How to check if current page is a parent item or child item in a given menu

Question

I'm trying to build a submenu into my theme, and I've got it working except it shows even on top-level pages that don't have children.

How could I check that the current page (or current menu item) is either a parent or a child item (I'm pretty sure that logic holds up) in a given menu?

At the moment I'm trying to do this check outside of the menu function, because it has wrapper divs and a title that I won't want to show at all in cases where the page isn't a parent or a child. However, if the best answer to my problem is to output the whole thing via a custom walker I'm open to how that might work too.

Thanks in advance.

Edit: I've gotten a bit further but am now stuck, I can capture the submenu as a variable, but I can't do the check "If there's nothing in the submenu, show nothing". In the below code, $submenu doesn't return false when there are no items even with that fallback_cb => false.

$main_menu_id = 2; // Change to the main menu id for this install
$top_menu_id = 4; // Change to the top menu id for this install
$in_menu = '';

// Check what menu the page is in, if any
if( coepio_is_in_menu( $main_menu_id ) ) {
    $in_menu = 'main-nav';
} else if( coepio_is_in_menu( $top_menu_id ) ) {
    $in_menu = 'top-nav';
}

// If the page is in a menu...
if( $in_menu != '' ) {

    // Capture submenu as a variable
    $args = array( 
                'theme_location' => $in_menu,
                'echo'           => '0',
                'sub_menu'       => true,
                'falback_cb'     => false
        );
    $submenu = wp_nav_menu( $args );

    // If the menu has items
    if( $submenu ) {
        echo 'Menu is good to go';
        ?>
        <div id="submenu" class="sidebar cf" role="navigation">

            <div class="widget">
                <div id="submenu-title" class="widget-title">
                    <h4>In This Section</h4>
                </div>
                <div class="widget-content">
                    <?php 
                    echo $submenu; 
                    ?>
                </div>
            </div>

        </div><!-- #submenu -->
        <?php
    }

}

You may have noticed that 'sub_menu' => true in the $args. Here's the code that handles that, taken from another SO post.

// Submenus - required for the submenu (above) to work
// filter_hook function to react on sub_menu flag
function coepio_submenu( $sorted_menu_items, $args ) {
  if ( isset( $args->sub_menu ) ) {
$root_id = 0;

// find the current menu item
foreach ( $sorted_menu_items as $menu_item ) {
  if ( $menu_item->current ) {
    // set the root id based on whether the current menu item has a parent or not
    $root_id = ( $menu_item->menu_item_parent ) ? $menu_item->menu_item_parent : $menu_item->ID;
    break;
  }
}

$menu_item_parents = array();
foreach ( $sorted_menu_items as $key => $item ) {
  // init menu_item_parents
  if ( $item->ID == $root_id ) $menu_item_parents[] = $item->ID;

  if ( in_array( $item->menu_item_parent, $menu_item_parents ) ) {
    // part of sub-tree: keep!
    $menu_item_parents[] = $item->ID;
  } else {
    // not part of sub-tree: away with it!
    unset( $sorted_menu_items[$key] );
  }
}

return $sorted_menu_items;
  } else {
return $sorted_menu_items;
  }
}
add_filter( 'wp_nav_menu_objects', 'coepio_submenu', 10, 2 );

Answer 1

I typically use something like:

$submenu = wp_list_pages(
    array(
        'child_of' => $post->ID,
        'echo'     => false,
    )
);

if ( $submenu )
    printf(
        '<ul class="submenu">%s</ul>',
        $submenu
    );

$submenu will evaluate false (empty) if there are no children for the current page.