class StringTextareaWidget

class StringTextareaWidget extends WidgetBase {
  
  /**
   * {@inheritdoc}
   */
  public static function defaultSettings() {
    return [
      'rows' => '5',
      'placeholder' => '',
    ] + parent::defaultSettings();
  }
  
  /**
   * {@inheritdoc}
   */
  public function settingsForm(array $form, FormStateInterface $form_state) {
    $element['rows'] = [
      '#type' => 'number',
      '#title' => $this->t('Rows'),
      '#default_value' => $this->getSetting('rows'),
      '#required' => TRUE,
      '#min' => 1,
    ];
    $element['placeholder'] = [
      '#type' => 'textarea',
      '#title' => $this->t('Placeholder'),
      '#default_value' => $this->getSetting('placeholder'),
      '#description' => $this->t('Text that will be shown inside the field until a value is entered. This hint is usually a sample value or a brief description of the expected format.'),
    ];
    return $element;
  }
  
  /**
   * {@inheritdoc}
   */
  public function settingsSummary() {
    $summary = [];
    $summary[] = $this->t('Number of rows: @rows', [
      '@rows' => $this->getSetting('rows'),
    ]);
    $placeholder = $this->getSetting('placeholder');
    if (!empty($placeholder)) {
      $summary[] = $this->t('Placeholder: @placeholder', [
        '@placeholder' => $placeholder,
      ]);
    }
    return $summary;
  }
  
  /**
   * {@inheritdoc}
   */
  public function formElement(FieldItemListInterface $items, $delta, array $element, array &$form, FormStateInterface $form_state) {
    $element['value'] = $element + [
      '#type' => 'textarea',
      '#default_value' => $items[$delta]->value,
      '#rows' => $this->getSetting('rows'),
      '#placeholder' => $this->getSetting('placeholder'),
      '#attributes' => [
        'class' => [
          'js-text-full',
          'text-full',
        ],
      ],
    ];
    return $element;
  }

}