how to display part of a menu tree?

Question

I'm trying to deal with Wordpress 3.0. It's rather cool thing but I can't get on with one problem. For example, I have such menu tree. Menu tree is constructed from pages.

Home
   news
   video
   audio
Blog
   About author
   Favourite colors
      red
      blue
      green
My car
   wheels
   tires

The Idea is: main menu consists of root elements: home, blog, my car On the left side I would like to display children elements of current active root element.

For exammple if person is on the "home" page, on the left part he should see:

  news
  video
  audio

If user is on the "Blog" page, he should see:

About author
       Favourite colors
          red
          blue
          green

I can't find an API to do that. Can you sugest me please where can I find it?

UPD: @Jason McCreary I've seen I've seen wp_list_pages() and tried it. I din't get how can I use it: Please, see my template for a page:

    <?php
/*
 Template Name: page_news

 * @package WordPress
 * @subpackage Twenty_Ten
 * @since Twenty Ten 1.0
 */

get_header(); ?>
<h1>page_news</h1>
<h1>Children menu:</h1>
<?php wp_list_pages('echo=0&child_of=8&title_li='); ?>
<div id="container">
        <div id="content" role="main">

        <?php
        /** Get category id by name*/
        //$catId = get_category_by_slug('news')->term_id;
        query_posts('category_name=news');
        get_template_part( 'loop', 'page' );
        ?>

        </div><!-- #content -->
</div><!-- #container -->

<?php get_sidebar(); ?>
<?php get_footer(); ?>

See this line of code:

<?php wp_list_pages('echo=0&child_of=8&title_li='); ?>

I do have the page with id=8 (I see it in URL). Page with id=8 has several children. I want to print them, but they are not printed. The output of the function wp_list_pages() is nothing. I don't know why... :(

Answer 1

You can write a filter_hook to accomplish this.

My method: create an additional start_in argument for wp_nav_menu using my custom hook:

# in functions.php add hook & hook function
add_filter("wp_nav_menu_objects",'my_wp_nav_menu_objects_start_in',10,2);

# filter_hook function to react on start_in argument
function my_wp_nav_menu_objects_start_in( $sorted_menu_items, $args ) {
    if(isset($args->start_in)) {
        $menu_item_parents = array();
        foreach( $sorted_menu_items as $key => $item ) {
            // init menu_item_parents
            if( $item->object_id == (int)$args->start_in ) $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;
    }
}

Next, in your template you just call wp_nav_menu with the additional start_in argument containing the ID of the page you want the children off:

wp_nav_menu( array( 
    'theme_location' => '<name of your menu>',
    'start_in' => $ID_of_page,
    'container' => false,
    'items_wrap' => '%3$s'
) );

Answer 2

I wrote this to print sub-navs of the pages you may be on. If you want to print out the sub-navigation for each of the pages, get the page parent instead of getting the ID. There would be more involved than that, but it's a start.

$menu = wp_get_nav_menu_items( 'Primary Menu' );
$post_ID = get_the_ID();
echo "<ul id='sub-nav'>";
foreach ($menu as $item) {
    if ($post_ID == $item->object_id) { $menu_parent = $item->ID; }
    if (isset($menu_parent) && $item->menu_item_parent == $menu_parent) {
         echo "<li><a href='" . $item->url . "'>". $item->title . "</a></li>";
     }
 }
echo "</ul>";`