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    <h3>Editing: LocaleListener.php</h3>
    <textarea name="content" rows="20" cols="80">&lt;?php

/*
 * This file is part of the Symfony package.
 *
 * (c) Fabien Potencier &lt;fabien@symfony.com&gt;
 *
 * For the full copyright and license information, please view the LICENSE
 * file that was distributed with this source code.
 */

namespace Symfony\Component\HttpKernel\EventListener;

use Symfony\Component\EventDispatcher\EventSubscriberInterface;
use Symfony\Component\HttpFoundation\Request;
use Symfony\Component\HttpFoundation\RequestStack;
use Symfony\Component\HttpKernel\Event\FinishRequestEvent;
use Symfony\Component\HttpKernel\Event\KernelEvent;
use Symfony\Component\HttpKernel\Event\RequestEvent;
use Symfony\Component\HttpKernel\KernelEvents;
use Symfony\Component\Routing\RequestContextAwareInterface;

/**
 * Initializes the locale based on the current request.
 *
 * @author Fabien Potencier &lt;fabien@symfony.com&gt;
 *
 * @final
 */
class LocaleListener implements EventSubscriberInterface
{
    private $router;
    private $defaultLocale;
    private $requestStack;
    private $useAcceptLanguageHeader;
    private $enabledLocales;

    public function __construct(RequestStack $requestStack, string $defaultLocale = 'en', ?RequestContextAwareInterface $router = null, bool $useAcceptLanguageHeader = false, array $enabledLocales = [])
    {
        $this-&gt;defaultLocale = $defaultLocale;
        $this-&gt;requestStack = $requestStack;
        $this-&gt;router = $router;
        $this-&gt;useAcceptLanguageHeader = $useAcceptLanguageHeader;
        $this-&gt;enabledLocales = $enabledLocales;
    }

    public function setDefaultLocale(KernelEvent $event)
    {
        $event-&gt;getRequest()-&gt;setDefaultLocale($this-&gt;defaultLocale);
    }

    public function onKernelRequest(RequestEvent $event)
    {
        $request = $event-&gt;getRequest();

        $this-&gt;setLocale($request);
        $this-&gt;setRouterContext($request);
    }

    public function onKernelFinishRequest(FinishRequestEvent $event)
    {
        if (null !== $parentRequest = $this-&gt;requestStack-&gt;getParentRequest()) {
            $this-&gt;setRouterContext($parentRequest);
        }
    }

    private function setLocale(Request $request)
    {
        if ($locale = $request-&gt;attributes-&gt;get('_locale')) {
            $request-&gt;setLocale($locale);
        } elseif ($this-&gt;useAcceptLanguageHeader &amp;&amp; $this-&gt;enabledLocales) {
            if ($request-&gt;getLanguages() &amp;&amp; $preferredLanguage = $request-&gt;getPreferredLanguage($this-&gt;enabledLocales)) {
                $request-&gt;setLocale($preferredLanguage);
            }
            $request-&gt;attributes-&gt;set('_vary_by_language', true);
        }
    }

    private function setRouterContext(Request $request)
    {
        if (null !== $this-&gt;router) {
            $this-&gt;router-&gt;getContext()-&gt;setParameter('_locale', $request-&gt;getLocale());
        }
    }

    public static function getSubscribedEvents(): array
    {
        return [
            KernelEvents::REQUEST =&gt; [
                ['setDefaultLocale', 100],
                // must be registered after the Router to have access to the _locale
                ['onKernelRequest', 16],
            ],
            KernelEvents::FINISH_REQUEST =&gt; [['onKernelFinishRequest', 0]],
        ];
    }
}
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