How to Register an Event Subscriber in Drupal 8

How to Register an Event Subscriber in Drupal 8Events in Drupal 8 allow for different components of the system to interact and communicate with each other. One system component dispatches the event at an appropriate time; many events are dispatched by Drupal core and the Symfony framework in every request. Other system components can register as event subscribers; when an event is dispatched, a method is called on each registered subscriber, allowing each one to react.

Most of the hooks from previous versions of Drupal were removed in Drupal 8 in favor of Events. Example: hook_init() or hook_boot() which now can be done by registering an event subscriber.

I will use the following structure for the example module:

my_event_subscriber/
  - my_event_subscriber.info.yml
  - my_event_subscriber.services.yml
  - src/
    - EventSubscriber/
      - MyEventSubscriber.php

Standard my_event_subscriber.info.yml file for the module:

name: Register an Event Subscriber
type: module
description: 'Example: How to Register an Event Subscriber in Drupal 8'
core: 8.x
package: Other

Define a service tagged with 'event_subscriber' in the my_event_subscriber.services.yml.

services:
  my_event_subscriber:
    class: '\Drupal\my_event_subscriber\EventSubscriber\MyEventSubscriber'
    tags:
      - { name: 'event_subscriber' }

src/EventSubscriber/MyEventSubscriber.php contains a class that implements \Symfony\Component\EventDispatcher\EventSubscriberInterface

/**
 * @file
 * Contains \Drupal\my_event_subscriber\EventSubscriber\MyEventSubscriber.
 */

namespace Drupal\my_event_subscriber\EventSubscriber;

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

/**
 * Event Subscriber MyEventSubscriber.
 */
class MyEventSubscriber implements EventSubscriberInterface {

  /**
   * Code that should be triggered on event specified 
   */
  public function onRespond(FilterResponseEvent $event) {
    // The RESPONSE event occurs once a response was created for replying to a request.
    // For example you could override or add extra HTTP headers in here
    $response = $event->getResponse();
    $response->headers->set('X-Custom-Header', 'MyValue');
  }

  /**
   * {@inheritdoc}
   */
  public static function getSubscribedEvents() {
    // For this example I am using KernelEvents constants (see below a full list).
    $events[KernelEvents::RESPONSE][] = ['onRespond'];
    return $events;
  }

}

Here is a list of KernelEvents constants:

KernelEvents::CONTROLLER; // The CONTROLLER event occurs once a controller was found for handling a request.
KernelEvents::EXCEPTION; // The EXCEPTION event occurs when an uncaught exception appears.
KernelEvents::FINISH_REQUEST; //The FINISH_REQUEST event occurs when a response was generated for a request.
KernelEvents::REQUEST; // The REQUEST event occurs at the very beginning of request dispatching.
KernelEvents::RESPONSE; // The RESPONSE event occurs once a response was created for replying to a request.
KernelEvents::TERMINATE; // The TERMINATE event occurs once a response was sent.
KernelEvents::VIEW; // The VIEW event occurs when the return value of a controller is not a Response instance.

There are more events available. More event constants.

How to dispatch an event

$dispatcher = \Drupal::service('event_dispatcher');
$dispatcher->dispatch($Event_Name, $Optional_Event_Object);

Useful links