ViewsExposedForm::buildForm

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

public function buildForm(array $form, FormStateInterface $form_state) {
  // Don't show the form when batch operations are in progress.
  if ($batch = batch_get() && isset($batch['current_set'])) {
    return array(
      // Set the theme callback to be nothing to avoid errors in template_preprocess_views_exposed_form().
      '#theme' => '',
    );
  }

  // Make sure that we validate because this form might be submitted
  // multiple times per page.
  $form_state->setValidationEnforced();
  /** @var \Drupal\views\ViewExecutable $view */
  $view = $form_state->get('view');
  $display = &$form_state->get('display');

  $form_state->setUserInput($view->getExposedInput());

  // Let form plugins know this is for exposed widgets.
  $form_state->set('exposed', TRUE);
  // Check if the form was already created
  if ($cache = $this->exposedFormCache->getForm($view->storage->id(), $view->current_display)) {
    return $cache;
  }

  $form['#info'] = array();

  // Go through each handler and let it generate its exposed widget.
  foreach ($view->display_handler->handlers as $type => $value) {
    /** @var \Drupal\views\Plugin\views\ViewsHandlerInterface $handler */
    foreach ($view->$type as $id => $handler) {
      if ($handler->canExpose() && $handler->isExposed()) {
        // Grouped exposed filters have their own forms.
        // Instead of render the standard exposed form, a new Select or
        // Radio form field is rendered with the available groups.
        // When an user choose an option the selected value is split
        // into the operator and value that the item represents.
        if ($handler->isAGroup()) {
          $handler->groupForm($form, $form_state);
          $id = $handler->options['group_info']['identifier'];
        }
        else {
          $handler->buildExposedForm($form, $form_state);
        }
        if ($info = $handler->exposedInfo()) {
          $form['#info']["$type-$id"] = $info;
        }
      }
    }
  }

  $form['actions'] = array(
    '#type' => 'actions'
  );
  $form['actions']['submit'] = array(
    // Prevent from showing up in \Drupal::request()->query.
    '#name' => '',
    '#type' => 'submit',
    '#value' => $this->t('Apply'),
    '#id' => Html::getUniqueId('edit-submit-' . $view->storage->id()),
  );

  $form['#action'] = $view->hasUrl() ? $view->getUrl()->toString() : Url::fromRoute('<current>')->toString();
  $form['#theme'] = $view->buildThemeFunctions('views_exposed_form');
  $form['#id'] = Html::cleanCssIdentifier('views_exposed_form-' . $view->storage->id() . '-' . $display['id']);

  /** @var \Drupal\views\Plugin\views\exposed_form\ExposedFormPluginInterface $exposed_form_plugin */
  $exposed_form_plugin = $view->display_handler->getPlugin('exposed_form');
  $exposed_form_plugin->exposedFormAlter($form, $form_state);

  // Save the form.
  $this->exposedFormCache->setForm($view->storage->id(), $view->current_display, $form);

  return $form;
}