最近在开发一款新主题，打算布局使用Bootstrap来做。
但是在具体开发的时候遇到了问题，就是WordPress的菜单函数 “wp_nav_menu”默认只能定义第一层菜单的样式，并不能很好的支持第二三级菜单的自定义。
在网上搜了下解决方案，发现可以自定义Walker_Nav_Menu类来到到修改第二三级菜单样式的目的。
在此记录下方法。
一、添加样式类
添加自定义的Walker_Nav_Menu 类到主题的function文件中，或者直接新建一个wp-bootstrap-navwalker.php 文件，放在主题目录中。
以下代码来自gitbub，大家也可以直接到github上下载最新版。
项目地址：https://github.com/wp-bootstrap/wp-bootstrap-navwalker
<?php /** * Created by PhpStorm. * User: lius * Date: 2017/7/15 * Time: 12:51 * From: 等英博客出品 http://www.waitig.com */ /** * Class Name: WP_Bootstrap_Navwalker * Plugin Name: WP Bootstrap Navwalker * Plugin URI: https://github.com/wp-bootstrap/wp-bootstrap-navwalker * Description: A custom WordPress nav walker class to implement the Bootstrap 3 navigation style in a custom theme using the WordPress built in menu manager. * Author: Edward McIntyre - @twittem, WP Bootstrap * Version: 2.0.5 * Author URI: https://github.com/wp-bootstrap * GitHub Plugin URI: https://github.com/wp-bootstrap/wp-bootstrap-navwalker * GitHub Branch: master * License: GPL-3.0+ * License URI: http://www.gnu.org/licenses/gpl-3.0.txt */ /* Check if Class Exists. */ if ( ! class_exists( 'WP_Bootstrap_Navwalker' ) ) { /** * WP_Bootstrap_Navwalker class. * * @extends Walker_Nav_Menu */ class WP_Bootstrap_Navwalker extends Walker_Nav_Menu { /** * Start Level. * * @see Walker::start_lvl() * @since 3.0.0 * * @access public * @param mixed $output Passed by reference. Used to append additional content. * @param int $depth (default: 0) Depth of page. Used for padding. * @param array $args (default: array()) Arguments. * @return void */ public function start_lvl( &$output, $depth = 0, $args = array() ) { $indent = str_repeat( "\t", $depth ); $output .= "\n$indent<ul role=\"menu\" class=\" dropdown-menu\" >\n"; } /** * Start El. * * @see Walker::start_el() * @since 3.0.0 * * @access public * @param mixed $output Passed by reference. Used to append additional content. * @param mixed $item Menu item data object. * @param int $depth (default: 0) Depth of menu item. Used for padding. * @param array $args (default: array()) Arguments. * @param int $id (default: 0) Menu item ID. * @return void */ public function start_el( &$output, $item, $depth = 0, $args = array(), $id = 0 ) { $indent = ( $depth ) ? str_repeat( "\t", $depth ) : ''; /** * Dividers, Headers or Disabled * ============================= * Determine whether the item is a Divider, Header, Disabled or regular * menu item. To prevent errors we use the strcasecmp() function to so a * comparison that is not case sensitive. The strcasecmp() function returns * a 0 if the strings are equal. */ if ( 0 === strcasecmp( $item->attr_title, 'divider' ) && 1 === $depth ) { $output .= $indent . '<li role="presentation" class="divider">'; } elseif ( 0 === strcasecmp( $item->title, 'divider' ) && 1 === $depth ) { $output .= $indent . '<li role="presentation" class="divider">'; } elseif ( 0 === strcasecmp( $item->attr_title, 'dropdown-header' ) && 1 === $depth ) { $output .= $indent . '<li role="presentation" class="dropdown-header">' . esc_attr( $item->title ); } elseif ( 0 === strcasecmp( $item->attr_title, 'disabled' ) ) { $output .= $indent . '<li role="presentation" class="disabled"><a href="#">' . esc_attr( $item->title ) . '</a>'; } else { $value = ''; $class_names = $value; $classes = empty( $item->classes ) ? array() : (array) $item->classes; $classes[] = 'menu-item-' . $item->ID; $class_names = join( ' ', apply_filters( 'nav_menu_css_class', array_filter( $classes ), $item, $args ) ); if ( $args->has_children ) { $class_names .= ' dropdown'; } if ( in_array( 'current-menu-item', $classes, true ) ) { $class_names .= ' active'; } $class_names = $class_names ? ' class="' . esc_attr( $class_names ) . '"' : ''; $id = apply_filters( 'nav_menu_item_id', 'menu-item-' . $item->ID, $item, $args ); $id = $id ? ' id="' . esc_attr( $id ) . '"' : ''; $output .= $indent . '<li itemscope="itemscope" itemtype="https://www.schema.org/SiteNavigationElement"' . $id . $value . $class_names . '>'; $atts = array(); if ( empty( $item->attr_title ) ) { $atts['title'] = ! empty( $item->title ) ? strip_tags( $item->title ) : ''; } else { $atts['title'] = $item->attr_title; } $atts['target'] = ! empty( $item->target ) ? $item->target : ''; $atts['rel'] = ! empty( $item->xfn ) ? $item->xfn : ''; // If item has_children add atts to a. if ( $args->has_children && 0 === $depth ) { $atts['href'] = '#'; $atts['data-toggle'] = 'dropdown'; $atts['class'] = 'dropdown-toggle'; $atts['aria-haspopup'] = 'true'; } else { $atts['href'] = ! empty( $item->url ) ? $item->url : ''; } $atts = apply_filters( 'nav_menu_link_attributes', $atts, $item, $args ); $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; /* * Glyphicons/Font-Awesome * =========== * Since the the menu item is NOT a Divider or Header we check the see * if there is a value in the attr_title property. If the attr_title * property is NOT null we apply it as the class name for the glyphicon. */ if ( ! empty( $item->attr_title ) ) { $pos = strpos( esc_attr( $item->attr_title ), 'glyphicon' ); if ( false !== $pos ) { $item_output .= '<a' . $attributes . '><span class="glyphicon ' . esc_attr( $item->attr_title ) . '" aria-hidden="true"></span> '; } else { $item_output .= '<a' . $attributes . '><i class="fa ' . esc_attr( $item->attr_title ) . '" aria-hidden="true"></i> '; } } else { $item_output .= '<a' . $attributes . '>'; } $item_output .= $args->link_before . apply_filters( 'the_title', $item->title, $item->ID ) . $args->link_after; $item_output .= ( $args->has_children && 0 === $depth ) ? ' <span class="caret"></span></a>' : '</a>'; $item_output .= $args->after; $output .= apply_filters( 'walker_nav_menu_start_el', $item_output, $item, $depth, $args ); } // End if(). } /** * Traverse elements to create list from elements. * * Display one element if the element doesn't have any children otherwise, * display the element and its children. Will only traverse up to the max * depth and no ignore elements under that depth. * * This method shouldn't be called directly, use the walk() method instead. * * @see Walker::start_el() * @since 2.5.0 * * @access public * @param mixed $element Data object. * @param mixed $children_elements List of elements to continue traversing. * @param mixed $max_depth Max depth to traverse. * @param mixed $depth Depth of current element. * @param mixed $args Arguments. * @param mixed $output Passed by reference. Used to append additional content. * @return null Null on failure with no changes to parameters. */ public function display_element( $element, &$children_elements, $max_depth, $depth, $args, &$output ) { if ( ! $element ) { return; } $id_field = $this->db_fields['id']; // Display this element. if ( is_object( $args[0] ) ) { $args[0]->has_children = ! empty( $children_elements[ $element->$id_field ] ); } parent::display_element( $element, $children_elements, $max_depth, $depth, $args, $output ); } /** * Menu Fallback * ============= * If this function is assigned to the wp_nav_menu's fallback_cb variable * and a menu has not been assigned to the theme location in the WordPress * menu manager the function with display nothing to a non-logged in user, * and will add a link to the WordPress menu manager if logged in as an admin. * * @param array $args passed from the wp_nav_menu function. */ public static function fallback( $args ) { if ( current_user_can( 'edit_theme_options' ) ) { /* Get Arguments. */ $container = $args['container']; $container_id = $args['container_id']; $container_class = $args['container_class']; $menu_class = $args['menu_class']; $menu_id = $args['menu_id']; if ( $container ) { echo '<' . esc_attr( $container ); if ( $container_id ) { echo ' id="' . esc_attr( $container_id ) . '"'; } if ( $container_class ) { echo ' class="' . sanitize_html_class( $container_class ) . '"'; } echo '>'; } echo '<ul'; if ( $menu_id ) { echo ' id="' . esc_attr( $menu_id ) . '"'; } if ( $menu_class ) { echo ' class="' . esc_attr( $menu_class ) . '"'; } echo '>'; echo '<li><a href="' . esc_url( admin_url( 'nav-menus.php' ) ) . '" title="">' . esc_attr( 'Add a menu', '' ) . '</a></li>'; echo '</ul>'; if ( $container ) { echo '</' . esc_attr( $container ) . '>'; } } } } } // End if().
接下来进行第二步
二、引入代码文件
如果你是新建文件的方式，则需要在function文件中引入那个文件，如果你是直接将代码写在function文件中，则不需要进行这步操作。
/** * Class BS_Walker_Nav_Menu * 使菜单使用Boostrap * From: 等英博客出品 http://www.waitig.com */ require_once 'wp-bootstrap-navwalker.php';
三、输出菜单样式
在输出菜单时使用自定的WP_Bootstrap_Navwalker样式类
官方推荐的样式：
<nav class="navbar navbar-default" role="navigation"> <div class="container-fluid"> <!-- Brand and toggle get grouped for better mobile display --> <div class="navbar-header"> <button type="button" class="navbar-toggle" data-toggle="collapse" data-target="#bs-example-navbar-collapse-1"> <span class="sr-only">Toggle navigation</span> <span class="icon-bar"></span> <span class="icon-bar"></span> <span class="icon-bar"></span> </button> <a class="navbar-brand" href="<?php echo home_url(); ?>"> <?php bloginfo('name'); ?> </a> </div> <?php wp_nav_menu( array( 'menu' => 'primary', 'theme_location' => 'primary', 'depth' => 2, 'container' => 'div', 'container_class' => 'collapse navbar-collapse', 'container_id' => 'bs-example-navbar-collapse-1', 'menu_class' => 'nav navbar-nav', 'fallback_cb' => 'WP_Bootstrap_Navwalker::fallback', 'walker' => new WP_Bootstrap_Navwalker()) ); ?> </div> </nav>
不出意外的话，这样WordPress的菜单就可以使用bootstrap默认的菜单样式了，如果你想自定义其样式的话，可以重写其中的样式。
如图所示：
