/**
* Returns a list of addressbook's groups.
*
* @return array addressbook's groups list.
*/
public function get_addressbook_groups()
{
// get addressbook's id from request
$address_book_id = get_input_value('address_book_id', RCUBE_INPUT_POST);
// get key from request. Key identifies the row wich sent the request
$key = get_input_value('key', RCUBE_INPUT_POST);
// get addressbook by id
$rcube_contacts = $this->rcmail->get_address_book($address_book_id);
// send back to client a group list in array form
$response = array('groups' => $rcube_contacts->list_groups(), 'key' => $key);
$this->rcmail->output->command('plugin.recipient_to_contact_get_addressbook_groups_response', $response);
}