JqueryUiTestAssetsController.php

<?php

namespace Drupal\jqueryui_library_assets_test\Controller;

use Drupal\Core\Controller\ControllerBase;

/**
 * Controller for testing jQuery UI asset loading order.
 */
class JqueryUiTestAssetsController extends ControllerBase {
    
    /**
     * Provides a page that loads a library.
     *
     * @param string $library
     *   A pipe delimited list of library names.
     *
     * @return array
     *   The render array.
     */
    public function build($library) {
        // If there are pipes in $library, they are separating multiple library
        // names.
        if (strpos($library, '|') !== FALSE) {
            $library = explode('|', $library);
            $library = array_map(function ($item) {
                return "core/{$item}";
            }, $library);
        }
        else {
            $library = "core/{$library}";
        }
        return [
            '#markup' => 'I am a page for testing jQuery UI asset loading order.',
            '#attached' => [
                'library' => $library,
            ],
        ];
    }

}