ConfigHandler::buildForm

public ConfigHandler::buildForm(array $form, FormStateInterface $form_state, Request $request = NULL)

public function buildForm(array $form, FormStateInterface $form_state, Request $request = NULL) {
  /** @var \Drupal\views\Entity\View $view */
  $view = $form_state->get('view');
  $display_id = $form_state->get('display_id');
  $type = $form_state->get('type');
  $id = $form_state->get('id');

  $form = array(
    'options' => array(
      '#tree' => TRUE,
      '#theme_wrappers' => array('container'),
      '#attributes' => array('class' => array('scroll'), 'data-drupal-views-scroll' => TRUE),
    ),
  );
  $executable = $view->getExecutable();
  $save_ui_cache = FALSE;
  if (!$executable->setDisplay($display_id)) {
    $form['markup'] = array('#markup' => $this->t('Invalid display id @display', array('@display' => $display_id)));
    return $form;
  }
  $item = $executable->getHandler($display_id, $type, $id);

  if ($item) {
    $handler = $executable->display_handler->getHandler($type, $id);
    if (empty($handler)) {
      $form['markup'] = array('#markup' => $this->t("Error: handler for @table > @field doesn't exist!", array('@table' => $item['table'], '@field' => $item['field'])));
    }
    else {
      $types = ViewExecutable::getHandlerTypes();

      // If this item can come from the default display, show a dropdown
      // that lets the user choose which display the changes should apply to.
      if ($executable->display_handler->defaultableSections($types[$type]['plural'])) {
        $section = $types[$type]['plural'];
        $form_state->set('section', $section);
        views_ui_standard_display_dropdown($form, $form_state, $section);
      }

      // A whole bunch of code to figure out what relationships are valid for
      // this item.
      $relationships = $executable->display_handler->getOption('relationships');
      $relationship_options = array();

      foreach ($relationships as $relationship) {
        // relationships can't link back to self. But also, due to ordering,
        // relationships can only link to prior relationships.
        if ($type == 'relationship' && $id == $relationship['id']) {
          break;
        }
        $relationship_handler = Views::handlerManager('relationship')->getHandler($relationship);
        // ignore invalid/broken relationships.
        if (empty($relationship_handler)) {
          continue;
        }

        // If this relationship is valid for this type, add it to the list.
        $data = Views::viewsData()->get($relationship['table']);
        if (isset($data[$relationship['field']]['relationship']['base']) && $base = $data[$relationship['field']]['relationship']['base']) {
          $base_fields = Views::viewsDataHelper()->fetchFields($base, $type, $executable->display_handler->useGroupBy());
          if (isset($base_fields[$item['table'] . '.' . $item['field']])) {
            $relationship_handler->init($executable, $executable->display_handler, $relationship);
            $relationship_options[$relationship['id']] = $relationship_handler->adminLabel();
          }
        }
      }

      if (!empty($relationship_options)) {
        // Make sure the existing relationship is even valid. If not, force
        // it to none.
        $base_fields = Views::viewsDataHelper()->fetchFields($view->get('base_table'), $type, $executable->display_handler->useGroupBy());
        if (isset($base_fields[$item['table'] . '.' . $item['field']])) {
          $relationship_options = array_merge(array('none' => $this->t('Do not use a relationship')), $relationship_options);
        }
        $rel = empty($item['relationship']) ? 'none' : $item['relationship'];
        if (empty($relationship_options[$rel])) {
          // Pick the first relationship.
          $rel = key($relationship_options);
          // We want this relationship option to get saved even if the user
          // skips submitting the form.
          $executable->setHandlerOption($display_id, $type, $id, 'relationship', $rel);
          $save_ui_cache = TRUE;
          // Re-initialize with new relationship.
          $item['relationship'] = $rel;
          $handler->init($executable, $executable->display_handler, $item);
        }

        $form['options']['relationship'] = array(
          '#type' => 'select',
          '#title' => $this->t('Relationship'),
          '#options' => $relationship_options,
          '#default_value' => $rel,
          '#weight' => -500,
        );
      }
      else {
        $form['options']['relationship'] = array(
          '#type' => 'value',
          '#value' => 'none',
        );
      }

      $form['#title'] = $this->t('Configure @type: @item', array('@type' => $types[$type]['lstitle'], '@item' => $handler->adminLabel()));

      if (!empty($handler->definition['help'])) {
        $form['options']['form_description'] = array(
          '#markup' => $handler->definition['help'],
          '#theme_wrappers' => array('container'),
          '#attributes' => array('class' => array('js-form-item form-item description')),
          '#weight' => -1000,
        );
      }

      $form['#section'] = $display_id . '-' . $type . '-' . $id;

      // Get form from the handler.
      $handler->buildOptionsForm($form['options'], $form_state);
      $form_state->set('handler', $handler);
    }

    $name = $form_state->get('update_name');

    $view->getStandardButtons($form, $form_state, 'views_ui_config_item_form', $name);
    // Add a 'remove' button.
    $form['actions']['remove'] = array(
      '#type' => 'submit',
      '#value' => $this->t('Remove'),
      '#submit' => array(array($this, 'remove')),
      '#limit_validation_errors' => array(array('override')),
      '#button_type' => 'danger',
    );
  }

  if ($save_ui_cache) {
    $view->cacheSet();
  }

  return $form;
}