Symfony\Component\HttpFoundation\Response::isNotModified PHP Method

    public function isNotModified(Request $request)
    {
        if (!$request->isMethodCacheable()) {
            return false;
        }

        $notModified = false;
        $lastModified = $this->headers->get('Last-Modified');
        $modifiedSince = $request->headers->get('If-Modified-Since');

        if ($etags = $request->getETags()) {
            $notModified = in_array($this->getEtag(), $etags) || in_array('*', $etags);
        }

        if ($modifiedSince && $lastModified) {
            $notModified = strtotime($modifiedSince) >= strtotime($lastModified) && (!$etags || $notModified);
        }

        if ($notModified) {
            $this->setNotModified();
        }

        return $notModified;
    }

 public function intercept(MethodInvocation $invocation)
 {
     /*
      * First let's make sure we can even bother caching this
      * if we can't then just return the controller response
      */
     if (!$this->decider->decide()) {
         return $invocation->proceed();
     }
     /* @var $controller ResourceController */
     $controller = $invocation->object;
     /* @var $request Request */
     $request = $invocation->arguments[0];
     // Current Resource
     $resource = $controller->findOr404($request);
     // Cache Options
     $options = $request->attributes->get('_sylius', []);
     $cacheOptions = array_key_exists('cache', $options) ? $options['cache'] : [];
     // Build the actual cache options since we'll need to use it twice
     $this->parser->process($cacheOptions, $resource);
     // Create a preliminary response to see if it is already cached
     $cachedResponse = new Response();
     $cachedResponse->setCache($cacheOptions);
     if ($cachedResponse->isNotModified($request)) {
         return $cachedResponse;
     }
     // If not we take the response back from the controller and modify it again
     $controllerResponse = $invocation->proceed();
     $controllerResponse->setCache($cacheOptions);
     return $controllerResponse;
 }