Doctrine Event Listeners and Subscribers

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Doctrine packages have a rich event system that fires events when almost anything happens inside the system. For you, this means that you can create arbitrary services and tell Doctrine to notify those objects whenever a certain action (e.g. prePersist()) happens within Doctrine. This could be useful, for example, to create an independent search index whenever an object in your database is saved.

Doctrine defines two types of objects that can listen to Doctrine events: listeners and subscribers. Both are very similar, but listeners are a bit more straightforward. For more, see The Event System on Doctrine's website.

The Doctrine website also explains all existing events that can be listened to.

Configuring the Listener/Subscriber

To register a service to act as an event listener or subscriber you just have to tag it with the appropriate name. Depending on your use-case, you can hook a listener into every DBAL connection and ORM entity manager or just into one specific DBAL connection and all the entity managers that use this connection.

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doctrine:
    dbal:
        default_connection: default
        connections:
            default:
                driver: pdo_sqlite
                memory: true

services:
    my.listener:
        class: AppBundle\EventListener\SearchIndexer
        tags:
            - { name: doctrine.event_listener, event: postPersist }
    my.listener2:
        class: AppBundle\EventListener\SearchIndexer2
        tags:
            - { name: doctrine.event_listener, event: postPersist, connection: default }
    my.subscriber:
        class: AppBundle\EventListener\SearchIndexerSubscriber
        tags:
            - { name: doctrine.event_subscriber, connection: default }

Creating the Listener Class

In the previous example, a service my.listener was configured as a Doctrine listener on the event postPersist. The class behind that service must have a postPersist() method, which will be called when the event is dispatched:

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// src/AppBundle/EventListener/SearchIndexer.php
namespace AppBundle\EventListener;

// for Doctrine < 2.4: use Doctrine\ORM\Event\LifecycleEventArgs;
use Doctrine\Common\Persistence\Event\LifecycleEventArgs;
use AppBundle\Entity\Product;

class SearchIndexer
{
    public function postPersist(LifecycleEventArgs $args)
    {
        $entity = $args->getObject();

        // only act on some "Product" entity
        if (!$entity instanceof Product) {
            return;
        }

        $entityManager = $args->getObjectManager();
        // ... do something with the Product
    }
}

In each event, you have access to a LifecycleEventArgs object, which gives you access to both the entity object of the event and the entity manager itself.

One important thing to notice is that a listener will be listening for all entities in your application. So, if you're interested in only handling a specific type of entity (e.g. a Product entity but not a BlogPost entity), you should check for the entity's class type in your method (as shown above).

Tip

In Doctrine 2.4, a feature called Entity Listeners was introduced. It is a lifecycle listener class used for an entity. You can read about it in the Doctrine Documentation.

Creating the Subscriber Class

A Doctrine event subscriber must implement the Doctrine\Common\EventSubscriber interface and have an event method for each event it subscribes to:

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// src/AppBundle/EventListener/SearchIndexerSubscriber.php
namespace AppBundle\EventListener;

use Doctrine\Common\EventSubscriber;
// for Doctrine < 2.4: use Doctrine\ORM\Event\LifecycleEventArgs;
use Doctrine\Common\Persistence\Event\LifecycleEventArgs;
use Doctrine\ORM\Events;
use AppBundle\Entity\Product;

class SearchIndexerSubscriber implements EventSubscriber
{
    public function getSubscribedEvents()
    {
        return array(
            Events::postPersist,
            Events::postUpdate,
        );
    }

    public function postUpdate(LifecycleEventArgs $args)
    {
        $this->index($args);
    }

    public function postPersist(LifecycleEventArgs $args)
    {
        $this->index($args);
    }

    public function index(LifecycleEventArgs $args)
    {
        $entity = $args->getObject();

        // perhaps you only want to act on some "Product" entity
        if ($entity instanceof Product) {
            $entityManager = $args->getObjectManager();
            // ... do something with the Product
        }
    }
}

Tip

Doctrine event subscribers cannot return a flexible array of methods to call for the events like the Symfony event subscriber can. Doctrine event subscribers must return a simple array of the event names they subscribe to. Doctrine will then expect methods on the subscriber with the same name as each subscribed event, just as when using an event listener.

For a full reference, see chapter The Event System in the Doctrine documentation.

Lazy loading for Event Listeners

One subtle difference between listeners and subscribers is that Symfony can load entity listeners lazily. This means that your listener class will only be fetched from the service container (and thus be instantiated) once the event it is linked to actually fires.

Lazy loading might give you a slight performance improvement when your listener runs for events that rarely fire. Also, it can help you when you run into circular dependency issues that may occur when your listener service in turn depends on the DBAL connection.

To mark a listener service as lazily loaded, just add the lazy attribute to the tag like so:

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services:
    my.listener:
        class: AppBundle\EventListener\SearchIndexer
        tags:
            - { name: doctrine.event_listener, event: postPersist, lazy: true }

Note

Marking an event listener as lazy has nothing to do with lazy service definitions which are described in their own article

Priorities for Event Listeners

In case you have multiple listeners for the same event you can control the order in which they are invoked using the priority attribute on the tag. Priorities are defined with positive or negative integers (they default to 0). Higher numbers mean that listeners are invoked earlier.

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services:
    my.listener.with_high_priority:
        class: AppBundle\EventListener\MyHighPriorityListener
        tags:
            - { name: doctrine.event_listener, event: postPersist, priority: 10 }

    my.listener.with_low_priority:
        class: AppBundle\EventListener\MyLowPriorityListener
        tags:
            - { name: doctrine.event_listener, event: postPersist, priority: 1 }
This work, including the code samples, is licensed under a Creative Commons BY-SA 3.0 license.
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