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I am trying to add some ARIA related stuff to the wp_nav_menu function. I use a custom walker class for this purpose:

class Walker_Nav_Menu_With_Aria extends Walker_Nav_Menu {
        function start_lvl( &$output, $depth = 0, $args = array() ) {
                $indent = str_repeat("\t", $depth);
                $output .= "\n$indent<ul class=\"sub-menu\" role=\"group\">\n";
        }

        function end_lvl( &$output, $depth = 0, $args = array() ) {
                $indent = str_repeat("\t", $depth);
                $output .= "$indent</ul>\n";
        }

        function start_el( &$output, $item, $depth = 0, $args = array(), $id = 0 ) {
                $indent = ( $depth ) ? str_repeat( "\t", $depth ) : '';

                $class_names = $value = '';

                $classes = empty( $item->classes ) ? array() : (array) $item->classes;
                $classes[] = 'menu-item-' . $item->ID;

                $class_names = join( ' ', apply_filters( 'nav_menu_css_class', array_filter( $classes ), $item, $args ) );
                $class_names = $class_names ? ' class="' . esc_attr( $class_names ) . '"' : '';

                $id = apply_filters( 'nav_menu_item_id', 'menu-item-'. $item->ID, $item, $args );
                $id = $id ? ' id="' . esc_attr( $id ) . '"' : '';

                // Add attributes for <li>
                $li_attributes  = ' role="treeitem"';
                $li_attributes .= ' aria-expanded="false"';

                $output .= $indent . '<li' . $id . $value . $class_names . $li_attributes .'>';

                $attributes  = ! empty( $item->attr_title ) ? ' title="'  . esc_attr( $item->attr_title ) .'"' : '';
                $attributes .= ! empty( $item->target )     ? ' target="' . esc_attr( $item->target     ) .'"' : '';
                $attributes .= ! empty( $item->xfn )        ? ' rel="'    . esc_attr( $item->xfn        ) .'"' : '';
                $attributes .= ! empty( $item->url )        ? ' href="'   . esc_attr( $item->url        ) .'"' : '';

                // Add attributes for <a>
                $attributes .= $depth == 0 ? ' tabindex="0"' : ' tabindex="-1"';

                $item_output = $args->before;
                $item_output .= '<a'. $attributes .'>';
                $item_output .= $args->link_before . apply_filters( 'the_title', $item->title, $item->ID ) . $args->link_after;
                $item_output .= '</a>';
                $item_output .= $args->after;

                $output .= apply_filters( 'walker_nav_menu_start_el', $item_output, $item, $depth, $args );
        }

        function end_el( &$output, $item, $depth = 0, $args = array() ) {
                $output .= "</li>\n";
        }
}

I just copy-pasted the original Walker_Nav_Menu class and added stuff, but I am getting errors because the $args variable treated like an object is really an array. But the really weird thing is that this error persists, even if I pass in the original walker like this:

wp_nav_menu( array(
    'theme_location' => 'main-nav',
    'walker'         => new Walker_Nav_Menu,
) );

These are the error messages I get:

NOTICE: TRYING TO GET PROPERTY OF NON-OBJECT IN /USERS/RUDOLF/SITES/LOCALHOST/WP/WP-INCLUDES/NAV-MENU-TEMPLATE.PHP ON LINE 88
NOTICE: TRYING TO GET PROPERTY OF NON-OBJECT IN /USERS/RUDOLF/SITES/LOCALHOST/WP/WP-INCLUDES/NAV-MENU-TEMPLATE.PHP ON LINE 90
NOTICE: TRYING TO GET PROPERTY OF NON-OBJECT IN /USERS/RUDOLF/SITES/LOCALHOST/WP/WP-INCLUDES/NAV-MENU-TEMPLATE.PHP ON LINE 90
NOTICE: TRYING TO GET PROPERTY OF NON-OBJECT IN /USERS/RUDOLF/SITES/LOCALHOST/WP/WP-INCLUDES/NAV-MENU-TEMPLATE.PHP ON LINE 92

The lines look like this (In the original file!):

(88) $item_output = $args->before;
(89) $item_output .= '<a'. $attributes .'>';
(90) $item_output .= $args->link_before . apply_filters( 'the_title', $item->title, $item->ID ) . $args->link_after;
(91) $item_output .= '</a>';
(92) $item_output .= $args->after;
share|improve this question
    
Do you get error if not passing walker at all? Arguments should be object, I think that that is what inline docs say and what wp_nav_menu() passes. –  Rarst Jun 16 '13 at 15:02
    
No, if I just use the default wp_nav_menu() function without a walker, it works perfectly. Yeah, but why is it an array then? I even var_dump'ed it, no object. –  Rudolf Jun 16 '13 at 15:26
    
I am not sure what is going on then, because passing new Walker_Nav_Menu should behave identically to not passing walker. Is anything else going on, any plugins hooking into / manipulating menu functionality? –  Rarst Jun 16 '13 at 15:27
    
I just did some additional testing, and I'm sure this is a WordPress bug. In my case, the nav menu I used wasn't assigned in the backend. Therefore the list pages fallback was used, it seems that then the args variable is an array. I will trace this to the source now, then tell you what I found. –  Rudolf Jun 16 '13 at 15:40
    
If I correctly assign a menu, it works! –  Rudolf Jun 16 '13 at 15:40

1 Answer 1

up vote 3 down vote accepted

I get this error when there are no menus defined or no menus set for the location at Appearance->Menus. When that occurs wp_nav_menu uses a page walker fallback.

  1. The fallback (default) for wp_nav_menu is wp_walker_page
  2. which uses wp_page_menu
  3. which uses wp_list_pages
  4. which uses walk_page_tree
  5. which uses Walker_Page not Walker_Nav_Menu.

And apparently the two walkers are not compatible. I don't know why it doesn't fail gracefully. That seems like a bug to me.

With a menu set at wp-admin->Appearance->Menus, your code works.

You can avoid the error by checking to see that there is a menu assigned to the location before trying to use the location.

$locations = get_nav_menu_locations();
if (0 !== $locations['main-nav']) {
  wp_nav_menu( array(
      'theme_location' => 'main-nav',
      'walker'         => new Walker_Nav_Menu_With_Aria,
  ) );
}

Or, if you'd prefer less antihistamine addled code ( thanks @Rarst ):

if (has_nav_menu('primary')) {
  wp_nav_menu( array(
      'theme_location' => 'primary',
      'walker'         => new Walker_Nav_Menu_With_Aria,
  ) );
}
share|improve this answer
    
Fallback is only used when all else fails to display pages. If menu works properly items are walked with walk_nav_menu_tree() which does use Walker_Nav_Menu by default. –  Rarst Jun 16 '13 at 14:57
    
Nice update! Didn't occur to me wrong function might be grabbing the walker (that's meh). Also there is has_nav_menu() for that check. –  Rarst Jun 16 '13 at 15:34
    
That is exactly what I just wanted to post :D The nav menu $args are converted to an object in wp-includes/nav-menu-template.php on line 145 by the wp_nav_menu() function. The wp_page_menu() function which is used as a fallback doesn't do that, but instead just passes the array to the Walker from the nav menu which needs the object. If you just add a $args = (object) $args on the top of the start_el function of the Walker class, you can fix this. Where would be the right place to report this to the WordPress team? –  Rudolf Jun 16 '13 at 15:57
    
@Rudolf core bugs should be reported at core.trac.wordpress.org –  Rarst Jun 16 '13 at 16:00
    
The WP_Post object that is passed as $item is different, too (Because of this, the pages have no names and the list just consists of empty <li> elements). Some serious refactoring would be needed to make these two walkers compatible. –  Rudolf Jun 16 '13 at 16:04

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