MenuForm::form

public MenuForm::form(array $form, FormStateInterface $form_state)

public function form(array $form, FormStateInterface $form_state) {
  $menu = $this->entity;

  if ($this->operation == 'edit') {
    $form['#title'] = $this->t('Edit menu %label', array('%label' => $menu->label()));
  }

  $form['label'] = array(
    '#type' => 'textfield',
    '#title' => $this->t('Title'),
    '#default_value' => $menu->label(),
    '#required' => TRUE,
  );
  $form['id'] = array(
    '#type' => 'machine_name',
    '#title' => $this->t('Menu name'),
    '#default_value' => $menu->id(),
    '#maxlength' => MENU_MAX_MENU_NAME_LENGTH_UI,
    '#description' => $this->t('A unique name to construct the URL for the menu. It must only contain lowercase letters, numbers and hyphens.'),
    '#machine_name' => array(
      'exists' => array($this, 'menuNameExists'),
      'source' => array('label'),
      'replace_pattern' => '[^a-z0-9-]+',
      'replace' => '-',
    ),
    // A menu's machine name cannot be changed.
    '#disabled' => !$menu->isNew() || $menu->isLocked(),
  );
  $form['description'] = array(
    '#type' => 'textfield',
    '#title' => t('Administrative summary'),
    '#maxlength' => 512,
    '#default_value' => $menu->getDescription(),
  );

  $form['langcode'] = array(
    '#type' => 'language_select',
    '#title' => t('Menu language'),
    '#languages' => LanguageInterface::STATE_ALL,
    '#default_value' => $menu->language()->getId(),
  );

  // Add menu links administration form for existing menus.
  if (!$menu->isNew() || $menu->isLocked()) {
    // Form API supports constructing and validating self-contained sections
    // within forms, but does not allow handling the form section's submission
    // equally separated yet. Therefore, we use a $form_state key to point to
    // the parents of the form section.
    // @see self::submitOverviewForm()
    $form_state->set('menu_overview_form_parents', ['links']);
    $form['links'] = array();
    $form['links'] = $this->buildOverviewForm($form['links'], $form_state);
  }

  return parent::form($form, $form_state);
}