* A class for displaying various tree-like structures.
* 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.
* What the class handles.
* Max number of pages walked by the paged walker
* Whether the current element has children or not.
* To be used in start_el().
* 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.
* @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.
* @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.
* @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.
* @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.
* @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 ) {
$id_field = $this->db_fields['id'];
$id = $element->$id_field;
$this->has_children = ! empty( $children_elements[ $id ] );
if ( isset( $args[0] ) && is_array( $args[0] ) ) {
$args[0]['has_children'] = $this->has_children; // Back-compat.
$this->start_el( $output, $element, $depth, ...array_values( $args ) );
// Descend only when the depth is right and there are children for this element.
if ( ( 0 == $max_depth || $max_depth > $depth + 1 ) && isset( $children_elements[ $id ] ) ) {
foreach ( $children_elements[ $id ] as $child ) {
if ( ! isset( $newlevel ) ) {
// Start the child delimiter.
$this->start_lvl( $output, $depth, ...array_values( $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.
$this->end_lvl( $output, $depth, ...array_values( $args ) );
$this->end_el( $output, $element, $depth, ...array_values( $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.
* $max_depth > 0 specifies the number of display levels.
* @since 5.3.0 Formalized the existing `...$args` parameter by adding it
* to the function signature.
* @param array $elements An array of elements.
* @param int $max_depth The maximum hierarchical depth.
* @param mixed ...$args Optional additional arguments.
* @return string The hierarchical item output.
public function walk( $elements, $max_depth, ...$args ) {
// Invalid parameter or nothing to walk.
if ( $max_depth < -1 || empty( $elements ) ) {
$parent_field = $this->db_fields['parent'];
if ( -1 == $max_depth ) {
foreach ( $elements as $e ) {
$this->display_element( $e, $empty_array, 1, 0, $args, $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;
$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 );
$top_level_elements = array();
$children_elements = array();
foreach ( $elements as $e ) {
if ( $root->$parent_field == $e->$parent_field ) {
$top_level_elements[] = $e;
$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 ( ( 0 == $max_depth ) && count( $children_elements ) > 0 ) {
foreach ( $children_elements as $orphans ) {
foreach ( $orphans as $op ) {
$this->display_element( $op, $empty_array, 1, 0, $args, $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.
* $max_depth > 0 specifies the number of display levels.
* @since 5.3.0 Formalized the existing `...$args` parameter by adding it
* to the function signature.
* @param int $max_depth The maximum hierarchical depth.
* @param int $page_num The specific page number, beginning with 1.
* @param mixed ...$args Optional additional arguments.
* @return string XHTML of the specified page of elements
public function paged_walk( $elements, $max_depth, $page_num, $per_page, ...$args ) {
if ( empty( $elements ) || $max_depth < -1 ) {
$parent_field = $this->db_fields['parent'];
if ( -1 == $max_depth ) {
$total_top = count( $elements );
if ( $page_num < 1 || $per_page < 0 ) {
if ( -1 == $max_depth ) {
$start = ( (int) $page_num - 1 ) * (int) $per_page;
$end = $start + $per_page;
if ( -1 == $max_depth ) {
$this->max_pages = ceil( $total_top / $per_page );
if ( -1 == $max_depth ) {
if ( ! empty( $args[0]['reverse_top_level'] ) ) {
$elements = array_reverse( $elements );
$start = $total_top - $end;
$end = $total_top - $oldstart;
foreach ( $elements as $e ) {
$this->display_element( $e, $empty_array, 1, 0, $args, $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;
$children_elements[ $e->$parent_field ][] = $e;
$total_top = count( $top_level_elements );
$this->max_pages = ceil( $total_top / $per_page );
if ( ! empty( $args[0]['reverse_top_level'] ) ) {
$top_level_elements = array_reverse( $top_level_elements );
$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 ) {
// 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 );
$this->display_element( $e, $children_elements, $max_depth, 0, $args, $output );
if ( $end >= $total_top && count( $children_elements ) > 0 ) {
foreach ( $children_elements as $orphans ) {
foreach ( $orphans as $op ) {
$this->display_element( $op, $empty_array, 1, 0, $args, $output );
* Calculates the total number of root elements.
* @param array $elements Elements to list.
* @return int Number of root elements.
public function get_number_of_root_elements( $elements ) {
$parent_field = $this->db_fields['parent'];
foreach ( $elements as $e ) {
if ( 0 == $e->$parent_field ) {
* Unset all the children for a given top level element.
* @param array $children_elements
public function unset_children( $e, &$children_elements ) {
if ( ! $e || ! $children_elements ) {
$id_field = $this->db_fields['id'];
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 ] );
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