Expand Page submenu items for current page Only

ok, so I don’t really know anything about this, walker function, but I’ve added this and called it in the theme, and this is it but the only problem is that it is hiding all the main items as well, but due to my lack of understanding this I’m not sure about how to edit the walker to fix that.. here is the walker code-

class Selective_Walker extends Walker_Nav_Menu
{
    function walk( $elements, $max_depth) {

        $args = array_slice(func_get_args(), 2);
        $output = '';

        if ($max_depth < -1) //invalid parameter
            return $output;

        if (empty($elements)) //nothing to walk
            return $output;

        $id_field = $this->db_fields['id'];
        $parent_field = $this->db_fields['parent'];

        // flat display
        if ( -1 == $max_depth ) {
            $empty_array = array();
            foreach ( $elements as $e )
                $this->display_element( $e, $empty_array, 1, 0, $args, $output );
            return $output;
        }

        /*
         * need to display in hierarchical order
         * separate elements into two buckets: top level and children elements
         * children_elements is two dimensional array, eg.
         * children_elements[10][] contains all sub-elements whose parent is 10.
         */
        $top_level_elements = array();
        $children_elements  = array();
        foreach ( $elements as $e) {
            if ( 0 == $e->$parent_field )
                $top_level_elements[] = $e;
            else
                $children_elements[ $e->$parent_field ][] = $e;
        }

        /*
         * when none of the elements is top level
         * assume the first one must be root of the sub elements
         */
        if ( empty($top_level_elements) ) {

            $first = array_slice( $elements, 0, 1 );
            $root = $first[0];

            $top_level_elements = array();
            $children_elements  = array();
            foreach ( $elements as $e) {
                if ( $root->$parent_field == $e->$parent_field )
                    $top_level_elements[] = $e;
                else
                    $children_elements[ $e->$parent_field ][] = $e;
            }
        }

        $current_element_markers = array( 'current-menu-item', 'current-menu-parent', 'current-menu-ancestor' );  //added by continent7
        foreach ( $top_level_elements as $e ){  //changed by continent7
            // descend only on current tree
            $descend_test = array_intersect( $current_element_markers, $e->classes );
            if ( !empty( $descend_test ) )
                $this->display_element( $e, $children_elements, 2, 0, $args, $output );
        }

        /*
         * if we are displaying all levels, and remaining children_elements is not empty,
         * then we got orphans, which should be displayed regardless
         */
         /* removed by continent7
        if ( ( $max_depth == 0 ) && count( $children_elements ) > 0 ) {
            $empty_array = array();
            foreach ( $children_elements as $orphans )
                foreach( $orphans as $op )
                    $this->display_element( $op, $empty_array, 1, 0, $args, $output );
         }
        */
         return $output;
    }
}

ok, so in my header I now have this:

<!-- begin #nav_main_wrap -->
<div id="nav_main_wrap"><div id="nav_main">
 <?php wp_nav_menu( array('theme_location'=>'primary','depth' => 1) ); ?>
<span class="clear">&nbsp;</span>
</div></div>
<!-- end #nav_main_wrap -->
<!-- begin #nav_sub_wrap -->
<div id="nav_sub_wrap"><div id="nav_sub">
	 <?php wp_nav_menu( array('theme_location'=>'primary','walker' =>new SH_Child_Only_Walker(),'depth' => 0) ); ?>
	<span class="clear">&nbsp;</span>
</div></div>
<!-- end #nav_sub_wrap -->

Which is displaying the top level of the menu fine, the walker for the sub menu seems to still be keeping it completely hidden, here is the walker function code

class SH_Child_Only_Walker extends Walker_Nav_Menu {  

    // Don't start the top level
    function start_lvl(&$output, $depth=0, $args=array()) {
        if( 0 == $depth )
            return;
        parent::start_lvl(&$output, $depth,$args);
    }  

    // Don't end the top level
    function end_lvl(&$output, $depth=0, $args=array()) {
        if( 0 == $depth )
            return;
        parent::end_lvl(&$output, $depth,$args);
    }  

    // Don't print top-level elements
    function start_el(&$output, $item, $depth=0, $args=array()) {
        if( 0 == $depth )
            return;
        parent::start_el(&$output, $item, $depth, $args);
    }  

    function end_el(&$output, $item, $depth=0, $args=array()) {
        if( 0 == $depth )
            return;
        parent::end_el(&$output, $item, $depth, $args);
    }  

    // Only follow down one branch
    function display_element( $element, &$children_elements, $max_depth, $depth=0, $args, &$output ) {  

        // Check if element as a 'current element' class
        $current_element_markers = array( 'current-menu-item', 'current-menu-parent', 'current-menu-ancestor' );
        $current_class = array_intersect( $current_element_markers, $element->classes ); 

        // If element has a 'current' class, it is an ancestor of the current element
        $ancestor_of_current = !empty($current_class);  

        // If this is a top-level link and not the current, or ancestor of the current menu item - stop here.
        if ( 0 == $depth && !$ancestor_of_current)
            return  

        parent::display_element( $element, &$children_elements, $max_depth, $depth, $args, &$output );
    }
}

Ok, I was able to get it working! just had to add a missing colon in the walker function, here is the working walker:

class SH_Child_Only_Walker extends Walker_Nav_Menu {  

    // Don't start the top level
    function start_lvl(&$output, $depth=0, $args=array()) {
        if( 0 == $depth )
            return;
        parent::start_lvl(&$output, $depth,$args);
    }  

    // Don't end the top level
    function end_lvl(&$output, $depth=0, $args=array()) {
        if( 0 == $depth )
            return;
        parent::end_lvl(&$output, $depth,$args);
    }  

    // Don't print top-level elements
    function start_el(&$output, $item, $depth=0, $args=array()) {
        if( 0 == $depth )
            return;
        parent::start_el(&$output, $item, $depth, $args);
    }  

    function end_el(&$output, $item, $depth=0, $args=array()) {
        if( 0 == $depth )
            return;
        parent::end_el(&$output, $item, $depth, $args);
    }  

    // Only follow down one branch
    function display_element( $element, &$children_elements, $max_depth, $depth=0, $args, &$output ) {  

        // Check if element as a 'current element' class
        $current_element_markers = array( 'current-menu-item', 'current-menu-parent', 'current-menu-ancestor' );
        $current_class = array_intersect( $current_element_markers, $element->classes ); 

        // If element has a 'current' class, it is an ancestor of the current element
        $ancestor_of_current = !empty($current_class);  

        // If this is a top-level link and not the current, or ancestor of the current menu item - stop here.
        if ( 0 == $depth && !$ancestor_of_current)
            return;  

        parent::display_element( $element, &$children_elements, $max_depth, $depth, $args, &$output );
    }
}