Add custom class to existing menu items from custom meta

Question

I would like to add a custom class to my existing menu items. The problem is, that I would like to do something like this:

<li class="<?php echo get_post_meta($post->ID, "_icon", true); ?>">...</li>
<li class="<?php echo get_post_meta($post->ID, "_icon", true); ?>">...</li>
<li class="<?php echo get_post_meta($post->ID, "_icon", true); ?>">...</li>

or

<li><a class="<?php echo get_post_meta($post->ID, "_icon", true); ?>">...</a></li>

So I have a custom meta tags for posts and pages and they supposed to be a classes of li or a elements.

I know that I have to use custom walker, and I found this article, but I dont know how to add custom meta tags to $item_output and this example adds new menu.

Greetings

Answer 1

I've got the filter just for you: nav_menu_css_class

function wpse_175057_nav_menu_css_class( $classes, $item ) {
    if ( $item->type === 'post_type' && $class = get_post_meta( $item->object_id, '_icon', true ) )
        $classes[] = $class;
    return $classes;
}

add_filter( 'nav_menu_css_class', 'wpse_175057_nav_menu_css_class', 10, 2 );