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]);
            }
        }
    }

}