Propose new alter hook: field_group_attach_groups to resolve display bug(s).

In instances where you have created field group styles with the API, you may have cases where you want the parent container processed last with all its children processed first.

In my case, I have a new custom Accordion and Tab style (http://drupal.org/project/foundation_group). When the groups are built initially, it is just read straight out of the database. Initially, nothing seems wrong. But when you add new items to put under the parent container, they get processed last, which breaks pre_render hooks and theming later on. Ideally, the group data should be built for the functions in the order in which they are nested (with the parent last), or have a hook that allows the group data to be altered so their ordering can be corrected before any pre rendering occurs.

What happens for me is the following:

Create groups (Parent Container) and create child groups (Container Items) and nest them. Everything works great.

Create more Container items, and nest into the parent. They don't show up, because the parent is processed before the new children (group ordering) which breaks the rest of the calls. I need the parent processed last, because it's using its own #theme callback.

I did a little work and could not find any existing hook throughout Drupal or Field Group to intercept this early on. Then, I added this to field_group_attach_groups, before the pre_render is attached to the $element:

drupal_alter('field_group_attach_groups', $element, $element['#groups']);

Then, in my module:

function foundation_group_field_group_attach_groups_alter(&$groups) {
  $types = _foundation_group_get_types(); // assigns 'parent' container types
  $parents = array();

  foreach ($groups as $key => $group) {
    if (in_array($group->format_type, $types)) {
      unset($groups[$key]);
      $parents[$key] = $group;
    }
  }

  if (count($parents)) {
    $groups += $parents;
  }
}

This allowed me to correctly order the groups to remove parent groups, then reinsert them at the end of the groups array. Then, the rest of the processing happens normally, where children are processed first, then parents (if you have a #theme callback, this is critical).

I think a new hook is a good idea, but maybe also the groups should be ordered in field_group_unpack? I am not sure what the latter suggestion would cause, so the alter hook seems like the best route. I attached a patch.