/**
  * {@inheritdoc}
  */
 public function settingsForm(array $form, FormStateInterface $form_state)
 {
     $element = parent::settingsForm($form, $form_state);
     $element['summary_rows'] = array('#type' => 'number', '#title' => t('Summary rows'), '#default_value' => $this->getSetting('summary_rows'), '#required' => TRUE, '#min' => 1);
     return $element;
 }