loadCachedFormState example

$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);
        }