Request-Scoped Feature Flags in Symfony with a Twig Extension and Controller Gate

<?php

namespace App\Twig;

use App\Feature\FeatureFlagManager;
use Twig\Extension\AbstractExtension;
use Twig\TwigFunction;

final class FeatureExtension extends AbstractExtension
{
    public function __construct(private readonly FeatureFlagManager $flags)
    {
    }

    public function getFunctions(): array
    {
        return [
            new TwigFunction('feature', $this->isEnabled(...)),
        ];
    }

    public function isEnabled(string $flag): bool
    {
        return $this->flags->isEnabled($flag);
    }
}

<?php

namespace App\Feature;

use Symfony\Component\EventDispatcher\EventSubscriberInterface;
use Symfony\Component\HttpKernel\Event\ControllerEvent;
use Symfony\Component\HttpKernel\Exception\NotFoundHttpException;
use Symfony\Component\HttpKernel\KernelEvents;

#[\Attribute(\Attribute::TARGET_METHOD | \Attribute::TARGET_CLASS)]
final class IsFeatureEnabled
{
    public function __construct(public readonly string $flag)
    {
    }
}

final class FeatureGateSubscriber implements EventSubscriberInterface
{
    public function __construct(private readonly FeatureFlagManager $flags)
    {
    }

    public static function getSubscribedEvents(): array
    {
        return [KernelEvents::CONTROLLER => 'onController'];
    }

    public function onController(ControllerEvent $event): void
    {
        foreach ($event->getAttributes(IsFeatureEnabled::class) as $attribute) {
            if (!$this->flags->isEnabled($attribute->flag)) {
                throw new NotFoundHttpException(sprintf('Feature "%s" is disabled.', $attribute->flag));
            }
        }
    }
}

<?php

namespace App\Controller;

use App\Feature\FeatureFlagManager;
use App\Feature\IsFeatureEnabled;
use Symfony\Bundle\FrameworkBundle\Controller\AbstractController;
use Symfony\Component\HttpFoundation\Response;
use Symfony\Component\Routing\Attribute\Route;

#[Route('/checkout')]
final class CheckoutController extends AbstractController
{
    #[Route('', name: 'checkout_start')]
    #[IsFeatureEnabled('new_checkout')]
    public function start(): Response
    {
        return $this->render('checkout/start.html.twig');
    }

    #[Route('/confirm', name: 'checkout_confirm', methods: ['POST'])]
    public function confirm(FeatureFlagManager $flags): Response
    {
        $template = $flags->isEnabled('one_click_confirm')
            ? 'checkout/confirm_one_click.html.twig'
            : 'checkout/confirm_classic.html.twig';

        return $this->render($template);
    }
}