// Show the ImageViewer with the first item
self.presentImageGallery(GalleryViewController(startIndex: 0, itemsDataSource: self))
// The GalleryItemsDataSource provides the items to show
extension ViewController: GalleryItemsDataSource {
    func itemCount() -> Int {
        return items.count
    }
    func provideGalleryItem(_ index: Int) -> GalleryItem {
        return items[index].galleryItem
    }
}