Twitter Bootstrap Walker for WordPress Menu with some simple mega-menu support

functions.php

/**
 * Bootstrap Walker for Wordpress Navigation Menu
 *  
 * Extends WordPress walker class to add support for Twitter Bootstrap navigation menu and JS dropdowns.
 * Caveats and features:
 * - Only top level elements with children will be turned into dropdowns
 * - Can be used in a Bootstrap nav but needs some CSS styling to improve presentation
 * - With extra CSS multiple columns can be shown in a dropdown (sort of a simple mega-menu)
 * 
 * Multi column dropdowns (mega-menu) via CSS:
 * - assign the class .columnizer to top level item in Wordpress
 * - only columns one level deep are supported (upon opening dropdown)
 * - each column can have dividers, and further 
 * - in stylesheet, assign:
 *      .columnizer .frame  { display: table }
 *      .columnizer .column { display: table-cell }
 * 
 * @todo improve support for native Bootstrap CSS
 * @todo improve outputted code indentation
 */
class Bootstrap_Walker_Nav_Menu extends Walker_Nav_Menu {

        // <ul> elements
        function start_lvl( &$output, $depth ) {
          
                // code indentation
                $indent = ( $depth > 0  ? str_repeat( "\t", $depth ) : '' ); 
                
                // build classes
                $classes = array(
                                    // only the top level will have dropdowns
                                    $depth == 0 ? 'dropdown-menu' : '',
                                    // assign a class to items nested deeper
                                    $depth >= 1 ? 'sub-menu' : ''
                                );
                $class_names = implode( ' ', $classes );
                
                // build html - <ul> becomes <div> at top levels
                if ( $depth == 0 )  $output .= "\n" . $indent . '<div class="' . $class_names . '">' .  "\n" . $indent . '<div class="frame">' . "\n";
                else $output .= "\n" . $indent . '<ul class="' . $class_names . '">' . "\n";
                
            
        }

        // <li> elements and <a> links
        function start_el( &$output, $item, $depth = 0, $args = array(), $id = 0 ) {
		
                // code indentation
		$indent = ( $depth ) ? str_repeat( "\t", $depth ) : '';
 
                // prepare
		$li_attributes = '';
		$class_names = $value = '';
		$classes = empty( $item->classes ) ? array() : (array) $item->classes;
		
		// build classes and attributes for <li> elements that have nested <ul> 
		if ( $args->has_children && $depth == 0 ) {
                    
                        $classes[] = 'dropdown';
			$li_attributes .= 'data-dropdown="dropdown"';
                        
		}
		$classes[] = 'menu-item-' . $item->ID;
		// if we are on the current page, add the active class to that menu item
		// $classes[] = ($item->current) ? 'active' : '';
		// add all of the wordpress classes, including those set by user in Wordpress
		$class_names = join( ' ', apply_filters( 'nav_menu_css_class', array_filter( $classes ), $item, $args ) );
		$class_names = ' class="' . esc_attr( $class_names ) . '"';
                
                // add id to <li> elements
		$id = apply_filters( 'nav_menu_item_id', 'menu-item-'. $item->ID, $item, $args );
		$id = strlen( $id ) ? ' id="' . esc_attr( $id ) . '"' : '';
                
                // build html - the <li> elements at top level become <div>
                if ( $depth == 0 ) $output .= $indent . '<li>' . "\n" . $indent . '<div ' . $id . $value . $class_names . $li_attributes . '>';
                // 
                elseif ( $depth == 1 ) $output .= $indent . '<div class="column">';
		else $output .= $indent . '<li' . $id . $value . $class_names . $li_attributes . '>';
 
		// build attributes and classes for <a> link elements
		$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 ) .'"' : '';
		$attributes .= ( $args->has_children && $depth == 0 ) ? ' class="dropdown-toggle" data-toggle="dropdown"' : ''; 
 
                // build <a> link html
		$item_output = $args->before;
		$item_output .= '<a'. $attributes .'>';
		$item_output .= $args->link_before . apply_filters( 'the_title', $item->title, $item->ID ) . $args->link_after;
                // add a bootstrap "caret" to top level dropdowns toggles
		$item_output .= ( $args->has_children && $depth == 0 ) ? ' <b class="caret"></b> ' : ''; 
		$item_output .= '</a>';
		$item_output .= $args->after;
 
                // final html output
                $output .= apply_filters( 'walker_nav_menu_start_el', $item_output, $item, $depth, $args );
                
	}
        
        function end_el(&$output, $item, $depth) {
            
                // closes elements opened in start_el
                if ( $depth == 0 ) $output .= '</div>' . "\n" . '</li>' . "\n";
                elseif ( $depth == 1 ) $output .= '</div>' . "\n";
		else $output .= "</li>\n";
                
	}
        
        function end_lvl(&$output, $depth) {
            
                // code indentation
		$indent = ( $depth > 0  ? str_repeat( "\t", $depth ) : '' );
                
                // closes elements opened in start_lvl
                if ( $depth == 0 ) $output .= $indent . '</div>' . "\n" . $indent . '</div>' . "\n";
		else $output .= $indent . '</ul>' . "\n";
                
	}
        
        // Overwrite display_element function to add has_children attribute @since WP 3.4
	function display_element( $element, &$children_elements, $max_depth, $depth=0, $args, &$output ) {
		
		if ( !$element )
			return;
		
		$id_field = $this->db_fields['id'];
 
		//display this element
		if ( is_array( $args[0] ) ) 
			$args[0]['has_children'] = ! empty( $children_elements[$element->$id_field] );
		else if ( is_object( $args[0] ) ) 
			$args[0]->has_children = ! empty( $children_elements[$element->$id_field] ); 
		$cb_args = array_merge( array(&$output, $element, $depth), $args);
		call_user_func_array(array(&$this, 'start_el'), $cb_args);
 
		$id = $element->$id_field;
 
		// descend only when the depth is right and there are children for this element
		if ( ($max_depth == 0 || $max_depth > $depth+1 ) && isset( $children_elements[$id]) ) {
 
			foreach( $children_elements[ $id ] as $child ){
 
				if ( !isset($newlevel) ) {
					$newlevel = true;
					//start the child delimiter
					$cb_args = array_merge( array(&$output, $depth), $args);
					call_user_func_array(array(&$this, 'start_lvl'), $cb_args);
				}
				$this->display_element( $child, $children_elements, $max_depth, $depth + 1, $args, $output );
			}
				unset( $children_elements[ $id ] );
		}
 
		if ( isset($newlevel) && $newlevel ){
			//end the child delimiter
			$cb_args = array_merge( array(&$output, $depth), $args);
			call_user_func_array(array(&$this, 'end_lvl'), $cb_args);
		}
 
		//end this element
		$cb_args = array_merge( array(&$output, $element, $depth), $args);
		call_user_func_array(array(&$this, 'end_el'), $cb_args);
		
	}

}

style.css

.columnizer .frame  { display: table }
.columnizer .column { display: table-cell }