Custom entity form submit handler not being invoked I'm trying to attach a submit handler to an entity form (language_delete_form) provided by the ConfigurableLanguage config entity. On the form alter function for that form, the top-level submit handler value looks like $form['#submit'] = ['::submitForm'].

I want to add an additional submission handler, so it looks like $form['#submit'] = ['::submitForm', 'locale_delete_language_submit']. However, locale_delete_language_submit is not being called during the submission process (I would expect it to be invoked after LanguageDeleteForm::submitForm is invoked). What am I missing here?

Based on Berdir's comment here (https://www.drupal.org/project/drupal/issues/2252165#comment-9437863) I would have expected the function to be invoked.

Here's my code:

/**
 * Implements hook_form_FORM_ID_alter() for language_delete_form().
 */
function locale_form_language_delete_form_alter(&amp;$form, FormStateInterface $form_state) {
  $x = 5;
  $form['#submit'][] = 'locale_delete_language_submit';
}

/**
 * @param $form
 * @param \Drupal\Core\Form\FormStateInterface $form_state
 */
function locale_delete_language_submit($form, FormStateInterface $form_state) {
  $defined_value = 'fifty';
}
As discussed in slack, you misunderstood my comment. You should add it to the form-level submit callbacks if and only if that form has no button-level submit callbacks.

If it has, then you need to add yours there as well.