Add levels selector class to wordpress nav menu

Question

I would like to add level classes to each li when echoing the results of wp_list_pages. Currently, I'm using:

<?php

   wp_nav_menu(array('theme_location' => 'main_menu', 'container' => '', 'menu_class' => 'fR clearfix', 'menu_id' => 'nav'));

<?php } ?>

The desired output would be:

<ul class="menu">
   <li class="page_item page-item-9 level-0 current_page_item"><a href="" title=""></a>
      <ul class="children expanded" style="display: block; ">
        <li class="page_item page-item-40 level-1"><a href="" title=""></a></li>
        <li class="page_item page-item-43 level-1"><a href="" title=""></a></li>
        <li class="page_item page-item-45 level-1"><a href="" title=""></a></li>
        <li class="page_item page-item-47 level-1"><a href="" title=""></a></li>
      </ul>
   </li>
   <!-- So on -->
</ul>

Is this possible to have the desired output? please help!

Answer 1

There isn't a direct way to do this.

You can use the wp_nav_menu_objects filter and manipulate the menu item's classes.

Here is the code for you:

<?php
add_filter('wp_nav_menu_objects' , 'my_menu_class');
function my_menu_class($menu) {
    $level = 0;
    $stack = array('0');
    foreach($menu as $key => $item) {
        while($item->menu_item_parent != array_pop($stack)) {
            $level--;
        }   
        $level++;
        $stack[] = $item->menu_item_parent;
        $stack[] = $item->ID;
        $menu[$key]->classes[] = 'level-'. ($level - 1);
    }                    
    return $menu;        
}

Answer 2

It's best to use a custom walker to add that class. See Cleaner output for wp_nav_menu() and Improve your Wordpress Navigation Menu Output and T5_Nav_Menu_Walker_Simple — Gist