class EventInfoController

class EventInfoController extends ControllerBase {
    
    /**
     * Event dispatcher service.
     *
     * @var \Symfony\Component\EventDispatcher\EventDispatcherInterface
     */
    protected $eventDispatcher;
    
    /**
     * EventInfoController constructor.
     *
     * @param \Symfony\Component\EventDispatcher\EventDispatcherInterface $event_dispatcher
     *   Event dispatcher service.
     */
    public function __construct(EventDispatcherInterface $event_dispatcher) {
        $this->eventDispatcher = $event_dispatcher;
    }
    
    /**
     * {@inheritdoc}
     */
    public static function create(ContainerInterface $container) {
        return new static($container->get('event_dispatcher'));
    }
    
    /**
     * Builds the events overview page.
     *
     * @return array
     *   A render array as expected by the renderer.
     */
    public function eventList() {
        $headers = [
            'name' => [
                'data' => $this->t('Event Name'),
                'class' => 'visually-hidden',
            ],
            'callable' => $this->t('Callable'),
            'priority' => $this->t('Priority'),
        ];
        $event_listeners = $this->eventDispatcher
            ->getListeners();
        ksort($event_listeners);
        $rows = [];
        foreach ($event_listeners as $event_name => $listeners) {
            $rows[][] = [
                'data' => $event_name,
                'class' => [
                    'devel-event-name-header',
                ],
                'filter' => TRUE,
                'colspan' => '3',
                'header' => TRUE,
            ];
            foreach ($listeners as $listener) {
                $row['name'] = [
                    'data' => $event_name,
                    'class' => [
                        'visually-hidden',
                    ],
                    'filter' => TRUE,
                ];
                $row['class'] = [
                    'data' => $this->resolveCallableName($listener),
                ];
                $row['priority'] = [
                    'data' => $this->eventDispatcher
                        ->getListenerPriority($event_name, $listener),
                ];
                $rows[] = $row;
            }
        }
        $output['events'] = [
            '#type' => 'devel_table_filter',
            '#filter_label' => $this->t('Search'),
            '#filter_placeholder' => $this->t('Enter event name'),
            '#filter_description' => $this->t('Enter a part of the event name to filter by.'),
            '#header' => $headers,
            '#rows' => $rows,
            '#empty' => $this->t('No events found.'),
            '#attributes' => [
                'class' => [
                    'devel-event-list',
                ],
            ],
        ];
        return $output;
    }
    
    /**
     * Helper function for resolve callable name.
     *
     * @param mixed $callable
     *   The for which resolve the name. Can be either the name of a function
     *   stored in a string variable, or an object and the name of a method
     *   within the object.
     *
     * @return string
     *   The resolved callable name or an empty string.
     */
    protected function resolveCallableName($callable) {
        if (is_callable($callable, TRUE, $callable_name)) {
            return $callable_name;
        }
        return '';
    }

}