wp_dropdown_categories( array|string $args = '' )

显示或检索类别的HTML下拉列表。
Display or retrieve the HTML dropdown list of categories.

说明（Description）

“hierarchy”参数（默认情况下是禁用的）将重写深度参数，除非它为true。当参数为false时，它将显示所有类别。当它被启用时，它将使用“depth”参数中的值。

源码（Source）

/**
 * Display or retrieve the HTML dropdown list of categories.
 *
 * The 'hierarchical' argument, which is disabled by default, will override the
 * depth argument, unless it is true. When the argument is false, it will
 * display all of the categories. When it is enabled it will use the value in
 * the 'depth' argument.
 *
 * @since 2.1.0
 * @since 4.2.0 Introduced the `value_field` argument.
 *
 * @param string|array $args {
 *     Optional. Array or string of arguments to generate a categories drop-down element.
 *
 *     @type string       $show_option_all   Text to display for showing all categories. Default empty.
 *     @type string       $show_option_none  Text to display for showing no categories. Default empty.
 *     @type string       $option_none_value Value to use when no category is selected. Default empty.
 *     @type string       $orderby           Which column to use for ordering categories. See get_terms() for a list
 *                                           of accepted values. Default 'id' (term_id).
 *     @type string       $order             Whether to order terms in ascending or descending order. Accepts 'ASC'
 *                                           or 'DESC'. Default 'ASC'.
 *     @type bool         $pad_counts        See get_terms() for an argument description. Default false.
 *     @type bool|int     $show_count        Whether to include post counts. Accepts 0, 1, or their bool equivalents.
 *                                           Default 0.
 *     @type bool|int     $hide_empty        Whether to hide categories that don't have any posts. Accepts 0, 1, or
 *                                           their bool equivalents. Default 1.
 *     @type int          $child_of          Term ID to retrieve child terms of. See get_terms(). Default 0.
 *     @type array|string $exclude           Array or comma/space-separated string of term ids to exclude.
 *                                           If `$include` is non-empty, `$exclude` is ignored. Default empty array.
 *     @type bool|int     $echo              Whether to echo or return the generated markup. Accepts 0, 1, or their
 *                                           bool equivalents. Default 1.
 *     @type bool|int     $hierarchical      Whether to traverse the taxonomy hierarchy. Accepts 0, 1, or their bool
 *                                           equivalents. Default 0.
 *     @type int          $depth             Maximum depth. Default 0.
 *     @type int          $tab_index         Tab index for the select element. Default 0 (no tabindex).
 *     @type string       $name              Value for the 'name' attribute of the select element. Default 'cat'.
 *     @type string       $id                Value for the 'id' attribute of the select element. Defaults to the value
 *                                           of `$name`.
 *     @type string       $class             Value for the 'class' attribute of the select element. Default 'postform'.
 *     @type int|string   $selected          Value of the option that should be selected. Default 0.
 *     @type string       $value_field       Term field that should be used to populate the 'value' attribute
 *                                           of the option elements. Accepts any valid term field: 'term_id', 'name',
 *                                           'slug', 'term_group', 'term_taxonomy_id', 'taxonomy', 'description',
 *                                           'parent', 'count'. Default 'term_id'.
 *     @type string       $taxonomy          Name of the category to retrieve. Default 'category'.
 *     @type bool         $hide_if_empty     True to skip generating markup if no categories are found.
 *                                           Default false (create select element even if no categories are found).
 * }
 * @return string HTML content only if 'echo' argument is 0.
 */
function wp_dropdown_categories( $args = '' ) {
	$defaults = array(
		'show_option_all' => '', 'show_option_none' => '',
		'orderby' => 'id', 'order' => 'ASC',
		'show_count' => 0,
		'hide_empty' => 1, 'child_of' => 0,
		'exclude' => '', 'echo' => 1,
		'selected' => 0, 'hierarchical' => 0,
		'name' => 'cat', 'id' => '',
		'class' => 'postform', 'depth' => 0,
		'tab_index' => 0, 'taxonomy' => 'category',
		'hide_if_empty' => false, 'option_none_value' => -1,
		'value_field' => 'term_id',
	);

	$defaults['selected'] = ( is_category() ) ? get_query_var( 'cat' ) : 0;

	// Back compat.
	if ( isset( $args['type'] ) && 'link' == $args['type'] ) {
		_deprecated_argument( __FUNCTION__, '3.0', '' );
		$args['taxonomy'] = 'link_category';
	}

	$r = wp_parse_args( $args, $defaults );
	$option_none_value = $r['option_none_value'];

	if ( ! isset( $r['pad_counts'] ) && $r['show_count'] && $r['hierarchical'] ) {
		$r['pad_counts'] = true;
	}

	$tab_index = $r['tab_index'];

	$tab_index_attribute = '';
	if ( (int) $tab_index > 0 ) {
		$tab_index_attribute = " tabindex="$tab_index"";
	}

	// Avoid clashes with the 'name' param of get_terms().
	$get_terms_args = $r;
	unset( $get_terms_args['name'] );
	$categories = get_terms( $r['taxonomy'], $get_terms_args );

	$name = esc_attr( $r['name'] );
	$class = esc_attr( $r['class'] );
	$id = $r['id'] ? esc_attr( $r['id'] ) : $name;

	if ( ! $r['hide_if_empty'] || ! empty( $categories ) ) {
		$output = "
";
	} else {
		$output = '';
	}
	if ( empty( $categories ) && ! $r['hide_if_empty'] && ! empty( $r['show_option_none'] ) ) {

		/**
		 * Filter a taxonomy drop-down display element.
		 *
		 * A variety of taxonomy drop-down display elements can be modified
		 * just prior to display via this filter. Filterable arguments include
		 * 'show_option_none', 'show_option_all', and various forms of the
		 * term name.
		 *
		 * @since 1.2.0
		 *
		 * @see wp_dropdown_categories()
		 *
		 * @param string $element Taxonomy element to list.
		 */
		$show_option_none = apply_filters( 'list_cats', $r['show_option_none'] );
		$output .= "	$show_option_none
";
	}

	if ( ! empty( $categories ) ) {

		if ( $r['show_option_all'] ) {

			/** This filter is documented in wp-includes/category-template.php */
			$show_option_all = apply_filters( 'list_cats', $r['show_option_all'] );
			$selected = ( '0' === strval($r['selected']) ) ? " selected='selected'" : '';
			$output .= "	$show_option_all
";
		}

		if ( $r['show_option_none'] ) {

			/** This filter is documented in wp-includes/category-template.php */
			$show_option_none = apply_filters( 'list_cats', $r['show_option_none'] );
			$selected = selected( $option_none_value, $r['selected'], false );
			$output .= "	$show_option_none
";
		}

		if ( $r['hierarchical'] ) {
			$depth = $r['depth'];  // Walk the full depth.
		} else {
			$depth = -1; // Flat.
		}
		$output .= walk_category_dropdown_tree( $categories, $depth, $r );
	}

	if ( ! $r['hide_if_empty'] || ! empty( $categories ) ) {
		$output .= "
";
	}
	/**
	 * Filter the taxonomy drop-down output.
	 *
	 * @since 2.1.0
	 *
	 * @param string $output HTML output.
	 * @param array  $r      Arguments used to build the drop-down.
	 */
	$output = apply_filters( 'wp_dropdown_cats', $output, $r );

	if ( $r['echo'] ) {
		echo $output;
	}
	return $output;
}

笔记（Notes）