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Event Subscribers

When to Use

When you need to react to events in Drupal — kernel events (request, response, exception), entity events (CRUD operations), or custom events.

Steps

  1. Create the event subscriber class 

    namespace Drupal\my_module\EventSubscriber;

use Symfony\Component\EventDispatcher\EventSubscriberInterface;
use Symfony\Component\HttpKernel\Event\RequestEvent;
use Symfony\Component\HttpKernel\KernelEvents;

class MySubscriber implements EventSubscriberInterface {

  public static function getSubscribedEvents(): array {
    return [
      KernelEvents::REQUEST => ['onRequest', 100],
      KernelEvents::RESPONSE => 'onResponse',
    ];
  }

  public function onRequest(RequestEvent $event): void {
    // React to request
  }

  public function onResponse($event): void {
    // React to response
  }
}

  2. Register as a service with event_subscriber tag 

    services:
  my_module.subscriber:
    class: Drupal\my_module\EventSubscriber\MySubscriber
    tags:
      - { name: event_subscriber }

  3. Alternative: Use autoconfigure (Drupal 11+) 

    services:
  _defaults:
    autoconfigure: true

  my_module.subscriber:
    class: Drupal\my_module\EventSubscriber\MySubscriber
    # No tags needed — autoconfigure detects EventSubscriberInterface

  4. Inject dependencies if needed 

    class MySubscriber implements EventSubscriberInterface {

  public function __construct(
    protected LoggerInterface $logger,
  ) {}

  public function onRequest(RequestEvent $event): void {
    $this->logger->info('Request received');
  }
}


services:
  my_module.subscriber:
    class: Drupal\my_module\EventSubscriber\MySubscriber
    arguments: ['@logger.channel.my_module']
    tags:
      - { name: event_subscriber }

Decision Points

At this step... If... Then...
Choosing event Need to modify request Listen to KernelEvents::REQUEST
Choosing event Need to modify response Listen to KernelEvents::RESPONSE
Choosing event Need entity CRUD hooks Listen to entity events (see entity.events service)
Setting priority Need to run before other subscribers Higher priority (positive number, e.g., 100)
Setting priority Need to run after other subscribers Lower priority (negative number, e.g., -100)

Pattern

Common Kernel Events: 

use Symfony\Component\HttpKernel\KernelEvents;

KernelEvents::REQUEST     // Before routing, controller execution
KernelEvents::CONTROLLER  // After controller resolver, before controller execution
KernelEvents::VIEW        // When controller returns non-Response
KernelEvents::RESPONSE    // Before sending response
KernelEvents::TERMINATE   // After sending response
KernelEvents::EXCEPTION   // When exception is thrown

Entity Events: 

use Drupal\Core\Entity\EntityTypeEvents;

EntityTypeEvents::CREATE   // New entity type created
EntityTypeEvents::UPDATE   // Entity type updated
EntityTypeEvents::DELETE   // Entity type deleted

For entity CRUD, use hooks (hook_entity_insert, hook_entity_update) or entity event subscribers via entity.events service.

Reference: /core/lib/Drupal/Core/EventSubscriber/ (core examples), Subscribe to and dispatch events

Common Mistakes

  • Returning array keys without event name — return [100 => 'onRequest'] is wrong; must be ['event.name' => ...]
  • Wrong priority order — Higher number = earlier execution (100 runs before 50)
  • Not stopping propagation when needed — Call $event->stopPropagation() if your subscriber should prevent others from running
  • Heavy processing in high-frequency events — KernelEvents::REQUEST runs on every request; keep it fast
  • Assuming event objects are mutable — Some events are immutable; check event class documentation
  • Forgetting to clear cache — New subscribers require cache rebuild

See Also