REST API: Support meta registration for specific object subtypes.

Introduce an `object_subtype` argument to the args array for `register_meta()` which can be used to limit meta registration to a single subtype (e.g. a custom post type or taxonomy, vs all posts or taxonomies).

Introduce `register_post_meta()` and `register_term_meta()` wrapper methods for `register_meta` to provide a convenient interface for the common case of registering meta for a specific taxonomy or post type. These methods work the way plugin developers have often expected `register_meta` to function, and should be used in place of direct `register_meta` where possible.

Props flixos90, tharsheblows, spacedmonkey.
Fixes #38323.


Built from https://develop.svn.wordpress.org/trunk@43378


git-svn-id: http://core.svn.wordpress.org/trunk@43206 1a063a9b-81f0-0310-95a4-ce76da25c4cd

wp-includes/rest-api/fields/class-wp-rest-comment-meta-fields.php

@@ -27,6 +27,17 @@ class WP_REST_Comment_Meta_Fields extends WP_REST_Meta_Fields {
 		return 'comment';
 	}
 
+	/**
+	 * Retrieves the object meta subtype.
+	 *
+	 * @since 5.0.0
+	 *
+	 * @return string 'comment' There are no subtypes.
+	 */
+	protected function get_meta_subtype() {
+		return 'comment';
+	}
+
 	/**
 	 * Retrieves the type for register_rest_field() in the context of comments.
 	 *

wp-includes/rest-api/fields/class-wp-rest-meta-fields.php

@@ -24,6 +24,17 @@ abstract class WP_REST_Meta_Fields {
 	 */
 	abstract protected function get_meta_type();
 
+	/**
+	 * Retrieves the object meta subtype.
+	 *
+	 * @since 5.0.0
+	 *
+	 * @return string Subtype for the meta type, or empty string if no specific subtype.
+	 */
+	protected function get_meta_subtype() {
+		return '';
+	}
+
 	/**
 	 * Retrieves the object type for register_rest_field().
 	 *
@@ -340,7 +351,15 @@ abstract class WP_REST_Meta_Fields {
 	protected function get_registered_fields() {
 		$registered = array();
 
-		foreach ( get_registered_meta_keys( $this->get_meta_type() ) as $name => $args ) {
+		$meta_type    = $this->get_meta_type();
+		$meta_subtype = $this->get_meta_subtype();
+
+		$meta_keys = get_registered_meta_keys( $meta_type );
+		if ( ! empty( $meta_subtype ) ) {
+			$meta_keys = array_merge( $meta_keys, get_registered_meta_keys( $meta_type, $meta_subtype ) );
+		}
+
+		foreach ( $meta_keys as $name => $args ) {
 			if ( empty( $args['show_in_rest'] ) ) {
 				continue;
 			}

wp-includes/rest-api/fields/class-wp-rest-post-meta-fields.php

@@ -46,6 +46,17 @@ class WP_REST_Post_Meta_Fields extends WP_REST_Meta_Fields {
 		return 'post';
 	}
 
+	/**
+	 * Retrieves the object meta subtype.
+	 *
+	 * @since 5.0.0
+	 *
+	 * @return string Subtype for the meta type, or empty string if no specific subtype.
+	 */
+	protected function get_meta_subtype() {
+		return $this->post_type;
+	}
+
 	/**
 	 * Retrieves the type for register_rest_field().
 	 *

wp-includes/rest-api/fields/class-wp-rest-term-meta-fields.php

@@ -46,6 +46,17 @@ class WP_REST_Term_Meta_Fields extends WP_REST_Meta_Fields {
 		return 'term';
 	}
 
+	/**
+	 * Retrieves the object meta subtype.
+	 *
+	 * @since 5.0.0
+	 *
+	 * @return string Subtype for the meta type, or empty string if no specific subtype.
+	 */
+	protected function get_meta_subtype() {
+		return $this->taxonomy;
+	}
+
 	/**
 	 * Retrieves the type for register_rest_field().
 	 *

wp-includes/rest-api/fields/class-wp-rest-user-meta-fields.php

@@ -27,6 +27,17 @@ class WP_REST_User_Meta_Fields extends WP_REST_Meta_Fields {
 		return 'user';
 	}
 
+	/**
+	 * Retrieves the object meta subtype.
+	 *
+	 * @since 5.0.0
+	 *
+	 * @return string 'user' There are no subtypes.
+	 */
+	protected function get_meta_subtype() {
+		return 'user';
+	}
+
 	/**
 	 * Retrieves the type for register_rest_field().
 	 *