class MappingTest

class MappingTest extends Mapping {
  
  /**
   * {@inheritdoc}
   */
  protected function defineMapping() {
    return [
      'title_field' => [
        '#title' => $this->t('Title field'),
        '#description' => $this->t('Choose the field with the custom title.'),
        '#toggle' => TRUE,
        '#required' => TRUE,
      ],
      'name_field' => [
        '#title' => $this->t('Name field'),
        '#description' => $this->t('Choose the field with the custom name.'),
      ],
      'numeric_field' => [
        '#title' => $this->t('Numeric field'),
        '#description' => $this->t('Select one or more numeric fields.'),
        '#multiple' => TRUE,
        '#toggle' => TRUE,
        '#filter' => 'filterNumericFields',
        '#required' => TRUE,
      ],
    ];
  }
  
  /**
   * Restricts the allowed fields to only numeric fields.
   *
   * @param array $fields
   *   An array of field labels, keyed by the field ID.
   */
  protected function filterNumericFields(&$fields) {
    foreach ($this->view->field as $id => $field) {
      if (!$field instanceof NumericField) {
        unset($fields[$id]);
      }
    }
  }

}