ViewsFormBase::getForm

public ViewsFormBase::getForm(ViewEntityInterface $view, $display_id, $js)

public function getForm(ViewEntityInterface $view, $display_id, $js) {
  $form_state = $this->getFormState($view, $display_id, $js);
  $view = $form_state->get('view');
  $key = $form_state->get('form_key');

  // @todo Remove the need for this.
  \Drupal::moduleHandler()->loadInclude('views_ui', 'inc', 'admin');

  // Reset the cache of IDs. Drupal rather aggressively prevents ID
  // duplication but this causes it to remember IDs that are no longer even
  // being used.
  Html::resetSeenIds();

  // check to see if this is the top form of the stack. If it is, pop
  // it off; if it isn't, the user clicked somewhere else and the stack is
  // now irrelevant.
  if (!empty($view->stack)) {
    $identifier = implode('-', array_filter([$key, $view->id(), $display_id, $form_state->get('type'), $form_state->get('id')]));
    // Retrieve the first form from the stack without changing the integer keys,
    // as they're being used for the "2 of 3" progress indicator.
    reset($view->stack);
    list($key, $top) = each($view->stack);
    unset($view->stack[$key]);

    if (array_shift($top) != $identifier) {
      $view->stack = array();
    }
  }

  // Automatically remove the form cache if it is set and the key does
  // not match. This way navigating away from the form without hitting
  // update will work.
  if (isset($view->form_cache) && $view->form_cache['key'] != $key) {
    unset($view->form_cache);
  }

  $form_class = get_class($form_state->getFormObject());
  $response = $this->ajaxFormWrapper($form_class, $form_state);

  // If the form has not been submitted, or was not set for rerendering, stop.
  if (!$form_state->isSubmitted() || $form_state->get('rerender')) {
    return $response;
  }

  // Sometimes we need to re-generate the form for multi-step type operations.
  if (!empty($view->stack)) {
    $stack = $view->stack;
    $top = array_shift($stack);

    // Build the new form state for the next form in the stack.
    $reflection = new \ReflectionClass($view::$forms[$top[1]]);
    /** @var $form_state \Drupal\Core\Form\FormStateInterface */
    $form_state = $reflection->newInstanceArgs(array_slice($top, 3, 2))->getFormState($view, $top[2], $form_state->get('ajax'));
    $form_class = get_class($form_state->getFormObject());

    $form_state->setUserInput(array());
    $form_url = views_ui_build_form_url($form_state);
    if (!$form_state->get('ajax')) {
      return new RedirectResponse($form_url->setAbsolute()->toString());
    }
    $form_state->set('url', $form_url);
    $response = $this->ajaxFormWrapper($form_class, $form_state);
  }
  elseif (!$form_state->get('ajax')) {
    // if nothing on the stack, non-js forms just go back to the main view editor.
    $display_id = $form_state->get('display_id');
    return new RedirectResponse($this->url('entity.view.edit_display_form', ['view' => $view->id(), 'display_id' => $display_id], ['absolute' => TRUE]));
  }
  else {
    $response = new AjaxResponse();
    $response->addCommand(new CloseModalDialogCommand());
    $response->addCommand(new ShowButtonsCommand(!empty($view->changed)));
    $response->addCommand(new TriggerPreviewCommand());
    if ($page_title = $form_state->get('page_title')) {
      $response->addCommand(new ReplaceTitleCommand($page_title));
    }
  }
  // If this form was for view-wide changes, there's no need to regenerate
  // the display section of the form.
  if ($display_id !== '') {
    \Drupal::entityManager()->getFormObject('view', 'edit')->rebuildCurrentTab($view, $response, $display_id);
  }

  return $response;
}