class ThemeTestSubscriber

class ThemeTestSubscriber implements EventSubscriberInterface {
    
    /**
     * The used container.
     *
     * @todo This variable is never initialized, so we don't know what it is.
     *   See https://www.drupal.org/node/2721315
     */
    protected $container;
    
    /**
     * The current route match.
     *
     * @var \Drupal\Core\Routing\RouteMatchInterface
     */
    protected $currentRouteMatch;
    
    /**
     * The renderer.
     *
     * @var \Drupal\Core\Render\RendererInterface
     */
    protected $renderer;
    
    /**
     * Constructs a new ThemeTestSubscriber.
     *
     * @param \Drupal\Core\Routing\RouteMatchInterface $current_route_match
     *   The route match handler.
     * @param \Drupal\Core\Render\RendererInterface $renderer
     *   The renderer.
     */
    public function __construct(RouteMatchInterface $current_route_match, RendererInterface $renderer) {
        $this->currentRouteMatch = $current_route_match;
        $this->renderer = $renderer;
    }
    
    /**
     * Generates themed output early in a page request.
     *
     * @see \Drupal\system\Tests\Theme\ThemeEarlyInitializationTest::testRequestListener()
     */
    public function onRequest(RequestEvent $event) {
        if ($this->currentRouteMatch
            ->getRouteName() === '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.
            $more_link = [
                '#type' => 'more_link',
                '#url' => Url::fromRoute('user.page'),
                '#attributes' => [
                    'title' => 'Themed output generated in a KernelEvents::REQUEST listener',
                ],
            ];
            $GLOBALS['theme_test_output'] = $this->renderer
                ->renderPlain($more_link);
        }
    }
    
    /**
     * Ensures that the theme registry was not initialized.
     */
    public function onView(RequestEvent $event) {
        $current_route = $this->currentRouteMatch
            ->getRouteName();
        $entity_autocomplete_route = [
            'system.entity_autocomplete',
        ];
        if (in_array($current_route, $entity_autocomplete_route)) {
            if ($this->container
                ->initialized('theme.registry')) {
                throw new \Exception('registry initialized');
            }
        }
    }
    
    /**
     * {@inheritdoc}
     */
    public static function getSubscribedEvents() {
        $events[KernelEvents::REQUEST][] = [
            'onRequest',
        ];
        $events[KernelEvents::VIEW][] = [
            'onView',
            -1000,
        ];
        return $events;
    }

}