“Drupal8表单构建元素（下拉列表）”

$form['example_select'] = [
  '#type' => 'select',
  '#title' => $this->t('Select element'),
  '#options' => [
    '1' => $this->t('One'),
    '2' => [
      '2.1' => $this->t('Two point one'),
      '2.2' => $this->t('Two point two'),
    ],
    '3' => $this->t('Three'),
  ],
];