Why aren't the radio buttons rendered?

Question

According to the documentation for Radios, the following code should output a group of radio buttons.

$form['settings']['active'] = array(
  '#type' => 'radios',
  '#title' => $this->t('Poll status'),
  '#default_value' => 1,
  '#options' => array(0 => $this->t('Closed'), 1 => $this->t('Active')),
);

However, when I use this code, I get an error.

public function radioComponent () {
  $form['posting_settings']['comment_preview'] = array(
    '#type' => 'radios',
    '#title' => t('Preview comment'),
    '#default_value' => '1',
    '#options' => array(t('Optional'), t('Required')),
  );

  return $form;
}

Notice: Undefined index: #title_display in Drupal\Core\Render\Element\Radios::preRenderCompositeFormElement() (line 20 of core\lib\Drupal\Core\Render\Element\CompositeFormElementTrait.php).

Notice: Undefined index: #id in Drupal\Core\Render\Element\Radios::preRenderCompositeFormElement() (line 30 of core\lib\Drupal\Core\Render\Element\CompositeFormElementTrait.php).

Even when I use the following code, the radio buttons aren't rendered.

public function radioComponent () {
  $form['posting_settings']['comment_preview'] = array(
    '#type' => 'radios',
    '#title' => t('Preview comment'),
    '#default_value' => '1',
    '#options' => array(t('Optional'), t('Required')),
    '#id' => 'al1',
    '#title_display' =>'title',
  );

  return $form;
}

Does anyone know why the radios don't render?

Answer 1

if you look at /core/lib/Drupal/Core/Render/Element/Radios.php you'll see it's not a render element, but a form element.

You can't use it without putting it in a form. If you search the code base for extends FormBase, you'll see many examples of forms that you can start from in building your own form, many of them quite simple.

For example: /core/modules/system/tests/modules/form_test/src/Form/FormTestProgrammaticForm.php