trait TreeStyleTrait in Views tree 8.2

trait TreeStyleTrait {

  /**
   * Gather common options.
   */
  protected function defineCommonOptions(array &$options) {
    $options['main_field'] = [
      'default' => '',
    ];
    $options['parent_field'] = [
      'default' => '',
    ];
  }

  /**
   * Builds common form elements for the options form.
   *
   * @param array $form
   *   The form definition.
   * @param \Drupal\Core\Form\FormStateInterface $form_state
   *   The form state.
   */
  protected function getCommonOptionsForm(array &$form, FormStateInterface $form_state) {
    $fields = [
      '' => $this
        ->t('<None>'),
    ];
    foreach ($this->displayHandler
      ->getHandlers('field') as $field => $handler) {
      $fields[$field] = $handler
        ->adminLabel();
    }
    $form['main_field'] = [
      '#type' => 'select',
      '#title' => $this
        ->t('Main field'),
      '#options' => $fields,
      '#default_value' => $this->options['main_field'],
      '#description' => $this
        ->t('Select the field with the unique identifier for each record.'),
      '#required' => TRUE,
    ];
    $form['parent_field'] = [
      '#type' => 'select',
      '#title' => $this
        ->t('Parent field'),
      '#options' => $fields,
      '#default_value' => $this->options['parent_field'],
      '#description' => $this
        ->t("Select the field that contains the unique identifier of the record's parent."),
    ];
  }

}