Drupal 7 add ajax listener directly to node form

Question

I'm trying to register an ajax listener to a checkbox on one of my standard node forms. I want to display certain regions of the form only in case, the field is checked. I can get the drupal ajax ecosystem to work, when I use it in a custom form, but I'm unable to make it work on my node forms.

First I hook into form_alter to check if I'm with the node type I want to add the listener for:

function interceptor_form_alter(&$form, &$form_state, $form_id) {
  if($form_id === 'film_node_form') {
    interceptor_dvd_listener($form, $form_state);
  }
}

Then I try to attach the ajax stuff to my checkbox, which name is field_dvd:

function interceptor_dvd_listener(&$form, &$form_state) {
  $form['field_dvd'] = array(
    '#title' => t('You want to display DVD informations too?'),
    '#type' => 'checkbox',
    '#ajax' => array(
      'callback' => 'interceptor_dvd_listener_callback',
      'wrapper' => 'checkboxes-div',
      'effect' => 'slide',
      'progress' => array('type' => 'none'),
    ),
  );
  return $form;
}

The function interceptor_dvd_listener_callback is never called. I try to print some debug information there, but nothing happens...

function interceptor_dvd_listener_callback($form, $form_state) {
  data_service_log_object($form);
}

UPDATE

After passing $form as reference to interceptor_dvd_listener the ajax callback function got called like expected. I edited my code above.

Answer 1

This interceptor_dvd_listener_callback() should return value, in documentation:

After form processing is complete, ajax_form_callback() calls the function named by #ajax['callback'], which returns the form element that has been updated and needs to be returned to the browser, or alternatively, an array of custom Ajax commands.

Why don't you use this: https://api.drupal.org/api/drupal/developer!topics!forms_api_reference.html/7#states

It is drupal 7 standard feature to hide or show some fields when some elements has some states.