/drupal-at-your-fingertips

Events & Event Subscribers

Source: Drupal at Your Fingertips - events Author: Selwyn Polit

Quick reference for Drupal events and event subscribers with practical code examples.

Core Concept

Events allow modules to react to actions without modifying code. Event subscribers listen for dispatched events and execute custom logic when events occur.

Event Subscriber Structure

Basic event subscriber:

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() {
    return [
      KernelEvents::REQUEST => ['onRequest', 100],
    ];
  }

  public function onRequest(RequestEvent $event) {
    // React to the request event
    \Drupal::logger('my_module')->notice('Request received');
  }
}

Register in my_module.services.yml:

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

Event Subscriber with DI

Inject services:

use Drupal\Core\Session\AccountProxyInterface;
use Symfony\Component\DependencyInjection\ContainerInterface;

class MySubscriber implements EventSubscriberInterface {

  protected AccountProxyInterface $currentUser;

  public function __construct(AccountProxyInterface $current_user) {
    $this->currentUser = $current_user;
  }

  public static function create(ContainerInterface $container) {
    return new static(
      $container->get('current_user')
    );
  }

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

  public function onRequest(RequestEvent $event) {
    if ($this->currentUser->isAuthenticated()) {
      // Do something for logged-in users
    }
  }
}

Service definition with DI:

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

Event Priorities

Priority order (higher numbers run first):

public static function getSubscribedEvents() {
  return [
    KernelEvents::REQUEST => [
      ['onRequestEarly', 100],    // Runs first
      ['onRequestLate', -100],    // Runs last
    ],
  ];
}

Default priority is 0 if not specified.

Commonly Used Events

KernelEvents Examples

Modify request:

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

public static function getSubscribedEvents() {
  return [KernelEvents::REQUEST => ['onRequest']];
}

public function onRequest(RequestEvent $event) {
  $request = $event->getRequest();

  // Add custom request attribute
  $request->attributes->set('custom_data', 'value');

  // Redirect based on condition
  if ($some_condition) {
    $response = new RedirectResponse('/custom-page');
    $event->setResponse($response);
  }
}

Modify response:

use Symfony\Component\HttpKernel\Event\ResponseEvent;
use Symfony\Component\HttpKernel\KernelEvents;

public static function getSubscribedEvents() {
  return [KernelEvents::RESPONSE => ['onResponse']];
}

public function onResponse(ResponseEvent $event) {
  $response = $event->getResponse();

  // Add custom header
  $response->headers->set('X-Custom-Header', 'MyValue');

  // Modify content
  $content = $response->getContent();
  $content = str_replace('old', 'new', $content);
  $response->setContent($content);
}

Custom Events

Define custom event:

namespace Drupal\my_module\Event;

use Symfony\Contracts\EventDispatcher\Event;
use Drupal\node\NodeInterface;

class NodeSaveEvent extends Event {

  const EVENT_NAME = 'my_module.node_save';

  protected NodeInterface $node;

  public function __construct(NodeInterface $node) {
    $this->node = $node;
  }

  public function getNode() {
    return $this->node;
  }
}

Dispatch custom event:

use Symfony\Component\EventDispatcher\EventDispatcherInterface;

class MyService {

  protected EventDispatcherInterface $eventDispatcher;

  public function __construct(EventDispatcherInterface $event_dispatcher) {
    $this->eventDispatcher = $event_dispatcher;
  }

  public function saveNode(NodeInterface $node) {
    $node->save();

    // Dispatch custom event
    $event = new NodeSaveEvent($node);
    $this->eventDispatcher->dispatch($event, NodeSaveEvent::EVENT_NAME);
  }
}

Subscribe to custom event:

use Drupal\my_module\Event\NodeSaveEvent;

public static function getSubscribedEvents() {
  return [
    NodeSaveEvent::EVENT_NAME => ['onNodeSave'],
  ];
}

public function onNodeSave(NodeSaveEvent $event) {
  $node = $event->getNode();
  \Drupal::logger('my_module')->notice('Node @title saved', [
    '@title' => $node->getTitle(),
  ]);
}

Stop Propagation

Prevent other subscribers from executing:

public function onRequest(RequestEvent $event) {
  if ($some_condition) {
    // Stop other subscribers from running
    $event->stopPropagation();

    // Set custom response
    $response = new JsonResponse(['error' => 'Access denied'], 403);
    $event->setResponse($response);
  }
}

Key Guidelines

✅ Use high priorities - For early intervention (100+) ✅ Use low priorities - For final modifications (-100) ✅ Inject dependencies - Use DI in subscribers ✅ Tag services - Always add event_subscriber tag ✅ Stop propagation - When needed to prevent later execution ✅ Create custom events - For module-specific actions ✅ Use constants - Define event names as class constants

❌ Don't use static calls - Inject services instead ❌ Don't forget service tags - Won't work without tag ❌ Don't overuse high priorities - Can break other modules ❌ Don't modify immutable data - Some event properties are read-only ❌ Don't use events for hooks - Use hooks when available

Full documentation: https://drupalatyourfingertips.com/events