Add ‘has_children’ class to parent li when modifying Walker_Nav_Menu

I’m writing a customised walker class for wp_nav_menu and want to be able to specify if an li contains a submenu. So I want my markup to be:

<li class="has_children [other-wordpress-classes]">
    <a class="parent-link">Some item</a>
    <ul class="sub-menu">

I know how to add and remove the classes fine, I just cant find anything to tell me if the current item has children items.

Any ideas?

Thanks in advance.

start_el() should get this information in its $args parameter, but it appears WordPress only fills this in if $args is an array, while for the custom navigation menus it is an object. This is reported in a Trac ticket. But no problem, you can fill this in yourself, if you also override the display_element() method in your custom walker (because this is the easiest place to access the child element array):

class WPSE16818_Walker extends Walker_Nav_Menu
{
    function display_element( $element, &$children_elements, $max_depth, $depth=0, $args, &$output )
    {
        $id_field = $this->db_fields['id'];
        if ( is_object( $args[0] ) ) {
            $args[0]->has_children = ! empty( $children_elements[$element->$id_field] );
        }
        return parent::display_element( $element, $children_elements, $max_depth, $depth, $args, $output );
    }

    function start_el( &$output, $item, $depth, $args ) {
        if ( $args->has_children ) {
            // ...
        }
    }