描述：
显示前台导航菜单
用法：
<?php
$defaults = array(
'theme_location' => '',
'menu' => '',
'container' => 'div',
'container_class' => '',
'container_id' => '',
'menu_class' => 'menu',
'menu_id' => '',
'echo' => true,
'fallback_cb' => 'wp_page_menu',
'before' => '',
'after' => '',
'link_before' => '',
'link_after' => '',
'items_wrap' => '<ul id="%1$s" class="%2$s">%3$s</ul>',
'depth' => 0,
'walker' => ''
);
wp_nav_menu( $defaults );
?>参数：
$theme_location
(string) (可选) 用于在调用导航菜单时指定注册过的某一个导航菜单名，如果没有指定，则显示第一个。
默认值: None
$menu
(string) (可选) 使用导航菜单的名称调用菜单，可以是 id, slug, name (按顺序匹配的) 。
默认值: None
$container
(string) (可选) ul 父节点（这里指导航菜单的容器）的标签类型，只支持div 和 nav 标签, 如果是其它值, ul 父节点的标签将不会被显示。也可以用false（container => false）去掉ul父节点标签。
默认值: div
$container_class
(string) (可选) ul 父节点的 class 属性值。
默认值: menu-{menu slug}-container
$container_id
(string) (可选) ul 父节点的 id 属性值。
默认值: None
$menu_class
(string) (可选) ul 节点的 class 属性值
默认值: menu
$menu_id
(string) (可选) ul 节点的 id 属性值。
默认值: menu-{menu slug};
$echo
(boolean) (可选) 确定直接显示导航菜单还是返回 HTML 片段，如果想将导航的代码作为赋值使用，可设置为false。
默认值: true
$fallback_cb
(string) (可选) 用于没有在后台设置导航时调的回调函数。
默认值: wp_page_menu
$before
(string) (可选) 显示在每个菜单链接前的文本。
默认值: None
$after
(string) (可选) 显示在每个菜单链接后的文本。
默认值: None
$link_before
(string) (可选) 显示在每个菜单链接文本前的文本。
默认值: None
$link_after
(string) (可选) 显示在每个菜单链接文本后的文本。
默认值: None
$items_wrap
(string) (可选) 使用字符串替换修改ul的class。
默认值: <ul id=”%1$s” class=”%2$s”>%3$s</ul>
$depth
(integer) (可选) 显示菜单的层级, 当数值为 0 时显示所有层级的菜单（就是你设置几级菜单都可以显示，一般最常用的都是二级菜单）。
默认值: 0
$walker
(object) (可选) 自定义walker
默认值: new Walker_Nav_Menu
示例：
默认菜单
<div class="access">
<?php wp_nav_menu(); ?>
</div>特定菜单
<?php wp_nav_menu( array('menu' => 'Project Nav' )); ?>源文件：
function wp_nav_menu( $args = array() ) {
static $menu_id_slugs = array();
$defaults = array(
'menu' => '',
'container' => 'div',
'container_class' => '',
'container_id' => '',
'menu_class' => 'menu',
'menu_id' => '',
'echo' => true,
'fallback_cb' => 'wp_page_menu',
'before' => '',
'after' => '',
'link_before' => '',
'link_after' => '',
'items_wrap' => '<ul id="%1$s" class="%2$s">%3$s</ul>',
'item_spacing' => 'preserve',
'depth' => 0,
'walker' => '',
'theme_location' => '',
);
$args = wp_parse_args( $args, $defaults );
if ( ! in_array( $args['item_spacing'], array( 'preserve', 'discard' ), true ) ) {
// invalid value, fall back to default.
$args['item_spacing'] = $defaults['item_spacing'];
}
/**
* Filters the arguments used to display a navigation menu.
*
* @since 3.0.0
*
* @see wp_nav_menu()
*
* @param array $args Array of wp_nav_menu() arguments.
*/
$args = apply_filters( 'wp_nav_menu_args', $args );
$args = (object) $args;
/**
* Filters whether to short-circuit the wp_nav_menu() output.
*
* Returning a non-null value to the filter will short-circuit
* wp_nav_menu(), echoing that value if $args->echo is true,
* returning that value otherwise.
*
* @since 3.9.0
*
* @see wp_nav_menu()
*
* @param string|null $output Nav menu output to short-circuit with. Default null.
* @param stdClass $args An object containing wp_nav_menu() arguments.
*/
$nav_menu = apply_filters( 'pre_wp_nav_menu', null, $args );
if ( null !== $nav_menu ) {
if ( $args->echo ) {
echo $nav_menu;
return;
}
return $nav_menu;
}
// Get the nav menu based on the requested menu
$menu = wp_get_nav_menu_object( $args->menu );
// Get the nav menu based on the theme_location
$locations = get_nav_menu_locations();
if ( ! $menu && $args->theme_location && $locations && isset( $locations[ $args->theme_location ] ) ) {
$menu = wp_get_nav_menu_object( $locations[ $args->theme_location ] );
}
// get the first menu that has items if we still can't find a menu
if ( ! $menu && ! $args->theme_location ) {
$menus = wp_get_nav_menus();
foreach ( $menus as $menu_maybe ) {
$menu_items = wp_get_nav_menu_items( $menu_maybe->term_id, array( 'update_post_term_cache' => false ) );
if ( $menu_items ) {
$menu = $menu_maybe;
break;
}
}
}
if ( empty( $args->menu ) ) {
$args->menu = $menu;
}
// If the menu exists, get its items.
if ( $menu && ! is_wp_error( $menu ) && ! isset( $menu_items ) ) {
$menu_items = wp_get_nav_menu_items( $menu->term_id, array( 'update_post_term_cache' => false ) );
}
/*
* If no menu was found:
* - Fall back (if one was specified), or bail.
*
* If no menu items were found:
* - Fall back, but only if no theme location was specified.
* - Otherwise, bail.
*/
if ( ( ! $menu || is_wp_error( $menu ) || ( isset( $menu_items ) && empty( $menu_items ) && ! $args->theme_location ) )
&& isset( $args->fallback_cb ) && $args->fallback_cb && is_callable( $args->fallback_cb ) ) {
return call_user_func( $args->fallback_cb, (array) $args );
}
if ( ! $menu || is_wp_error( $menu ) ) {
return false;
}
$nav_menu = '';
$items = '';
$show_container = false;
if ( $args->container ) {
/**
* Filters the list of HTML tags that are valid for use as menu containers.
*
* @since 3.0.0
*
* @param array $tags The acceptable HTML tags for use as menu containers.
* Default is array containing 'div' and 'nav'.
*/
$allowed_tags = apply_filters( 'wp_nav_menu_container_allowedtags', array( 'div', 'nav' ) );
if ( is_string( $args->container ) && in_array( $args->container, $allowed_tags ) ) {
$show_container = true;
$class = $args->container_class ? ' class="' . esc_attr( $args->container_class ) . '"' : ' class="menu-' . $menu->slug . '-container"';
$id = $args->container_id ? ' id="' . esc_attr( $args->container_id ) . '"' : '';
$nav_menu .= '<' . $args->container . $id . $class . '>';
}
}
// Set up the $menu_item variables
_wp_menu_item_classes_by_context( $menu_items );
$sorted_menu_items = array();
$menu_items_with_children = array();
foreach ( (array) $menu_items as $menu_item ) {
$sorted_menu_items[ $menu_item->menu_order ] = $menu_item;
if ( $menu_item->menu_item_parent ) {
$menu_items_with_children[ $menu_item->menu_item_parent ] = true;
}
}
// Add the menu-item-has-children class where applicable
if ( $menu_items_with_children ) {
foreach ( $sorted_menu_items as &$menu_item ) {
if ( isset( $menu_items_with_children[ $menu_item->ID ] ) ) {
$menu_item->classes[] = 'menu-item-has-children';
}
}
}
unset( $menu_items, $menu_item );
/**
* Filters the sorted list of menu item objects before generating the menu's HTML.
*
* @since 3.1.0
*
* @param array $sorted_menu_items The menu items, sorted by each menu item's menu order.
* @param stdClass $args An object containing wp_nav_menu() arguments.
*/
$sorted_menu_items = apply_filters( 'wp_nav_menu_objects', $sorted_menu_items, $args );
$items .= walk_nav_menu_tree( $sorted_menu_items, $args->depth, $args );
unset( $sorted_menu_items );
// Attributes
if ( ! empty( $args->menu_id ) ) {
$wrap_id = $args->menu_id;
} else {
$wrap_id = 'menu-' . $menu->slug;
while ( in_array( $wrap_id, $menu_id_slugs ) ) {
if ( preg_match( '#-(\d+)$#', $wrap_id, $matches ) ) {
$wrap_id = preg_replace( '#-(\d+)$#', '-' . ++$matches[1], $wrap_id );
} else {
$wrap_id = $wrap_id . '-1';
}
}
}
$menu_id_slugs[] = $wrap_id;
$wrap_class = $args->menu_class ? $args->menu_class : '';
/**
* Filters the HTML list content for navigation menus.
*
* @since 3.0.0
*
* @see wp_nav_menu()
*
* @param string $items The HTML list content for the menu items.
* @param stdClass $args An object containing wp_nav_menu() arguments.
*/
$items = apply_filters( 'wp_nav_menu_items', $items, $args );
/**
* Filters the HTML list content for a specific navigation menu.
*
* @since 3.0.0
*
* @see wp_nav_menu()
*
* @param string $items The HTML list content for the menu items.
* @param stdClass $args An object containing wp_nav_menu() arguments.
*/
$items = apply_filters( "wp_nav_menu_{$menu->slug}_items", $items, $args );
// Don't print any markup if there are no items at this point.
if ( empty( $items ) ) {
return false;
}
$nav_menu .= sprintf( $args->items_wrap, esc_attr( $wrap_id ), esc_attr( $wrap_class ), $items );
unset( $items );
if ( $show_container ) {
$nav_menu .= '</' . $args->container . '>';
}
/**
* Filters the HTML content for navigation menus.
*
* @since 3.0.0
*
* @see wp_nav_menu()
*
* @param string $nav_menu The HTML content for the navigation menu.
* @param stdClass $args An object containing wp_nav_menu() arguments.
*/
$nav_menu = apply_filters( 'wp_nav_menu', $nav_menu, $args );
if ( $args->echo ) {
echo $nav_menu;
} else {
return $nav_menu;
}
}
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