extended Walker_Page doesn’t output custom start_lvl() or end_lvl()

Question

I have a custom Walker_Page class that I have extended like this:

class List_Nav extends Walker_Page {
  function start_lvl( &$output, $depth = 0, $args = array() ) {
    $indent = str_repeat("\t", $depth);
    $output .= "\n$indent<ul class='ListNav'>\n";
  }

  function start_el(&$output, $page, $depth = 0, $args = array(), $current_page = 0) {
    $output .= '<li class="ListNav-item">';
    $output .= '<a class="ListNav-link" href="' . get_permalink($page->ID) . '">' . apply_filters( 'the_title', $page->post_title, $page->ID ) . '</a>';
    $output .= '</li>';
  }

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

But I’m not getting any output from the start_lvl or end_lvl functions. Is there something I’m missing here or that I need to return? I’m getting the <li> output from start_el().

---

Update with usage

Here’s how I’m using the walker:

if ($post->post_parent) {
  $ancestors=get_post_ancestors($post->ID);
  $root = count($ancestors) - 1;
  $top_parent = $ancestors[$root];
} else {
  $top_parent = $post->ID;
}

$page_list_args = array(
  'child_of'     => $top_parent,
  'depth'        => 0,
  'title_li'     => false,
  'walker'       => new List_Nav
);

wp_list_pages($page_list_args);

Answer 1

Looks like start_lvl() and end_lvl() is always called within the loop and never on a first level. This goes for all WordPress Walkers like Walker_Nav_Menu, Walker_Page and Walker_Category.

It’s not super clear but you could guess it when you look at the Core Walker code or when you read the Walker documentation about start_lvl().

But in the documentation for Walker::start_lvl it just says that it...

Starts the list before the elements are added.

So perhaps what should be done is an update in the docs.