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services:
  subscriber.locale_listener:
    class: AppBundle\EventListener\LocaleSubscriber
    arguments: ['%kernel.default_locale%']
    tags:
      - { name: kernel.event_subscriber }

<?php

namespace AppBundle\EventListener;

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

class LocaleSubscriber implements EventSubscriberInterface
{
    /**
     * @var string
     */
    private $defaultLocale;

    /**
     * @param string $defaultLocale
     */
    public function __construct($defaultLocale = 'en')
    {
        $this->defaultLocale = $defaultLocale;
    }

    /**
     * @param GetResponseEvent $event
     */
    public function onKernelRequest(GetResponseEvent $event)
    {
        $request = $event->getRequest();
        if (!$request->hasPreviousSession()) {
            return;
        }

        if ($locale = $request->query->get('_locale')) {
            $request->getSession()->set('_locale', $locale);
            $request->setLocale($locale);
        } else {
            $request->setLocale($request->getSession()->get('_locale', $this->defaultLocale));
        }
    }

    /**
     * @return array
     */
    public static function getSubscribedEvents()
    {
        return [
            KernelEvents::REQUEST => [['onKernelRequest', 17]]
        ];
    }
}