REST API: Add _fields parameter to selectively include fields in response JSON.

Allows REST API consumers to specify the specific fields needed in their application code, whitelisting those fields and omitting all others from the returned JSON response object.
This permits applications that only need for example the ID and title of posts to avoid having to transfer the entire rendered post content over the wire alongside the desired fields.
While this whitelisting has no affect on the queries run when preparing the response, it can yield significant reductions in the bandwidth required to transfer a response payload for simple applications.

Props adamsilverstein, TimothyBlynJacobs, svrooij.
Fixes #38131.



git-svn-id: https://develop.svn.wordpress.org/trunk@41744 602fd350-edb4-49c9-b593-d223f7449a82

src/wp-includes/rest-api.php

@@ -165,6 +165,7 @@ function rest_api_default_filters() {
 	// Default serving.
 	add_filter( 'rest_pre_serve_request', 'rest_send_cors_headers' );
 	add_filter( 'rest_post_dispatch', 'rest_send_allow_header', 10, 3 );
+	add_filter( 'rest_post_dispatch', 'rest_filter_response_fields', 10, 3 );
 
 	add_filter( 'rest_pre_dispatch', 'rest_handle_options_request', 10, 3 );
 }
@@ -632,6 +633,49 @@ function rest_send_allow_header( $response, $server, $request ) {
 	return $response;
 }
 
+/**
+ * Filter the API response to include only a white-listed set of response object fields.
+ *
+ * @since 4.8.0
+ *
+ * @param WP_REST_Response $response Current response being served.
+ * @param WP_REST_Server   $server   ResponseHandler instance (usually WP_REST_Server).
+ * @param WP_REST_Request  $request  The request that was used to make current response.
+ *
+ * @return WP_REST_Response Response to be served, trimmed down to contain a subset of fields.
+ */
+function rest_filter_response_fields( $response, $server, $request ) {
+	if ( ! isset( $request['_fields'] ) || $response->is_error() ) {
+		return $response;
+	}
+
+	$data = $response->get_data();
+
+	$fields = is_array( $request['_fields']  ) ? $request['_fields'] : preg_split( '/[\s,]+/', $request['_fields'] );
+
+	if ( 0 === count( $fields ) ) {
+		return $response;
+	}
+
+	// Trim off outside whitespace from the comma delimited list.
+	$fields = array_map( 'trim', $fields );
+
+	$fields_as_keyed = array_combine( $fields, array_fill( 0, count( $fields ), true ) );
+
+	if ( wp_is_numeric_array( $data ) ) {
+		$new_data = array();
+		foreach ( $data as $item ) {
+			$new_data[] = array_intersect_key( $item, $fields_as_keyed );
+		}
+	} else {
+		$new_data = array_intersect_key( $data, $fields_as_keyed );
+	}
+
+	$response->set_data( $new_data );
+
+	return $response;
+}
+
 /**
  * Adds the REST API URL to the WP RSD endpoint.
  *