FormBuilder::retrieveForm

public FormBuilder::retrieveForm($form_id, FormStateInterface &$form_state)

public function retrieveForm($form_id, FormStateInterface &$form_state) {
  // Record the $form_id.
  $form_state->addBuildInfo('form_id', $form_id);

  // We save two copies of the incoming arguments: one for modules to use
  // when mapping form ids to constructor functions, and another to pass to
  // the constructor function itself.
  $build_info = $form_state->getBuildInfo();
  $args = $build_info['args'];

  $callback = [$form_state->getFormObject(), 'buildForm'];

  $form = array();
  // Assign a default CSS class name based on $form_id.
  // This happens here and not in self::prepareForm() in order to allow the
  // form constructor function to override or remove the default class.
  $form['#attributes']['class'][] = Html::getClass($form_id);
  // Same for the base form ID, if any.
  if (isset($build_info['base_form_id'])) {
    $form['#attributes']['class'][] = Html::getClass($build_info['base_form_id']);
  }

  // We need to pass $form_state by reference in order for forms to modify it,
  // since call_user_func_array() requires that referenced variables are
  // passed explicitly.
  $args = array_merge(array($form, &$form_state), $args);

  $form = call_user_func_array($callback, $args);
  // If the form returns a response, skip subsequent page construction by
  // throwing an exception.
  // @see Drupal\Core\EventSubscriber\EnforcedFormResponseSubscriber
  //
  // @todo Exceptions should not be used for code flow control. However, the
  //   Form API currently allows any form builder functions to return a
  //   response.
  //   @see https://www.drupal.org/node/2363189
  if ($form instanceof Response) {
    throw new EnforcedResponseException($form);
  }
  $form['#form_id'] = $form_id;

  return $form;
}