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

    public function isFresh()
    {
        return $this->getTtl() > 0;
    }

 /**
  * {@inheritDoc}
  */
 protected function isFreshEnough(Request $request, Response $entry)
 {
     if (!$entry->isFresh() && !$this->isFreshCacheEntry($entry)) {
         return $this->lock($request, $entry);
     }
     return true;
 }