AddHandler::buildForm

public AddHandler::buildForm(array $form, FormStateInterface $form_state)

public function buildForm(array $form, FormStateInterface $form_state) {
  $view = $form_state->get('view');
  $display_id = $form_state->get('display_id');
  $type = $form_state->get('type');

  $form = array(
    'options' => array(
      '#theme_wrappers' => array('container'),
      '#attributes' => array('class' => array('scroll'), 'data-drupal-views-scroll' => TRUE),
    ),
  );

  $executable = $view->getExecutable();
  if (!$executable->setDisplay($display_id)) {
    $form['markup'] = array('#markup' => $this->t('Invalid display id @display', array('@display' => $display_id)));
    return $form;
  }
  $display = &$executable->displayHandlers->get($display_id);

  $types = ViewExecutable::getHandlerTypes();
  $ltitle = $types[$type]['ltitle'];
  $section = $types[$type]['plural'];

  if (!empty($types[$type]['type'])) {
    $type = $types[$type]['type'];
  }

  $form['#title'] = $this->t('Add @type', array('@type' => $ltitle));
  $form['#section'] = $display_id . 'add-handler';

  // Add the display override dropdown.
  views_ui_standard_display_dropdown($form, $form_state, $section);

  // Figure out all the base tables allowed based upon what the relationships provide.
  $base_tables = $executable->getBaseTables();
  $options = Views::viewsDataHelper()->fetchFields(array_keys($base_tables), $type, $display->useGroupBy(), $form_state->get('type'));

  if (!empty($options)) {
    $form['override']['controls'] = array(
      '#theme_wrappers' => array('container'),
      '#id' => 'views-filterable-options-controls',
      '#attributes' => ['class' => ['form--inline', 'views-filterable-options-controls']],
    );
    $form['override']['controls']['options_search'] = array(
      '#type' => 'textfield',
      '#title' => $this->t('Search'),
    );

    $groups = array('all' => $this->t('- All -'));
    $form['override']['controls']['group'] = array(
      '#type' => 'select',
      '#title' => $this->t('Category'),
      '#options' => array(),
    );

    $form['options']['name'] = array(
      '#prefix' => '<div class="views-radio-box form-checkboxes views-filterable-options">',
      '#suffix' => '</div>',
      '#type' => 'tableselect',
      '#header' => array(
        'title' => $this->t('Title'),
        'group' => $this->t('Category'),
        'help' => $this->t('Description'),
      ),
      '#js_select' => FALSE,
    );

    $grouped_options = array();
    foreach ($options as $key => $option) {
      $group = preg_replace('/[^a-z0-9]/', '-', strtolower($option['group']));
      $groups[$group] = $option['group'];
      $grouped_options[$group][$key] = $option;
      if (!empty($option['aliases']) && is_array($option['aliases'])) {
        foreach ($option['aliases'] as $id => $alias) {
          if (empty($alias['base']) || !empty($base_tables[$alias['base']])) {
            $copy = $option;
            $copy['group'] = $alias['group'];
            $copy['title'] = $alias['title'];
            if (isset($alias['help'])) {
              $copy['help'] = $alias['help'];
            }

            $group = preg_replace('/[^a-z0-9]/', '-', strtolower($copy['group']));
            $groups[$group] = $copy['group'];
            $grouped_options[$group][$key . '$' . $id] = $copy;
          }
        }
      }
    }

    foreach ($grouped_options as $group => $group_options) {
      foreach ($group_options as $key => $option) {
        $form['options']['name']['#options'][$key] = array(
          '#attributes' => array(
            'class' => array('filterable-option', $group),
          ),
          'title' => array(
            'data' => array(
              '#title' => $option['title'],
              '#plain_text' => $option['title'],
            ),
            'class' => array('title'),
          ),
          'group' => $option['group'],
          'help' => array(
            'data' => $option['help'],
            'class' => array('description'),
          ),
        );
      }
    }

    $form['override']['controls']['group']['#options'] = $groups;
  }
  else {
    $form['options']['markup'] = array(
      '#markup' => '<div class="js-form-item form-item">' . $this->t('There are no @types available to add.', array('@types' => $ltitle)) . '</div>',
    );
  }
  // Add a div to show the selected items
  $form['selected'] = array(
    '#type' => 'item',
    '#markup' => '<span class="views-ui-view-title">' . $this->t('Selected:') . '</span> ' . '<div class="views-selected-options"></div>',
    '#theme_wrappers' => array('form_element', 'views_ui_container'),
    '#attributes' => array(
      'class' => array('container-inline', 'views-add-form-selected', 'views-offset-bottom'),
      'data-drupal-views-offset' => 'bottom',
    ),
  );
  $view->getStandardButtons($form, $form_state, 'views_ui_add_handler_form', $this->t('Add and configure @types', array('@types' => $ltitle)));

  // Remove the default submit function.
  $form['actions']['submit']['#submit'] = array_filter($form['actions']['submit']['#submit'], function($var) {
    return !(is_array($var) && isset($var[1]) && $var[1] == 'standardSubmit');
  });
  $form['actions']['submit']['#submit'][] = array($view, 'submitItemAdd');

  return $form;
}