class SessionTestSubscriber

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

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

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

Drupal\session_test\EventSubscriber\SessionTestSubscriber

class SessionTestSubscriber implements EventSubscriberInterface {

  /*
   * Stores whether $_SESSION is empty at the beginning of the request.
   */
  protected $emptySession;

  /**
   * Set header for session testing.
   *
   * @param \Symfony\Component\HttpKernel\Event\GetResponseEvent $event
   *   The Event to process.
   */
  public function onKernelRequestSessionTest(GetResponseEvent $event) {
    $this->emptySession = intval(empty($_SESSION));
  }

  /**
   * Set header for session testing.
   *
   * @param \Symfony\Component\HttpKernel\Event\FilterResponseEvent $event
   *   The Event to process.
   */
  public function onKernelResponseSessionTest(FilterResponseEvent $event) {
    $event
      ->getResponse()->headers
      ->set('X-Session-Empty', $this->emptySession);
  }

  /**
   * Registers the methods in this class that should be listeners.
   *
   * @return array
   *   An array of event listener definitions.
   */
  static function getSubscribedEvents() {
    $events[KernelEvents::RESPONSE][] = array(
      'onKernelResponseSessionTest',
      300,
    );
    $events[KernelEvents::REQUEST][] = array(
      'onKernelRequestSessionTest',
      300,
    );
    return $events;
  }

}