Error messages aren't displayed when submitting the form

Question

I want to display an error message when I submit a form. I added an AJAX callback to the form, and the error message appears after I refreshed the page.

I first tried the following code, but it doesn't work.

$response = new \Drupal\Core\Ajax\AjaxResponse();
$message = drupal_get_messages();
$output = $response->addCommand(new \Drupal\Core\Ajax\PrependCommand(".region-highlighted",$message));
return $output;

I also tried the following code.

$status_messages = array('#type' => 'status_messages');
$text = \Drupal::service('renderer')->render($status_messages);
return $text;

Answer 1

You should retrieve the errors with $form_state->getErrors().

So, you should have something like:

/**
   * Ajax submit handler.
   *
   * @param array $form
   * @param \Drupal\Core\Form\FormStateInterface $form_state
   *
   * @return \Drupal\Core\Ajax\AjaxResponse
   */
  public function ajaxSubmit(array &$form, FormStateInterface $form_state) {
    drupal_get_messages();
    $response = new AjaxResponse();
    if ($form_state->getErrors()) {
      unset($form['#prefix']);
      unset($form['#suffix']);
      $form['status_messages'] = [
        '#type' => 'status_messages',
        '#weight' => -10,
      ];
      $response->addCommand(new ReplaceCommand('#my-id', $form));
    }

    return $response;
  }

Check Examples for Developers for more details.

Answer 2

this works for form messages (in a regular ajax form callback):

    $form['status_messages'] = [
      '#type' => 'status_messages',
      '#weight' => -10,
    ];
    $response->addCommand(new ReplaceCommand('#yourFormWrapperElement', $form));

but using drupal_get_messages() screw's things somehow up .... sometimes. plus you need enable showing messages here: /admin/config/development/logging

So - your second try should also work !

$status_messages = array('#type' => 'status_messages');
$text = \Drupal::service('renderer')->render($status_messages);
return $text;

Maybe the mentioned settings are needed ..