Walker::walk( array $elements, int $max_depth, mixed $args ): string

194	public function walk( $elements, $max_depth, ...$args ) {195		$output = '';196 197		$max_depth = (int) $max_depth;198 199		// Invalid parameter or nothing to walk.200		if ( $max_depth < -1 || empty( $elements ) ) {201			return $output;202		}203 204		$parent_field = $this->db_fields['parent'];205 206		// Flat display.207		if ( -1 === $max_depth ) {208			$empty_array = array();209			foreach ( $elements as $e ) {210				$this->display_element( $e, $empty_array, 1, 0, $args, $output );211			}212			return $output;213		}214 215		/*216		 * Need to display in hierarchical order.217		 * Separate elements into two buckets: top level and children elements.218		 * Children_elements is two dimensional array. Example:219		 * Children_elements[10][] contains all sub-elements whose parent is 10.220		 */221		$top_level_elements = array();222		$children_elements  = array();223		foreach ( $elements as $e ) {224			if ( empty( $e->$parent_field ) ) {225				$top_level_elements[] = $e;226			} else {227				$children_elements[ $e->$parent_field ][] = $e;228			}229		}230 231		/*232		 * When none of the elements is top level.233		 * Assume the first one must be root of the sub elements.234		 */235		if ( empty( $top_level_elements ) ) {236 237			$first = array_slice( $elements, 0, 1 );238			$root  = $first[0];239 240			$top_level_elements = array();241			$children_elements  = array();242			foreach ( $elements as $e ) {243				if ( $root->$parent_field === $e->$parent_field ) {244					$top_level_elements[] = $e;245				} else {246					$children_elements[ $e->$parent_field ][] = $e;247				}248			}249		}250 251		foreach ( $top_level_elements as $e ) {252			$this->display_element( $e, $children_elements, $max_depth, 0, $args, $output );253		}254 255		/*256		 * If we are displaying all levels, and remaining children_elements is not empty,257		 * then we got orphans, which should be displayed regardless.258		 */259		if ( ( 0 === $max_depth ) && count( $children_elements ) > 0 ) {260			$empty_array = array();261			foreach ( $children_elements as $orphans ) {262				foreach ( $orphans as $op ) {263					$this->display_element( $op, $empty_array, 1, 0, $args, $output );264				}265			}266		}267 268		return $output;269	}