AutocompleteController.php

<?php

declare (strict_types=1);
namespace Drupal\form_test;

use Symfony\Component\HttpFoundation\JsonResponse;

/**
 * Defines a controller class with methods for autocompletion.
 */
class AutocompleteController {
    
    /**
     * Returns some autocompletion content with a slight delay.
     *
     * The delay is present so tests can make assertions on the "processing"
     * layout of autocompletion.
     *
     * @return \Symfony\Component\HttpFoundation\JsonResponse
     *   A JSON response.
     */
    public function delayed_autocomplete() {
        sleep(1);
        return new JsonResponse([
            [
                'value' => 'value',
                'label' => 'label',
            ],
        ]);
    }

}