hook_field_formatter_third_party_settings_form doesn't fire

Question

I'm attempting to add a checkbox to a field settings form in D9. I have the following code:

function my_module_field_formatter_third_party_settings_form(\Drupal\Core\Field\FormatterInterface $plugin, \Drupal\Core\Field\FieldDefinitionInterface $field_definition, $view_mode, $form, \Drupal\Core\Form\FormStateInterface $form_state) {
  $element = array();
  print '<h1>HERE</h1>';

  if ($plugin
    ->getPluginId() == 'text_textarea_with_summary') {
    $element['my_setting'] = array(
      '#type' => 'checkbox',
      '#title' => t('test'),
      '#default_value' => $plugin
        ->getThirdPartySetting('my_module', 'my_setting'),
    );
  }
  return $element;
}

I then edit an instance of a body field (admin/structure/types/manage/article/fields/node.article.body), and the hook doesn't fire...no debug text nor stopping at the breakpoint on that same line.

Answer 1

There are two problems.

/admin/structure/types/manage/article/fields/node.article.body is the path of a field setting. To trigger the hook you need to visit /admin/structure/types/manage/article/display.

The plugin ID is wrong, too. text_textarea_with_summary is the plugin ID of a widget. What you are looking for is probably the formatter text_summary_or_trimmed.

If this is neither formatter nor widget, but the content type specific field definition, then use a normal form alter hook, because there is no specific hook to add third party settings:

use Drupal\Core\Form\FormStateInterface;

/**
 * Implements hook_form_FORM_ID_form_alter().
 */
function my_module_form_field_config_edit_form_alter(array &$form, FormStateInterface $form_state) {
  /** @var \Drupal\Core\Field\FieldConfigInterface $field */
  $field = $form_state->getFormObject()->getEntity();
  if ($field->getType() == 'text_with_summary') {
    $form['third_party_settings']['my_module']['my_setting'] = [
      '#type' => 'checkbox',
      '#title' => t('test'),
      '#default_value' => $field->getThirdPartySetting('my_module', 'my_setting'),
    ];
  }
}

Edit: The form keys $form['third_party_settings']['my_module']['my_setting'] are important. They are used to submit the third party settings.