extension ViewController: ThumbnailViewDataSource {
    func numberOfItems(in thumbnailView: ThumbnailView) -> Int {
        return images.count
    }

    func thumbnailView(_ thumbnailView: ThumbnailView, thumbnailForItemAt index: Int, size: CGSize, completion: @escaping (UIImage?) -> Void) {
        // load image
        completion(image)
    }
}