<?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

	 * @var array

	 */

	protected $pending_objects;

	/**

	 * Settings for supported object types.

	 *

	 * @since 4.5.0

	 * @var array

	 */

	protected $settings = array();

	/**

	 * Constructor.

	 *

	 * @since 4.5.0

	 */

	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

	 *

	 * @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 void|WP_Error 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  Array of 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

	 *

	 * @param string $object_type Object type. Accepts 'comment' or 'term'.

	 * @return void|WP_Error 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

	 *

	 * @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 {@see '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;

	}

}