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How to create an Event Listener

Symfony has various events and hooks that can be used to trigger custom behavior in your application. Those events are thrown by the HttpKernel component and can be viewed in the KernelEvents class.

To hook into an event and add your own custom logic, you have to create a service that will act as an event listener on that event. In this entry, you will create a service that will act as an Exception Listener, allowing you to modify how exceptions are shown by your application. The KernelEvents::EXCEPTION event is just one of the core kernel events:

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// src/Acme/DemoBundle/EventListener/AcmeExceptionListener.php
namespace Acme\DemoBundle\EventListener;

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

class AcmeExceptionListener
{
    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);
        }

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

New in version 2.4: Support for HTTP status code constants was introduced in Symfony 2.4.

Tip

Each event receives a slightly different type of $event object. For the kernel.exception event, it is GetResponseForExceptionEvent. To see what type of object each event listener receives, see KernelEvents.

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":

  • YAML
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    # app/config/config.yml
    services:
        kernel.listener.your_listener_name:
            class: Acme\DemoBundle\EventListener\AcmeExceptionListener
            tags:
                - { name: kernel.event_listener, event: kernel.exception, method: onKernelException }
    
  • XML
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    <!-- app/config/config.xml -->
    <service id="kernel.listener.your_listener_name" class="Acme\DemoBundle\EventListener\AcmeExceptionListener">
        <tag name="kernel.event_listener" event="kernel.exception" method="onKernelException" />
    </service>
    
  • PHP
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    // app/config/config.php
    $container
        ->register('kernel.listener.your_listener_name', 'Acme\DemoBundle\EventListener\AcmeExceptionListener')
        ->addTag('kernel.event_listener', array('event' => 'kernel.exception', 'method' => 'onKernelException'))
    ;
    

Note

There is an additional tag option priority that is optional and defaults to 0. This value can be from -255 to 255, and the listeners will be executed in the order of their priority (highest to lowest). This is useful when you need to guarantee that one listener is executed before another.

Request events, checking types

A single page can make several requests (one master request, and then multiple sub-requests), which is why when working with the KernelEvents::REQUEST event, you might need to check the type of the request. This can be easily done as follow:

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// src/Acme/DemoBundle/EventListener/AcmeRequestListener.php
namespace Acme\DemoBundle\EventListener;

use Symfony\Component\HttpKernel\Event\GetResponseEvent;
use Symfony\Component\HttpKernel\HttpKernel;

class AcmeRequestListener
{
    public function onKernelRequest(GetResponseEvent $event)
    {
        if (HttpKernel::MASTER_REQUEST != $event->getRequestType()) {
            // don't do anything if it's not the master request
            return;
        }

        // ...
    }
}

Tip

Two types of request are available in the HttpKernelInterface interface: HttpKernelInterface::MASTER_REQUEST and HttpKernelInterface::SUB_REQUEST.