Walker

A class for displaying various tree-like structures.


Description Description

Extend the Walker class to use it, see examples below. Child classes do not need to implement all of the abstract methods in the class. The child only needs to implement the methods that are needed.


Source Source

File: wp-includes/class-wp-walker.php

class Walker {
	/**
	 * What the class handles.
	 *
	 * @since 2.1.0
	 * @var string
	 */
	public $tree_type;

	/**
	 * DB fields to use.
	 *
	 * @since 2.1.0
	 * @var array
	 */
	public $db_fields;

	/**
	 * Max number of pages walked by the paged walker
	 *
	 * @since 2.7.0
	 * @var int
	 */
	public $max_pages = 1;

	/**
	 * Whether the current element has children or not.
	 *
	 * To be used in start_el().
	 *
	 * @since 4.0.0
	 * @var bool
	 */
	public $has_children;

	/**
	 * Starts the list before the elements are added.
	 *
	 * The $args parameter holds additional values that may be used with the child
	 * class methods. This method is called at the start of the output list.
	 *
	 * @since 2.1.0
	 * @abstract
	 *
	 * @param string $output Used to append additional content (passed by reference).
	 * @param int    $depth  Depth of the item.
	 * @param array  $args   An array of additional arguments.
	 */
	public function start_lvl( &$output, $depth = 0, $args = array() ) {}

	/**
	 * Ends the list of after the elements are added.
	 *
	 * The $args parameter holds additional values that may be used with the child
	 * class methods. This method finishes the list at the end of output of the elements.
	 *
	 * @since 2.1.0
	 * @abstract
	 *
	 * @param string $output Used to append additional content (passed by reference).
	 * @param int    $depth  Depth of the item.
	 * @param array  $args   An array of additional arguments.
	 */
	public function end_lvl( &$output, $depth = 0, $args = array() ) {}

	/**
	 * Start the element output.
	 *
	 * The $args parameter holds additional values that may be used with the child
	 * class methods. Includes the element output also.
	 *
	 * @since 2.1.0
	 * @abstract
	 *
	 * @param string $output            Used to append additional content (passed by reference).
	 * @param object $object            The data object.
	 * @param int    $depth             Depth of the item.
	 * @param array  $args              An array of additional arguments.
	 * @param int    $current_object_id ID of the current item.
	 */
	public function start_el( &$output, $object, $depth = 0, $args = array(), $current_object_id = 0 ) {}

	/**
	 * Ends the element output, if needed.
	 *
	 * The $args parameter holds additional values that may be used with the child class methods.
	 *
	 * @since 2.1.0
	 * @abstract
	 *
	 * @param string $output Used to append additional content (passed by reference).
	 * @param object $object The data object.
	 * @param int    $depth  Depth of the item.
	 * @param array  $args   An array of additional arguments.
	 */
	public function end_el( &$output, $object, $depth = 0, $args = array() ) {}

	/**
	 * 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. It is possible to set the
	 * max depth to include all depths, see walk() method.
	 *
	 * This method should not be called directly, use the walk() method instead.
	 *
	 * @since 2.5.0
	 *
	 * @param object $element           Data object.
	 * @param array  $children_elements List of elements to continue traversing (passed by reference).
	 * @param int    $max_depth         Max depth to traverse.
	 * @param int    $depth             Depth of current element.
	 * @param array  $args              An array of arguments.
	 * @param string $output            Used to append additional content (passed by reference).
	 */
	public function display_element( $element, &$children_elements, $max_depth, $depth, $args, &$output ) {
		if ( ! $element ) {
			return;
		}

		$id_field = $this->db_fields['id'];
		$id       = $element->$id_field;

		//display this element
		$this->has_children = ! empty( $children_elements[ $id ] );
		if ( isset( $args[0] ) && is_array( $args[0] ) ) {
			$args[0]['has_children'] = $this->has_children; // Back-compat.
		}

		$cb_args = array_merge( array(&$output, $element, $depth), $args);
		call_user_func_array(array($this, 'start_el'), $cb_args);

		// descend only when the depth is right and there are childrens 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);
	}

	/**
	 * Display array of elements hierarchically.
	 *
	 * Does not assume any existing order of elements.
	 *
	 * $max_depth = -1 means flatly display every element.
	 * $max_depth = 0 means display all levels.
	 * $max_depth > 0 specifies the number of display levels.
	 *
	 * @since 2.1.0
	 *
	 * @param array $elements  An array of elements.
	 * @param int   $max_depth The maximum hierarchical depth.
	 * @return string The hierarchical item output.
	 */
	public function walk( $elements, $max_depth ) {
		$args = array_slice(func_get_args(), 2);
		$output = '';

		//invalid parameter or nothing to walk
		if ( $max_depth < -1 || empty( $elements ) ) {
			return $output;
		}

		$parent_field = $this->db_fields['parent'];

		// flat display
		if ( -1 == $max_depth ) {
			$empty_array = array();
			foreach ( $elements as $e )
				$this->display_element( $e, $empty_array, 1, 0, $args, $output );
			return $output;
		}

		/*
		 * Need to display in hierarchical order.
		 * Separate elements into two buckets: top level and children elements.
		 * Children_elements is two dimensional array, eg.
		 * Children_elements[10][] contains all sub-elements whose parent is 10.
		 */
		$top_level_elements = array();
		$children_elements  = array();
		foreach ( $elements as $e) {
			if ( empty( $e->$parent_field ) )
				$top_level_elements[] = $e;
			else
				$children_elements[ $e->$parent_field ][] = $e;
		}

		/*
		 * When none of the elements is top level.
		 * Assume the first one must be root of the sub elements.
		 */
		if ( empty($top_level_elements) ) {

			$first = array_slice( $elements, 0, 1 );
			$root = $first[0];

			$top_level_elements = array();
			$children_elements  = array();
			foreach ( $elements as $e) {
				if ( $root->$parent_field == $e->$parent_field )
					$top_level_elements[] = $e;
				else
					$children_elements[ $e->$parent_field ][] = $e;
			}
		}

		foreach ( $top_level_elements as $e )
			$this->display_element( $e, $children_elements, $max_depth, 0, $args, $output );

		/*
		 * If we are displaying all levels, and remaining children_elements is not empty,
		 * then we got orphans, which should be displayed regardless.
		 */
		if ( ( $max_depth == 0 ) && count( $children_elements ) > 0 ) {
			$empty_array = array();
			foreach ( $children_elements as $orphans )
				foreach ( $orphans as $op )
					$this->display_element( $op, $empty_array, 1, 0, $args, $output );
		 }

		 return $output;
	}

	/**
 	 * paged_walk() - produce a page of nested elements
 	 *
 	 * Given an array of hierarchical elements, the maximum depth, a specific page number,
 	 * and number of elements per page, this function first determines all top level root elements
 	 * belonging to that page, then lists them and all of their children in hierarchical order.
 	 *
	 * $max_depth = 0 means display all levels.
	 * $max_depth > 0 specifies the number of display levels.
	 *
 	 * @since 2.7.0
	 *
	 * @param array $elements
	 * @param int   $max_depth The maximum hierarchical depth.
	 * @param int   $page_num The specific page number, beginning with 1.
	 * @param int   $per_page
	 * @return string XHTML of the specified page of elements
	 */
	public function paged_walk( $elements, $max_depth, $page_num, $per_page ) {
		if ( empty( $elements ) || $max_depth < -1 ) {
			return '';
		}

		$args = array_slice( func_get_args(), 4 );
		$output = '';

		$parent_field = $this->db_fields['parent'];

		$count = -1;
		if ( -1 == $max_depth )
			$total_top = count( $elements );
		if ( $page_num < 1 || $per_page < 0  ) {
			// No paging
			$paging = false;
			$start = 0;
			if ( -1 == $max_depth )
				$end = $total_top;
			$this->max_pages = 1;
		} else {
			$paging = true;
			$start = ( (int)$page_num - 1 ) * (int)$per_page;
			$end   = $start + $per_page;
			if ( -1 == $max_depth )
				$this->max_pages = ceil($total_top / $per_page);
		}

		// flat display
		if ( -1 == $max_depth ) {
			if ( !empty($args[0]['reverse_top_level']) ) {
				$elements = array_reverse( $elements );
				$oldstart = $start;
				$start = $total_top - $end;
				$end = $total_top - $oldstart;
			}

			$empty_array = array();
			foreach ( $elements as $e ) {
				$count++;
				if ( $count < $start )
					continue;
				if ( $count >= $end )
					break;
				$this->display_element( $e, $empty_array, 1, 0, $args, $output );
			}
			return $output;
		}

		/*
		 * Separate elements into two buckets: top level and children elements.
		 * Children_elements is two dimensional array, e.g.
		 * $children_elements[10][] contains all sub-elements whose parent is 10.
		 */
		$top_level_elements = array();
		$children_elements  = array();
		foreach ( $elements as $e) {
			if ( 0 == $e->$parent_field )
				$top_level_elements[] = $e;
			else
				$children_elements[ $e->$parent_field ][] = $e;
		}

		$total_top = count( $top_level_elements );
		if ( $paging )
			$this->max_pages = ceil($total_top / $per_page);
		else
			$end = $total_top;

		if ( !empty($args[0]['reverse_top_level']) ) {
			$top_level_elements = array_reverse( $top_level_elements );
			$oldstart = $start;
			$start = $total_top - $end;
			$end = $total_top - $oldstart;
		}
		if ( !empty($args[0]['reverse_children']) ) {
			foreach ( $children_elements as $parent => $children )
				$children_elements[$parent] = array_reverse( $children );
		}

		foreach ( $top_level_elements as $e ) {
			$count++;

			// For the last page, need to unset earlier children in order to keep track of orphans.
			if ( $end >= $total_top && $count < $start )
					$this->unset_children( $e, $children_elements );

			if ( $count < $start )
				continue;

			if ( $count >= $end )
				break;

			$this->display_element( $e, $children_elements, $max_depth, 0, $args, $output );
		}

		if ( $end >= $total_top && count( $children_elements ) > 0 ) {
			$empty_array = array();
			foreach ( $children_elements as $orphans )
				foreach ( $orphans as $op )
					$this->display_element( $op, $empty_array, 1, 0, $args, $output );
		}

		return $output;
	}

	/**
	 * Calculates the total number of root elements.
	 *
	 * @since 2.7.0
	 *
	 * @param array $elements Elements to list.
	 * @return int Number of root elements.
	 */
	public function get_number_of_root_elements( $elements ){
		$num = 0;
		$parent_field = $this->db_fields['parent'];

		foreach ( $elements as $e) {
			if ( 0 == $e->$parent_field )
				$num++;
		}
		return $num;
	}

	/**
	 * Unset all the children for a given top level element.
	 *
	 * @since 2.7.0
	 *
	 * @param object $e
	 * @param array $children_elements
	 */
	public function unset_children( $e, &$children_elements ){
		if ( ! $e || ! $children_elements ) {
			return;
		}

		$id_field = $this->db_fields['id'];
		$id = $e->$id_field;

		if ( !empty($children_elements[$id]) && is_array($children_elements[$id]) )
			foreach ( (array) $children_elements[$id] as $child )
				$this->unset_children( $child, $children_elements );

		unset( $children_elements[ $id ] );
	}

} // Walker

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Changelog Changelog

Changelog
Version Description
2.1.0 Introduced.


Top ↑

Methods Methods

  • __get — Make private properties readable for backwards compatibility.
  • __isset — Make private properties checkable for backwards compatibility.
  • __set — Make private properties settable for backwards compatibility.
  • __unset — Make private properties un-settable for backwards compatibility.
  • display_element — Traverse elements to create list from elements.
  • end_el — Ends the element output, if needed.
  • end_lvl — Ends the list of after the elements are added.
  • get_number_of_root_elements — Calculates the total number of root elements.
  • paged_walk — paged_walk() - produce a page of nested elements
  • start_el — Start the element output.
  • start_lvl — Starts the list before the elements are added.
  • unset_children — Unset all the children for a given top level element.
  • walk — Display array of elements hierarchically.

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User Contributed Notes User Contributed Notes

  1. Skip to note content
    Contributed by chetansatasiya

    Tip: In this case, you could choose to extend Walker_Nav_Menu instead of Walker, and then you wouldn’t need to define $db_fields manually.

    In order to utilize this custom walker class, you would call wp_nav_menu() (likely from within a theme file) and pass it a new instance of the custom Walker child class.

    <ul>
        <?php
        wp_nav_menu(array(
            'menu'    => 2, //menu id
            'walker'  => new Walker_Quickstart_Menu() //use our custom walker
        ));
        ?>
    </ul>
    
  2. Skip to note content
    Contributed by chetansatasiya

    This example would shows how you might set up the Walker’s $db_fields property for handling a tree of page objects. Since we know the parent and id properties for all post objects (pages included), we just need to match those up using the Walker’s $db_fields property. Like so…

    class Walker_Page extends Walker {
        var $db_fields = array (
            'parent' => 'post_parent', 
            'id'     => 'ID'
        );
    
        // define abstract methods here
    }
    
  3. Skip to note content
    Contributed by chetansatasiya

    This example shows one of the simplest (and most common) implementations of the walker class. In this case, the Walker will be used to generate a custom menu in combination with wp_nav_menu(). The first block shows the example Walker child class, the second block demonstrates how this class is utilized.

    class Walker_Quickstart_Menu extends Walker {
    
        // Tell Walker where to inherit it's parent and id values
        var $db_fields = array(
            'parent' => 'menu_item_parent', 
            'id'     => 'db_id' 
        );
    
        /**
         * At the start of each element, output a <li> and <a> tag structure.
         * 
         * Note: Menu objects include url and title properties, so we will use those.
         */
        function start_el( &$output, $item, $depth = 0, $args = array(), $id = 0 ) {
            $output .= sprintf( "\n<li><a href='%1$s'%2$s>%3$s</a></li>\n",
                esc_url( $item->url ),
                ( $item->object_id === get_the_ID() ) ? esc_attr( ' class="current"' ) : '',
                esc_html( $item->title )
            );
        }
    
    }
    

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