollection(userInterfaceIdiom: .phone)
    let isCompact = UITraitCollection(verticalSizeClass: .compact)
    let allTraits = UITraitCollection(traitsFrom: [isPhone, isCompact])
    photoView.doneButton = photoView.traitCollection.containsTraits(in: allTraits) ? nil : UIBarButtonItem(barButtonSystemItem: .done, target: nil, action: nil)
}