WordPress Walker Nav extended to add data attributes for Drop support

class-drop-walker-nav.php

<?php
/**
 * Extends WordPress nav menu by adding data attributes and 
 * markup for Drop by Chris Ferdinandi
 * 
 * Version 6.1.1
 * 
 * drop source: https://github.com/cferdinandi/drop
 */

class Drop_Menu_Walker extends Walker_Nav_Menu {

    function start_lvl( &$output, $depth = 0, $args = array() ) {
        $indent = str_repeat("\t", $depth);
        $output .= "\n$indent<ul class=\"sub-menu\" data-dropdown-menu>\n";
    }

    // adds a has_children conditional to $args
    function display_element( $element, &$children_elements, $max_depth, $depth=0, $args, &$output ) {

        $id_field = $this->db_fields['id'];
        if ( is_object( $args[0] ) ) {
            $args[0]->has_children = ! empty( $children_elements[$element->$id_field] );
        }
        return parent::display_element( $element, $children_elements, $max_depth, $depth, $args, $output );
    }


    function start_el( &$output, $item, $depth = 0, $args = array(), $id = 0 ) {
        // call global db query
        global $wpdb;

        $indent = ( $depth ) ? str_repeat( "\t", $depth ) : '';

        $classes = empty( $item->classes ) ? array() : (array) $item->classes;
        $classes[] = 'menu-item-' . $item->ID;

        /**
         * Filter the CSS class(es) applied to a menu item's list item element.
         *
         * @since 3.0.0
         * @since 4.1.0 The `$depth` parameter was added.
         *
         * @param array  $classes The CSS classes that are applied to the menu item's `<li>` element.
         * @param object $item    The current menu item.
         * @param array  $args    An array of {@see wp_nav_menu()} arguments.
         * @param int    $depth   Depth of menu item. Used for padding.
         */
        $class_names = join( ' ', apply_filters( 'nav_menu_css_class', array_filter( $classes ), $item, $args, $depth ) );
        $class_names = $class_names ? ' class="' . esc_attr( $class_names ) . '"' : '';


        /**
         * Filter the ID applied to a menu item's list item element.
         *
         * @since 3.0.1
         * @since 4.1.0 The `$depth` parameter was added.
         *
         * @param string $menu_id The ID that is applied to the menu item's `<li>` element.
         * @param object $item    The current menu item.
         * @param array  $args    An array of {@see wp_nav_menu()} arguments.
         * @param int    $depth   Depth of menu item. Used for padding.
         */
        $id = apply_filters( 'nav_menu_item_id', 'menu-item-'. $item->ID, $item, $args, $depth );
        $id = $id ? ' id="' . esc_attr( $id ) . '"' : '';

        // if the list item has children add a data attribute
        $li_attributes = $args->has_children ? ' data-dropdown' : '';

        $output .= $indent . '<li' . $id . $class_names . $li_attributes . '>';

        $atts = array();
        $atts['title']  = ! empty( $item->attr_title ) ? $item->attr_title : '';
        $atts['target'] = ! empty( $item->target )     ? $item->target     : '';
        $atts['rel']    = ! empty( $item->xfn )        ? $item->xfn        : '';
        $atts['href']   = ! empty( $item->url )        ? $item->url        : '';

        /**
         * Filter the HTML attributes applied to a menu item's anchor element.
         *
         * @since 3.6.0
         * @since 4.1.0 The `$depth` parameter was added.
         *
         * @param array $atts {
         *     The HTML attributes applied to the menu item's `<a>` element, empty strings are ignored.
         *
         *     @type string $title  Title attribute.
         *     @type string $target Target attribute.
         *     @type string $rel    The rel attribute.
         *     @type string $href   The href attribute.
         * }
         * @param object $item  The current menu item.
         * @param array  $args  An array of {@see wp_nav_menu()} arguments.
         * @param int    $depth Depth of menu item. Used for padding.
         */
        $atts = apply_filters( 'nav_menu_link_attributes', $atts, $item, $args, $depth );

        $attributes = '';
        foreach ( $atts as $attr => $value ) {
            if ( ! empty( $value ) ) {
                $value = ( 'href' === $attr ) ? esc_url( $value ) : esc_attr( $value );
                $attributes .= ' ' . $attr . '="' . $value . '"';
            }
        }

        $item_output = $args->before;
        $item_output .= '<a'. $attributes .'>';
        /** This filter is documented in wp-includes/post-template.php */
        $item_output .= $args->link_before . apply_filters( 'the_title', $item->title, $item->ID ) . $args->link_after;
        $item_output .= '</a>';
        $item_output .= $args->after;

        /**
         * Filter a menu item's starting output.
         *
         * The menu item's starting output only includes `$args->before`, the opening `<a>`,
         * the menu item's title, the closing `</a>`, and `$args->after`. Currently, there is
         * no filter for modifying the opening and closing `<li>` for a menu item.
         *
         * @since 3.0.0
         *
         * @param string $item_output The menu item's starting HTML output.
         * @param object $item        Menu item data object.
         * @param int    $depth       Depth of menu item. Used for padding.
         * @param array  $args        An array of {@see wp_nav_menu()} arguments.
         */
        $output .= apply_filters( 'walker_nav_menu_start_el', $item_output, $item, $depth, $args );
    }

}

/**
 * Front End Markup for the navigation menu
 * 
 * The markup below assumes you will add your own styling and theme location
 * for the menu. Otherwise you can use drops basic styling and change the 
 * class and IDs accordingly
 **/
 
?>
        
	<nav id="menu" class="nav" role="navigation" itemscope="itemscope" itemtype="http://schema.org/SiteNavigationElement">
		<?php wp_nav_menu( 
			array( 
				'theme_location' => 'primary',
				'container'	 => false,
				'menu_id'	 => 'nav-primary',
				'fallback_cb'	 => '',
				"walker"	 => new Drop_Menu_Walker(),
			)
		); ?>
        </nav>
        
<?php

Pure awesome, great code. I really like lots of Chris' code and your walker made Drop fit seamlessly into WordPress with just a bit of tweaking to suit my needs. Thanks!