Symfony locale listener

LocaleListener.php

<?php

namespace Acme\Bundle\I18nBundle\EventListener;

use Symfony\Component\HttpKernel\HttpKernelInterface;
use Symfony\Component\DependencyInjection\ContainerInterface;
use Symfony\Component\HttpKernel\Event\GetResponseEvent;

class LocaleListener
{

    private $container;
    private $locales;

    public function __construct(ContainerInterface $container, array $locales)
    {
        $this->container = $container;
        $this->locales = $locales;
    }

    public function onKernelRequest(GetResponseEvent $event)
    {
        if (HttpKernelInterface::MASTER_REQUEST !== $event->getRequestType()) {
            return;
        }

        $request = $event->getRequest();
        $locale = $request->getPreferredLanguage($this->locales);

        if($this->container->has('session')) {
            $session = $this->container->get('session');

            if($session->get('_locale', false)) {
                $request->setLocale($session->get('_locale'));
                return;
            }

            $session->set('_locale', $locale);
        }

        $request->setLocale($locale);

    }

}