+<?php
+/**
+ * Meta API: WP_Metadata_Lazyloader class
+ *
+ * @package WordPress
+ * @subpackage Meta
+ * @since 4.5.0
+ */
+
+/**
+ * Core class used for lazy-loading object metadata.
+ *
+ * When loading many objects of a given type, such as posts in a WP_Query loop, it often makes
+ * sense to prime various metadata caches at the beginning of the loop. This means fetching all
+ * relevant metadata with a single database query, a technique that has the potential to improve
+ * performance dramatically in some cases.
+ *
+ * In cases where the given metadata may not even be used in the loop, we can improve performance
+ * even more by only priming the metadata cache for affected items the first time a piece of metadata
+ * is requested - ie, by lazy-loading it. So, for example, comment meta may not be loaded into the
+ * cache in the comments section of a post until the first time get_comment_meta() is called in the
+ * context of the comment loop.
+ *
+ * WP uses the WP_Metadata_Lazyloader class to queue objects for metadata cache priming. The class
+ * then detects the relevant get_*_meta() function call, and queries the metadata of all queued objects.
+ *
+ * Do not access this class directly. Use the wp_metadata_lazyloader() function.
+ *
+ * @since 4.5.0
+ */
+class WP_Metadata_Lazyloader {
+ /**
+ * Pending objects queue.
+ *
+ * @since 4.5.0
+ * @access protected
+ * @var array
+ */
+ protected $pending_objects;
+
+ /**
+ * Settings for supported object types.
+ *
+ * @since 4.5.0
+ * @access protected
+ * @var array
+ */
+ protected $settings = array();
+
+ /**
+ * Constructor.
+ *
+ * @since 4.5.0
+ * @access public
+ */
+ public function __construct() {
+ $this->settings = array(
+ 'term' => array(
+ 'filter' => 'get_term_metadata',
+ 'callback' => array( $this, 'lazyload_term_meta' ),
+ ),
+ 'comment' => array(
+ 'filter' => 'get_comment_metadata',
+ 'callback' => array( $this, 'lazyload_comment_meta' ),
+ ),
+ );
+ }
+
+ /**
+ * Adds objects to the metadata lazy-load queue.
+ *
+ * @since 4.5.0
+ * @access public
+ *
+ * @param string $object_type Type of object whose meta is to be lazy-loaded. Accepts 'term' or 'comment'.
+ * @param array $object_ids Array of object IDs.
+ * @return bool|WP_Error True on success, WP_Error on failure.
+ */
+ public function queue_objects( $object_type, $object_ids ) {
+ if ( ! isset( $this->settings[ $object_type ] ) ) {
+ return new WP_Error( 'invalid_object_type', __( 'Invalid object type' ) );
+ }
+
+ $type_settings = $this->settings[ $object_type ];
+
+ if ( ! isset( $this->pending_objects[ $object_type ] ) ) {
+ $this->pending_objects[ $object_type ] = array();
+ }
+
+ foreach ( $object_ids as $object_id ) {
+ // Keyed by ID for faster lookup.
+ if ( ! isset( $this->pending_objects[ $object_type ][ $object_id ] ) ) {
+ $this->pending_objects[ $object_type ][ $object_id ] = 1;
+ }
+ }
+
+ add_filter( $type_settings['filter'], $type_settings['callback'] );
+
+ /**
+ * Fires after objects are added to the metadata lazy-load queue.
+ *
+ * @since 4.5.0
+ *
+ * @param array $object_ids Object IDs.
+ * @param string $object_type Type of object being queued.
+ * @param WP_Metadata_Lazyloader $lazyloader The lazy-loader object.
+ */
+ do_action( 'metadata_lazyloader_queued_objects', $object_ids, $object_type, $this );
+ }
+
+ /**
+ * Resets lazy-load queue for a given object type.
+ *
+ * @since 4.5.0
+ * @access public
+ *
+ * @param string $object_type Object type. Accepts 'comment' or 'term'.
+ * @return bool|WP_Error True on success, WP_Error on failure.
+ */
+ public function reset_queue( $object_type ) {
+ if ( ! isset( $this->settings[ $object_type ] ) ) {
+ return new WP_Error( 'invalid_object_type', __( 'Invalid object type' ) );
+ }
+
+ $type_settings = $this->settings[ $object_type ];
+
+ $this->pending_objects[ $object_type ] = array();
+ remove_filter( $type_settings['filter'], $type_settings['callback'] );
+ }
+
+ /**
+ * Lazy-loads term meta for queued terms.
+ *
+ * This method is public so that it can be used as a filter callback. As a rule, there
+ * is no need to invoke it directly.
+ *
+ * @since 4.5.0
+ * @access public
+ *
+ * @param mixed $check The `$check` param passed from the 'get_term_metadata' hook.
+ * @return mixed In order not to short-circuit `get_metadata()`. Generally, this is `null`, but it could be
+ * another value if filtered by a plugin.
+ */
+ public function lazyload_term_meta( $check ) {
+ if ( ! empty( $this->pending_objects['term'] ) ) {
+ update_termmeta_cache( array_keys( $this->pending_objects['term'] ) );
+
+ // No need to run again for this set of terms.
+ $this->reset_queue( 'term' );
+ }
+
+ return $check;
+ }
+
+ /**
+ * Lazy-loads comment meta for queued comments.
+ *
+ * This method is public so that it can be used as a filter callback. As a rule, there is no need to invoke it
+ * directly, from either inside or outside the `WP_Query` object.
+ *
+ * @since 4.5.0
+ *
+ * @param mixed $check The `$check` param passed from the 'get_comment_metadata' hook.
+ * @return mixed The original value of `$check`, so as not to short-circuit `get_comment_metadata()`.
+ */
+ public function lazyload_comment_meta( $check ) {
+ if ( ! empty( $this->pending_objects['comment'] ) ) {
+ update_meta_cache( 'comment', array_keys( $this->pending_objects['comment'] ) );
+
+ // No need to run again for this set of comments.
+ $this->reset_queue( 'comment' );
+ }
+
+ return $check;
+ }
+}
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