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    <h3>Editing: CacheAttributeListener.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\ExpressionLanguage\ExpressionLanguage;
use Symfony\Component\HttpFoundation\Request;
use Symfony\Component\HttpFoundation\Response;
use Symfony\Component\HttpKernel\Attribute\Cache;
use Symfony\Component\HttpKernel\Event\ControllerArgumentsEvent;
use Symfony\Component\HttpKernel\Event\ResponseEvent;
use Symfony\Component\HttpKernel\KernelEvents;

/**
 * Handles HTTP cache headers configured via the Cache attribute.
 *
 * @author Fabien Potencier &lt;fabien@symfony.com&gt;
 */
class CacheAttributeListener implements EventSubscriberInterface
{
    /**
     * @var \SplObjectStorage&lt;Request, \DateTimeInterface&gt;
     */
    private \SplObjectStorage $lastModified;

    /**
     * @var \SplObjectStorage&lt;Request, string&gt;
     */
    private \SplObjectStorage $etags;

    public function __construct(
        private ?ExpressionLanguage $expressionLanguage = null,
    ) {
        $this-&gt;lastModified = new \SplObjectStorage();
        $this-&gt;etags = new \SplObjectStorage();
    }

    /**
     * Handles HTTP validation headers.
     */
    public function onKernelControllerArguments(ControllerArgumentsEvent $event)
    {
        $request = $event-&gt;getRequest();

        if (!\is_array($attributes = $request-&gt;attributes-&gt;get('_cache') ?? $event-&gt;getAttributes()[Cache::class] ?? null)) {
            return;
        }

        $request-&gt;attributes-&gt;set('_cache', $attributes);
        $response = null;
        $lastModified = null;
        $etag = null;

        /** @var Cache[] $attributes */
        foreach ($attributes as $cache) {
            if (null !== $cache-&gt;lastModified) {
                $lastModified = $this-&gt;getExpressionLanguage()-&gt;evaluate($cache-&gt;lastModified, array_merge($request-&gt;attributes-&gt;all(), $event-&gt;getNamedArguments()));
                ($response ??= new Response())-&gt;setLastModified($lastModified);
            }

            if (null !== $cache-&gt;etag) {
                $etag = hash('sha256', $this-&gt;getExpressionLanguage()-&gt;evaluate($cache-&gt;etag, array_merge($request-&gt;attributes-&gt;all(), $event-&gt;getNamedArguments())));
                ($response ??= new Response())-&gt;setEtag($etag);
            }
        }

        if ($response?-&gt;isNotModified($request)) {
            $event-&gt;setController(static fn () =&gt; $response);
            $event-&gt;stopPropagation();

            return;
        }

        if (null !== $etag) {
            $this-&gt;etags[$request] = $etag;
        }
        if (null !== $lastModified) {
            $this-&gt;lastModified[$request] = $lastModified;
        }
    }

    /**
     * Modifies the response to apply HTTP cache headers when needed.
     */
    public function onKernelResponse(ResponseEvent $event)
    {
        $request = $event-&gt;getRequest();

        /** @var Cache[] $attributes */
        if (!\is_array($attributes = $request-&gt;attributes-&gt;get('_cache'))) {
            return;
        }
        $response = $event-&gt;getResponse();

        // http://tools.ietf.org/html/draft-ietf-httpbis-p4-conditional-12#section-3.1
        if (!\in_array($response-&gt;getStatusCode(), [200, 203, 300, 301, 302, 304, 404, 410])) {
            unset($this-&gt;lastModified[$request]);
            unset($this-&gt;etags[$request]);

            return;
        }

        if (isset($this-&gt;lastModified[$request]) &amp;&amp; !$response-&gt;headers-&gt;has('Last-Modified')) {
            $response-&gt;setLastModified($this-&gt;lastModified[$request]);
        }

        if (isset($this-&gt;etags[$request]) &amp;&amp; !$response-&gt;headers-&gt;has('Etag')) {
            $response-&gt;setEtag($this-&gt;etags[$request]);
        }

        unset($this-&gt;lastModified[$request]);
        unset($this-&gt;etags[$request]);
        $hasVary = $response-&gt;headers-&gt;has('Vary');

        foreach (array_reverse($attributes) as $cache) {
            if (null !== $cache-&gt;smaxage &amp;&amp; !$response-&gt;headers-&gt;hasCacheControlDirective('s-maxage')) {
                $response-&gt;setSharedMaxAge($this-&gt;toSeconds($cache-&gt;smaxage));
            }

            if ($cache-&gt;mustRevalidate) {
                $response-&gt;headers-&gt;addCacheControlDirective('must-revalidate');
            }

            if (null !== $cache-&gt;maxage &amp;&amp; !$response-&gt;headers-&gt;hasCacheControlDirective('max-age')) {
                $response-&gt;setMaxAge($this-&gt;toSeconds($cache-&gt;maxage));
            }

            if (null !== $cache-&gt;maxStale &amp;&amp; !$response-&gt;headers-&gt;hasCacheControlDirective('max-stale')) {
                $response-&gt;headers-&gt;addCacheControlDirective('max-stale', $this-&gt;toSeconds($cache-&gt;maxStale));
            }

            if (null !== $cache-&gt;staleWhileRevalidate &amp;&amp; !$response-&gt;headers-&gt;hasCacheControlDirective('stale-while-revalidate')) {
                $response-&gt;headers-&gt;addCacheControlDirective('stale-while-revalidate', $this-&gt;toSeconds($cache-&gt;staleWhileRevalidate));
            }

            if (null !== $cache-&gt;staleIfError &amp;&amp; !$response-&gt;headers-&gt;hasCacheControlDirective('stale-if-error')) {
                $response-&gt;headers-&gt;addCacheControlDirective('stale-if-error', $this-&gt;toSeconds($cache-&gt;staleIfError));
            }

            if (null !== $cache-&gt;expires &amp;&amp; !$response-&gt;headers-&gt;has('Expires')) {
                $response-&gt;setExpires(new \DateTimeImmutable('@'.strtotime($cache-&gt;expires, time())));
            }

            if (!$hasVary &amp;&amp; $cache-&gt;vary) {
                $response-&gt;setVary($cache-&gt;vary, false);
            }
        }

        foreach ($attributes as $cache) {
            if (true === $cache-&gt;public) {
                $response-&gt;setPublic();
            }

            if (false === $cache-&gt;public) {
                $response-&gt;setPrivate();
            }
        }
    }

    public static function getSubscribedEvents(): array
    {
        return [
            KernelEvents::CONTROLLER_ARGUMENTS =&gt; ['onKernelControllerArguments', 10],
            KernelEvents::RESPONSE =&gt; ['onKernelResponse', -10],
        ];
    }

    private function getExpressionLanguage(): ExpressionLanguage
    {
        return $this-&gt;expressionLanguage ??= class_exists(ExpressionLanguage::class)
            ? new ExpressionLanguage()
            : throw new \LogicException('Unable to use expressions as the Symfony ExpressionLanguage component is not installed. Try running &quot;composer require symfony/expression-language&quot;.');
    }

    private function toSeconds(int|string $time): int
    {
        if (!is_numeric($time)) {
            $now = time();
            $time = strtotime($time, $now) - $now;
        }

        return $time;
    }
}
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