FormBuilder::rebuildForm

public FormBuilder::rebuildForm($form_id, FormStateInterface &$form_state, $old_form = NULL)

public function rebuildForm($form_id, FormStateInterface &$form_state, $old_form = NULL) {
  $form = $this->retrieveForm($form_id, $form_state);

  // Only GET and POST are valid form methods. If the form receives its input
  // via POST, then $form_state must be persisted when it is rebuilt between
  // submissions. If the form receives its input via GET, then persisting
  // state is forbidden by $form_state->setCached(), and the form must use
  // the URL itself to transfer its state across steps. Although $form_state
  // throws an exception based on the request method rather than the form's
  // method, we base the decision to cache on the form method, because:
  // - It's the form method that defines what the form needs to do to manage
  //   its state.
  // - rebuildForm() should only be called after successful input processing,
  //   which means the request method matches the form method, and if not,
  //   there's some other error, so it's ok if an exception is thrown.
  if ($form_state->isMethodType('POST')) {
    $form_state->setCached();
  }

  // If only parts of the form will be returned to the browser (e.g., Ajax or
  // RIA clients), or if the form already had a new build ID regenerated when
  // it was retrieved from the form cache, reuse the existing #build_id.
  // Otherwise, a new #build_id is generated, to not clobber the previous
  // build's data in the form cache; also allowing the user to go back to an
  // earlier build, make changes, and re-submit.
  // @see self::prepareForm()
  $rebuild_info = $form_state->getRebuildInfo();
  $enforce_old_build_id = isset($old_form['#build_id']) && !empty($rebuild_info['copy']['#build_id']);
  $old_form_is_mutable_copy = isset($old_form['#build_id_old']);
  if ($enforce_old_build_id || $old_form_is_mutable_copy) {
    $form['#build_id'] = $old_form['#build_id'];
    if ($old_form_is_mutable_copy) {
      $form['#build_id_old'] = $old_form['#build_id_old'];
    }
  }
  else {
    if (isset($old_form['#build_id'])) {
      $form['#build_id_old'] = $old_form['#build_id'];
    }
    $form['#build_id'] = 'form-' . Crypt::randomBytesBase64();
  }

  // #action defaults to $request->getRequestUri(), but in case of Ajax and
  // other partial rebuilds, the form is submitted to an alternate URL, and
  // the original #action needs to be retained.
  if (isset($old_form['#action']) && !empty($rebuild_info['copy']['#action'])) {
    $form['#action'] = $old_form['#action'];
  }

  $this->prepareForm($form_id, $form, $form_state);

  // Caching is normally done in self::processForm(), but what needs to be
  // cached is the $form structure before it passes through
  // self::doBuildForm(), so we need to do it here.
  // @todo For Drupal 8, find a way to avoid this code duplication.
  if ($form_state->isCached()) {
    $this->setCache($form['#build_id'], $form, $form_state);
  }

  // Clear out all group associations as these might be different when
  // re-rendering the form.
  $form_state->setGroups([]);

  // Return a fully built form that is ready for rendering.
  return $this->doBuildForm($form_id, $form, $form_state);
}