\events
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Overview of event dispatch and subscribing

Overview of event dispatch and subscribing

Introduction and terminology

Events are part of the Symfony framework: they allow for different components of the system to interact and communicate with each other. Each event has a unique string name. 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. For more on the general concept of events, see http://symfony.com/doc/current/components/event_dispatcher/introduction.html

Dispatching events

To dispatch an event, call the \Symfony\Component\EventDispatcher\EventDispatchInterface::dispatch() method on the 'event_dispatcher' service (see the @link container Services topic @endlink for more information about how to interact with services). The first argument is the unique event name, which you should normally define as a constant in a separate static class (see \Symfony\Component\HttpKernel\KernelEvents and \Drupal\Core\Config\ConfigEvents for examples). The second argument is a \Symfony\Component\EventDispatcher\Event object; normally you will need to extend this class, so that your event class can provide data to the event subscribers.

Registering event subscribers

Here are the steps to register an event subscriber:

  • Define a service in your module, tagged with 'event_subscriber' (see the @link container Services topic @endlink for instructions).
  • Define a class for your subscriber service that implements \Symfony\Component\EventDispatcher\EventSubscriberInterface
  • In your class, the getSubscribedEvents method returns a list of the events this class is subscribed to, and which methods on the class should be called for each one. Example:

  public static function getSubscribedEvents() {
    // Subscribe to kernel terminate with priority 100.
    $events[KernelEvents::TERMINATE][] = array('onTerminate', 100);
    // Subscribe to kernel request with default priority of 0.
    $events[KernelEvents::REQUEST][] = array('onRequest');
    return $events;
  }
  
  • Write the methods that respond to the events; each one receives the event object provided in the dispatch as its one argument. In the above example, you would need to write onTerminate() and onRequest() methods.

Note that in your getSubscribedEvents() method, you can optionally set the priority of your event subscriber (see terminate example above). Event subscribers with higher priority numbers get executed first; the default priority is zero. If two event subscribers for the same event have the same priority, the one defined in a module with a lower module weight will fire first. Subscribers defined in the same services file are fired in definition order. If order matters defining a priority is strongly advised instead of relying on these two tie breaker rules as they might change in a minor release.

Comments

mathieso’s picture

Should...

public function getSubscribedEvents() {

...be...

public static function getSubscribedEvents() {

jp.stacey’s picture

This looks like it's been fixed! Thanks for the feedback, btw: for anyone else reading this in future it might be better to open an issue in the core Drupal project queue, in the "documentation" component, for any API documentation bugs. It tends to get more eyeballs than comments here.