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    <h3>Editing: ResponseListener.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\HttpKernel\Event\ResponseEvent;
use Symfony\Component\HttpKernel\KernelEvents;

/**
 * ResponseListener fixes the Response headers based on the Request.
 *
 * @author Fabien Potencier &lt;fabien@symfony.com&gt;
 *
 * @final
 */
class ResponseListener implements EventSubscriberInterface
{
    private string $charset;
    private bool $addContentLanguageHeader;

    public function __construct(string $charset, bool $addContentLanguageHeader = false)
    {
        $this-&gt;charset = $charset;
        $this-&gt;addContentLanguageHeader = $addContentLanguageHeader;
    }

    /**
     * Filters the Response.
     */
    public function onKernelResponse(ResponseEvent $event)
    {
        if (!$event-&gt;isMainRequest()) {
            return;
        }

        $response = $event-&gt;getResponse();

        if (null === $response-&gt;getCharset()) {
            $response-&gt;setCharset($this-&gt;charset);
        }

        if ($this-&gt;addContentLanguageHeader &amp;&amp; !$response-&gt;isInformational() &amp;&amp; !$response-&gt;isEmpty() &amp;&amp; !$response-&gt;headers-&gt;has('Content-Language')) {
            $response-&gt;headers-&gt;set('Content-Language', $event-&gt;getRequest()-&gt;getLocale());
        }

        if ($event-&gt;getRequest()-&gt;attributes-&gt;get('_vary_by_language')) {
            $response-&gt;setVary('Accept-Language', false);
        }

        $response-&gt;prepare($event-&gt;getRequest());
    }

    public static function getSubscribedEvents(): array
    {
        return [
            KernelEvents::RESPONSE =&gt; 'onKernelResponse',
        ];
    }
}
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