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// src/Controller/DefaultController.php
namespace App\Controller;

use Symfony\Bundle\FrameworkBundle\Controller\AbstractController;
use Symfony\Component\HttpFoundation\Request;
use Symfony\Component\HttpFoundation\Response;

class DefaultController extends AbstractController
{
    public function homepage(Request $request): Response
    {
        $response = $this->render('static/homepage.html.twig');
        $response->setEtag(md5($response->getContent()));
        $response->setPublic(); // make sure the response is public/cacheable
        $response->isNotModified($request);

        return $response;
    }
}

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// src/Controller/ArticleController.php
namespace App\Controller;

// ...
use App\Entity\Article;
use Symfony\Component\HttpFoundation\Request;
use Symfony\Component\HttpFoundation\Response;

class ArticleController extends AbstractController
{
    public function show(Article $article, Request $request): Response
    {
        $author = $article->getAuthor();

        $articleDate = new \DateTime($article->getUpdatedAt());
        $authorDate = new \DateTime($author->getUpdatedAt());

        $date = $authorDate > $articleDate ? $authorDate : $articleDate;

        $response = new Response();
        $response->setLastModified($date);
        // Set response as public. Otherwise it will be private by default.
        $response->setPublic();

        if ($response->isNotModified($request)) {
            return $response;
        }

        // ... do more work to populate the response with the full content

        return $response;
    }
}

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// src/Controller/ArticleController.php
namespace App\Controller;

// ...
use Symfony\Component\HttpFoundation\Request;
use Symfony\Component\HttpFoundation\Response;

class ArticleController extends AbstractController
{
    public function show(string $articleSlug, Request $request): Response
    {
        // Get the minimum information to compute
        // the ETag or the Last-Modified value
        // (based on the Request, data is retrieved from
        // a database or a key-value store for instance)
        $article = ...;

        // create a Response with an ETag and/or a Last-Modified header
        $response = new Response();
        $response->setEtag($article->computeETag());
        $response->setLastModified($article->getPublishedAt());

        // Set response as public. Otherwise it will be private by default.
        $response->setPublic();

        // Check that the Response is not modified for the given Request
        if ($response->isNotModified($request)) {
            // return the 304 Response immediately
            return $response;
        }

        // do more work here - like retrieving more data
        $comments = ...;

        // or render a template with the $response you've already started
        return $this->render('article/show.html.twig', [
            'article' => $article,
            'comments' => $comments,
        ], $response);
    }
}