class FormCache

class FormCache implements FormCacheInterface {
    
    /**
     * The factory for expirable key value stores used by form cache.
     *
     * @var \Drupal\Core\KeyValueStore\KeyValueExpirableFactoryInterface
     */
    protected $keyValueExpirableFactory;
    
    /**
     * The CSRF token generator to validate the form token.
     *
     * @var \Drupal\Core\Access\CsrfTokenGenerator
     */
    protected $csrfToken;
    
    /**
     * The current user.
     *
     * @var \Drupal\Core\Session\AccountInterface
     */
    protected $currentUser;
    
    /**
     * The module handler.
     *
     * @var \Drupal\Core\Extension\ModuleHandlerInterface
     */
    protected $moduleHandler;
    
    /**
     * Logger channel.
     *
     * @var \Drupal\Core\Logger\LoggerChannelInterface
     */
    protected $logger;
    
    /**
     * The config factory.
     *
     * @var \Drupal\Core\Config\ConfigFactoryInterface
     */
    protected $configFactory;
    
    /**
     * The request stack.
     *
     * @var \Symfony\Component\HttpFoundation\RequestStack
     */
    protected $requestStack;
    
    /**
     * A policy rule determining the cacheability of a request.
     *
     * @var \Drupal\Core\PageCache\RequestPolicyInterface
     */
    protected $requestPolicy;
    
    /**
     * The app root.
     *
     * @var string
     */
    protected $root;
    
    /**
     * Constructs a new FormCache.
     *
     * @param string $root
     *   The app root.
     * @param \Drupal\Core\KeyValueStore\KeyValueExpirableFactoryInterface $key_value_expirable_factory
     *   The key value expirable factory, used to create key value expirable
     *   stores for the form cache and form state cache.
     * @param \Drupal\Core\Extension\ModuleHandlerInterface $module_handler
     *   The module handler.
     * @param \Drupal\Core\Session\AccountInterface $current_user
     *   The current user.
     * @param \Drupal\Core\Access\CsrfTokenGenerator $csrf_token
     *   The CSRF token generator.
     * @param \Psr\Log\LoggerInterface $logger
     *   A logger instance.
     * @param \Symfony\Component\HttpFoundation\RequestStack $request_stack
     *   The request stack.
     * @param \Drupal\Core\PageCache\RequestPolicyInterface $request_policy
     *   A policy rule determining the cacheability of a request.
     */
    public function __construct($root, KeyValueExpirableFactoryInterface $key_value_expirable_factory, ModuleHandlerInterface $module_handler, AccountInterface $current_user, CsrfTokenGenerator $csrf_token, LoggerInterface $logger, RequestStack $request_stack, RequestPolicyInterface $request_policy) {
        $this->root = $root;
        $this->keyValueExpirableFactory = $key_value_expirable_factory;
        $this->moduleHandler = $module_handler;
        $this->currentUser = $current_user;
        $this->logger = $logger;
        $this->csrfToken = $csrf_token;
        $this->requestStack = $request_stack;
        $this->requestPolicy = $request_policy;
    }
    
    /**
     * {@inheritdoc}
     */
    public function getCache($form_build_id, FormStateInterface $form_state) {
        if ($form = $this->keyValueExpirableFactory
            ->get('form')
            ->get($form_build_id)) {
            if (isset($form['#cache_token']) && $this->csrfToken
                ->validate($form['#cache_token']) || !isset($form['#cache_token']) && $this->currentUser
                ->isAnonymous()) {
                $this->loadCachedFormState($form_build_id, $form_state);
                // Generate a new #build_id if the cached form was rendered on a
                // cacheable page.
                $build_info = $form_state->getBuildInfo();
                if (!empty($build_info['immutable'])) {
                    $form['#build_id_old'] = $form['#build_id'];
                    $form['#build_id'] = 'form-' . Crypt::randomBytesBase64();
                    $form['form_build_id']['#value'] = $form['#build_id'];
                    $form['form_build_id']['#id'] = $form['#build_id'];
                    unset($build_info['immutable']);
                    $form_state->setBuildInfo($build_info);
                }
                return $form;
            }
        }
    }
    
    /**
     * Loads the cached form state.
     *
     * @param string $form_build_id
     *   The unique form build ID.
     * @param \Drupal\Core\Form\FormStateInterface $form_state
     *   The current state of the form.
     */
    protected function loadCachedFormState($form_build_id, FormStateInterface $form_state) {
        if ($stored_form_state = $this->keyValueExpirableFactory
            ->get('form_state')
            ->get($form_build_id)) {
            // Re-populate $form_state for subsequent rebuilds.
            $form_state->setFormState($stored_form_state);
            // If the original form is contained in include files, load the files.
            // @see \Drupal\Core\Form\FormStateInterface::loadInclude()
            $build_info = $form_state->getBuildInfo();
            $build_info += [
                'files' => [],
            ];
            foreach ($build_info['files'] as $file) {
                if (is_array($file)) {
                    $file += [
                        'type' => 'inc',
                        'name' => $file['module'],
                    ];
                    $this->moduleHandler
                        ->loadInclude($file['module'], $file['type'], $file['name']);
                }
                elseif (file_exists($file)) {
                    require_once $this->root . '/' . $file;
                }
            }
        }
    }
    
    /**
     * {@inheritdoc}
     */
    public function setCache($form_build_id, $form, FormStateInterface $form_state) {
        // Cache forms for 6 hours by default.
        $expire = Settings::get('form_cache_expiration', 21600);
        // Ensure that the form build_id embedded in the form structure is the same
        // as the one passed in as a parameter. This is an additional safety measure
        // to prevent legacy code operating directly with
        // \Drupal::formBuilder()->getCache() and \Drupal::formBuilder()->setCache()
        // from accidentally overwriting immutable form state.
        if (isset($form['#build_id']) && $form['#build_id'] != $form_build_id) {
            $this->logger
                ->error('Form build-id mismatch detected while attempting to store a form in the cache.');
            return;
        }
        // Cache form structure.
        if (isset($form)) {
            if ($this->currentUser
                ->isAuthenticated()) {
                $form['#cache_token'] = $this->csrfToken
                    ->get();
            }
            unset($form['#build_id_old']);
            $this->keyValueExpirableFactory
                ->get('form')
                ->setWithExpire($form_build_id, $form, $expire);
        }
        if ($data = $form_state->getCacheableArray()) {
            $this->keyValueExpirableFactory
                ->get('form_state')
                ->setWithExpire($form_build_id, $data, $expire);
        }
    }
    
    /**
     * {@inheritdoc}
     */
    public function deleteCache($form_build_id) {
        $this->keyValueExpirableFactory
            ->get('form')
            ->delete($form_build_id);
        $this->keyValueExpirableFactory
            ->get('form_state')
            ->delete($form_build_id);
    }

}