Use class directly and remove static cache

src/wp-includes/class-hierarchical-sort.php

@@ -14,19 +14,22 @@
  */
 class Hierarchical_Sort {
 
-	private static $post_ids = array();
-	private static $levels   = array();
-	private static $instance;
-
-	public static function get_instance() {
-		if ( null === self::$instance ) {
-			self::$instance = new self();
+	/**
+	 * Check if the request is eligible for hierarchical sorting.
+	 *
+	 * @param array $request The request data.
+	 *
+	 * @return bool Return true if the request is eligible for hierarchical sorting.
+	 */
+	public static function is_eligible( $request ) {
+		if ( ! isset( $request['orderby_hierarchy'] ) || true !== $request['orderby_hierarchy'] ) {
+			return false;
 		}
 
-		return self::$instance;
+		return true;
 	}
 
-	public function run( $args ) {
+	public static function run( $args ) {
 		$new_args = array_merge(
 			$args,
 			array(
@@ -36,28 +39,11 @@ public function run( $args ) {
 		);
 		$query    = new WP_Query( $new_args );
 		$posts    = $query->posts;
-		$result   = self::sort( $posts );
-
-		self::$post_ids = $result['post_ids'];
-		self::$levels   = $result['levels'];
-	}
-
-	/**
-	 * Check if the request is eligible for hierarchical sorting.
-	 *
-	 * @param array $request The request data.
-	 *
-	 * @return bool Return true if the request is eligible for hierarchical sorting.
-	 */
-	public static function is_eligible( $request ) {
-		if ( ! isset( $request['orderby_hierarchy'] ) || true !== $request['orderby_hierarchy'] ) {
-			return false;
-		}
 
-		return true;
+		return self::sort( $posts );
 	}
 
-	public static function get_ancestor( $post_id ) {
+	private static function get_ancestor( $post_id ) {
 		return get_post( $post_id )->post_parent ?? 0;
 	}
 
@@ -93,7 +79,7 @@ public static function get_ancestor( $post_id ) {
 	 *     @type array $levels   Array of levels for the corresponding post ID in the same index
 	 * }
 	 */
-	public static function sort( $posts ) {
+	private static function sort( $posts ) {
 		/*
 		 * Arrange pages in two arrays:
 		 *
@@ -152,38 +138,4 @@ private static function add_hierarchical_ids( &$ids, &$levels, $level, $to_proce
 			}
 		}
 	}
-
-	public static function get_post_ids() {
-		return self::$post_ids;
-	}
-
-	public static function get_levels() {
-		return self::$levels;
-	}
-}
-
-function rest_page_query_hierarchical_sort_filter_by_post_in( $args, $request ) {
-	if ( ! Hierarchical_Sort::is_eligible( $request ) ) {
-		return $args;
-	}
-
-	$hs = Hierarchical_Sort::get_instance();
-	$hs->run( $args );
-
-	// Reconfigure the args to display only the ids in the list.
-	$args['post__in'] = $hs->get_post_ids();
-	$args['orderby']  = 'post__in';
-
-	return $args;
-}
-
-function rest_prepare_page_hierarchical_sort_add_levels( $response, $post, $request ) {
-	if ( ! Hierarchical_Sort::is_eligible( $request ) ) {
-		return $response;
-	}
-
-	$hs                      = Hierarchical_Sort::get_instance();
-	$response->data['level'] = $hs->get_levels()[ $post->ID ];
-
-	return $response;
 }

src/wp-includes/rest-api/endpoints/class-wp-rest-posts-controller.php

@@ -47,6 +47,13 @@ class WP_REST_Posts_Controller extends WP_REST_Controller {
 	 */
 	protected $allow_batch = array( 'v1' => true );
 
+	/**
+	 * Holds information about each post's level.
+	 * Level means the depth of the post in the hierarchy:
+	 * top-level posts have level 0, their children have level 1, and so on.
+	 */
+	protected $levels = array();
+
 	/**
 	 * Constructor.
 	 *
@@ -402,7 +409,13 @@ static function ( $format ) {
 		// Force the post_type argument, since it's not a user input variable.
 		$args['post_type'] = $this->post_type;
 
-		$args = rest_page_query_hierarchical_sort_filter_by_post_in( $args, $request );
+		if ( Hierarchical_Sort::is_eligible( $request ) ) {
+			$result       = Hierarchical_Sort::run( $args );
+			$this->levels = $result[ 'levels' ];
+
+			$args[ 'post__in' ] = $result[ 'post_ids' ];
+			$args[ 'orderby' ]  = 'post__in';
+		}
 
 		/**
 		 * Filters WP_Query arguments when querying posts via the REST API.
@@ -2092,7 +2105,9 @@ public function prepare_item_for_response( $item, $request ) {
 			}
 		}
 
-		$response = rest_prepare_page_hierarchical_sort_add_levels( $response, $post, $request );
+		if ( Hierarchical_Sort::is_eligible( $request ) ) {
+			$response->data['level'] = $this->levels[ $post->ID ];
+		}
 
 		/**
 		 * Filters the post data for a REST API response.