REST API: Introduce rest_{$this->post_type}_item_schema filter to enable manipulation of schema values.

register_rest_field can be used to add properties to a schema, but no mechanism existed to alter existing properties like "content".
Running the schema through this filter lets plugins append additional sub-properties to existing schema definitions.

Props luisherranz, TimothyBlynJacobs, swissspidy, westonruter, kadamwhite.
Fixes #47779.


Built from https://develop.svn.wordpress.org/trunk@47265


git-svn-id: http://core.svn.wordpress.org/trunk@47065 1a063a9b-81f0-0310-95a4-ce76da25c4cd

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

@@ -2406,6 +2406,27 @@ class WP_REST_Posts_Controller extends WP_REST_Controller {
 			$schema['links'] = $schema_links;
 		}
 
+		// Take a snapshot of which fields are in the schema pre-filtering.
+		$schema_fields = array_keys( $schema['properties'] );
+
+		/**
+		 * Filter the post's schema.
+		 *
+		 * The dynamic portion of the filter, `$this->post_type`, refers to the
+		 * post type slug for the controller.
+		 *
+		 * @since 5.4.0
+		 *
+		 * @param array $schema Item schema data.
+		 */
+		$schema = apply_filters( "rest_{$this->post_type}_item_schema", $schema );
+
+		// Emit a _doing_it_wrong warning if user tries to add new properties using this filter.
+		$new_fields = array_diff( array_keys( $schema['properties'] ), $schema_fields );
+		if ( count( $new_fields ) > 0 ) {
+			_doing_it_wrong( __METHOD__, __( 'Please use register_rest_field to add new schema properties.' ), '5.4.0' );
+		}
+
 		$this->schema = $schema;
 
 		return $this->add_additional_fields_schema( $this->schema );

wp-includes/version.php

@@ -13,7 +13,7 @@
  *
  * @global string $wp_version
  */
-$wp_version = '5.4-alpha-47264';
+$wp_version = '5.4-alpha-47265';
 
 /**
  * Holds the WordPress DB revision, increments when changes are made to the WordPress DB schema.