private async void Button_Add_Last(object sender, RoutedEventArgs e)
        {
            BasemapGalleryItem item = await BasemapGalleryItem.CreateAsync(new Basemap());

            item.Name      = "With Thumbnail";
            item.Tooltip   = Guid.NewGuid().ToString();
            item.Thumbnail = new ArcGISRuntime.UI.RuntimeImage(new Uri("https://www.esri.com/content/dam/esrisites/en-us/home/homepage-tile-arcgis-collaboration.jpg"));
            Gallery.AvailableBasemaps.Add(item);

            BasemapGalleryItem item2 = await BasemapGalleryItem.CreateAsync(new Basemap());

            item2.Name = "Without Thumbnail";
            Gallery.AvailableBasemaps.Add(item2);
        }
 private void Gallery_BasemapSelected(object sender, BasemapGalleryItem e)
 {
     LastSelectedDateLabel.Text = DateTime.Now.ToLongTimeString();
 }