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',
      ],
    ]);
  }

}