Events and Event Listeners

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During the execution of a Symfony application, lots of event notifications are triggered. Your application can listen to these notifications and respond to them by executing any piece of code.

Symfony triggers several events related to the kernel while processing the HTTP Request. Third-party bundles may also dispatch events, and you can even dispatch custom events from your own code.

All the examples shown in this article use the same KernelEvents::EXCEPTION event for consistency purposes. In your own application, you can use any event and even mix several of them in the same subscriber.

Creating an Event Listener

The most common way to listen to an event is to register an event listener:

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

use Symfony\Component\HttpFoundation\Response;
use Symfony\Component\HttpKernel\Event\GetResponseForExceptionEvent;
use Symfony\Component\HttpKernel\Exception\HttpExceptionInterface;

class ExceptionListener
{
    public function onKernelException(GetResponseForExceptionEvent $event)
    {
        // You get the exception object from the received event
        $exception = $event->getException();
        $message = sprintf(
            'My Error says: %s with code: %s',
            $exception->getMessage(),
            $exception->getCode()
        );

        // Customize your response object to display the exception details
        $response = new Response();
        $response->setContent($message);

        // HttpExceptionInterface is a special type of exception that
        // holds status code and header details
        if ($exception instanceof HttpExceptionInterface) {
            $response->setStatusCode($exception->getStatusCode());
            $response->headers->replace($exception->getHeaders());
        } else {
            $response->setStatusCode(Response::HTTP_INTERNAL_SERVER_ERROR);
        }

        // sends the modified response object to the event
        $event->setResponse($response);
    }
}

Tip

Each event receives a slightly different type of $event object. For the kernel.exception event, it is GetResponseForExceptionEvent. Check out the Symfony events reference to see what type of object each event provides.

Now that the class is created, you just need to register it as a service and notify Symfony that it is a "listener" on the kernel.exception event by using a special "tag":

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# app/config/services.yml
services:
    AppBundle\EventListener\ExceptionListener:
        tags:
            - { name: kernel.event_listener, event: kernel.exception }

Note

There is an optional tag attribute called method which defines which method to execute when the event is triggered. By default, the name of the method is on + "camel-cased event name". If the event is kernel.exception the method executed by default is onKernelException().

The other optional tag attribute is called priority, which defaults to 0 and it controls the order in which listeners are executed for a given event (the higher the number the earlier a listener is executed). This is useful when you need to guarantee that one listener is executed before another. The priorities of the internal Symfony listeners usually range from -255 to 255 but your own listeners can use any positive or negative integer.

Creating an Event Subscriber

Another way to listen to events is via an event subscriber, which is a class that defines one or more methods that listen to one or various events. The main difference with the event listeners is that subscribers always know which events they are listening to.

If different event subscriber methods listen to the same event, their order is defined by the priority parameter. This value is a positive or negative integer which defaults to 0. The higher the number, the earlier the method is called. Priority is aggregated for all listeners and subscribers, so your methods could be executed before or after the methods defined in other listeners and subscribers. To learn more about event subscribers, read The EventDispatcher Component.

The following example shows an event subscriber that defines several methods which listen to the same kernel.exception event:

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// src/AppBundle/EventSubscriber/ExceptionSubscriber.php
namespace AppBundle\EventSubscriber;

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

class ExceptionSubscriber implements EventSubscriberInterface
{
    public static function getSubscribedEvents()
    {
        // return the subscribed events, their methods and priorities
        return [
            KernelEvents::EXCEPTION => [
                ['processException', 10],
                ['logException', 0],
                ['notifyException', -10],
            ],
        ];
    }

    public function processException(GetResponseForExceptionEvent $event)
    {
        // ...
    }

    public function logException(GetResponseForExceptionEvent $event)
    {
        // ...
    }

    public function notifyException(GetResponseForExceptionEvent $event)
    {
        // ...
    }
}

That's it! Your services.yml file should already be setup to load services from the EventSubscriber directory. Symfony takes care of the rest.

Tip

If your methods are not called when an exception is thrown, double-check that you're loading services from the EventSubscriber directory and have autoconfigure enabled. You can also manually add the kernel.event_subscriber tag.

Request Events, Checking Types

A single page can make several requests (one master request, and then multiple sub-requests - typically by How to Embed Controllers in a Template). For the core Symfony events, you might need to check to see if the event is for a "master" request or a "sub request":

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

use Symfony\Component\HttpKernel\Event\GetResponseEvent;

class RequestListener
{
    public function onKernelRequest(GetResponseEvent $event)
    {
        if (!$event->isMasterRequest()) {
            // don't do anything if it's not the master request
            return;
        }

        // ...
    }
}

Certain things, like checking information on the real request, may not need to be done on the sub-request listeners.

Listeners or Subscribers

Listeners and subscribers can be used in the same application indistinctly. The decision to use either of them is usually a matter of personal taste. However, there are some minor advantages for each of them:

  • Subscribers are easier to reuse because the knowledge of the events is kept in the class rather than in the service definition. This is the reason why Symfony uses subscribers internally;
  • Listeners are more flexible because bundles can enable or disable each of them conditionally depending on some configuration value.

Debugging Event Listeners

You can find out what listeners are registered in the event dispatcher using the console. To show all events and their listeners, run:

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$ php bin/console debug:event-dispatcher

You can get registered listeners for a particular event by specifying its name:

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$ php bin/console debug:event-dispatcher kernel.exception

Learn more

This work, including the code samples, is licensed under a Creative Commons BY-SA 3.0 license.
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