class ThemeTestSubscriber

theme_test.services.yml in drupal/core/modules/system/tests/modules/theme_test/theme_test.services.yml

drupal/core/modules/system/tests/modules/theme_test/theme_test.services.yml

theme_test.subscriber in drupal/core/modules/system/tests/modules/theme_test/theme_test.services.yml

Drupal\theme_test\EventSubscriber\ThemeTestSubscriber

class ThemeTestSubscriber implements EventSubscriberInterface {

  /**
   * Generates themed output early in a page request.
   *
   * @see \Drupal\system\Tests\Theme\ThemeEarlyInitializationTest::testRequestListener()
   */
  public function onRequest(GetResponseEvent $event) {
    $request = $event
      ->getRequest();
    $current_path = $request->attributes
      ->get('system_path');
    if ($current_path == 'theme-test/request-listener') {

      // First, force the theme registry to be rebuilt on this page request.
      // This allows us to test a full initialization of the theme system in
      // the code below.
      drupal_theme_rebuild();

      // Next, initialize the theme system by storing themed text in a global
      // variable. We will use this later in
      // theme_test_request_listener_page_callback() to test that even when the
      // theme system is initialized this early, it is still capable of
      // returning output and theming the page as a whole.
      $GLOBALS['theme_test_output'] = theme('more_link', array(
        'url' => 'user',
        'title' => 'Themed output generated in a KernelEvents::REQUEST listener',
      ));
    }
    if (strpos($current_path, 'user/autocomplete') === 0) {

      // Register a fake registry loading callback. If it gets called by
      // theme_get_registry(), the registry has not been initialized yet.
      _theme_registry_callback('_theme_test_load_registry', array());
    }
  }

  /**
   * {@inheritdoc}
   */
  static function getSubscribedEvents() {
    $events[KernelEvents::REQUEST][] = array(
      'onRequest',
    );
    return $events;
  }

}