REST API: Link to the REST route for the currently queried resource.

This allows for programatically determining the REST version of the current page. The links also aid human discovery of the REST API in general.

Props dshanske, tfrommen, TimothyBlynJacobs.
Fixes #49116.

Built from https://develop.svn.wordpress.org/trunk@48273


git-svn-id: http://core.svn.wordpress.org/trunk@48042 1a063a9b-81f0-0310-95a4-ce76da25c4cd

wp-includes/class-wp-taxonomy.php

@@ -209,6 +209,16 @@ final class WP_Taxonomy {
 	 */
 	public $rest_controller_class;
 
+	/**
+	 * The controller instance for this taxonomy's REST API endpoints.
+	 *
+	 * Lazily computed. Should be accessed using {@see WP_Taxonomy::get_rest_controller()}.
+	 *
+	 * @since 5.5.0
+	 * @var WP_REST_Controller $rest_controller
+	 */
+	public $rest_controller;
+
 	/**
 	 * Whether it is a built-in taxonomy.
 	 *
@@ -452,4 +462,40 @@ final class WP_Taxonomy {
 	public function remove_hooks() {
 		remove_filter( 'wp_ajax_add-' . $this->name, '_wp_ajax_add_hierarchical_term' );
 	}
+
+	/**
+	 * Gets the REST API controller for this taxonomy.
+	 *
+	 * Will only instantiate the controller class once per request.
+	 *
+	 * @since 5.5.0
+	 *
+	 * @return WP_REST_Controller|null The controller instance, or null if the taxonomy
+	 *                                 is set not to show in rest.
+	 */
+	public function get_rest_controller() {
+		if ( ! $this->show_in_rest ) {
+			return null;
+		}
+
+		$class = $this->rest_controller_class ? $this->rest_controller_class : WP_REST_Terms_Controller::class;
+
+		if ( ! class_exists( $class ) ) {
+			return null;
+		}
+
+		if ( ! is_subclass_of( $class, WP_REST_Controller::class ) ) {
+			return null;
+		}
+
+		if ( ! $this->rest_controller ) {
+			$this->rest_controller = new $class( $this->name );
+		}
+
+		if ( ! ( $this->rest_controller instanceof $class ) ) {
+			return null;
+		}
+
+		return $this->rest_controller;
+	}
 }

wp-includes/rest-api.php

@@ -234,13 +234,9 @@ function create_initial_rest_routes() {
 
 	// Terms.
 	foreach ( get_taxonomies( array( 'show_in_rest' => true ), 'object' ) as $taxonomy ) {
-		$class = ! empty( $taxonomy->rest_controller_class ) ? $taxonomy->rest_controller_class : 'WP_REST_Terms_Controller';
+		$controller = $taxonomy->get_rest_controller();
 
-		if ( ! class_exists( $class ) ) {
-			continue;
-		}
-		$controller = new $class( $taxonomy->name );
-		if ( ! is_subclass_of( $controller, 'WP_REST_Controller' ) ) {
+		if ( ! $controller ) {
 			continue;
 		}
 
@@ -873,7 +869,13 @@ function rest_output_link_wp_head() {
 		return;
 	}
 
-	echo "<link rel='https://api.w.org/' href='" . esc_url( $api_root ) . "' />\n";
+	printf( '<link rel="https://api.w.org/" href="%s" />', esc_url( $api_root ) );
+
+	$resource = rest_get_queried_resource_route();
+
+	if ( $resource ) {
+		printf( '<link rel="alternate" type="application/json" href="%s" />', esc_url( rest_url( $resource ) ) );
+	}
 }
 
 /**
@@ -892,7 +894,13 @@ function rest_output_link_header() {
 		return;
 	}
 
-	header( 'Link: <' . esc_url_raw( $api_root ) . '>; rel="https://api.w.org/"', false );
+	header( sprintf( 'Link: <%s>; rel="https://api.w.org/"', esc_url_raw( $api_root ) ), false );
+
+	$resource = rest_get_queried_resource_route();
+
+	if ( $resource ) {
+		header( sprintf( 'Link: <%s>; rel="alternate"; type="application/json"', esc_url_raw( rest_url( $resource ) ) ), false );
+	}
 }
 
 /**
@@ -1823,3 +1831,121 @@ function rest_default_additional_properties_to_false( $schema ) {
 
 	return $schema;
 }
+
+/**
+ * Gets the REST API route for a post.
+ *
+ * @since 5.5.0
+ *
+ * @param int|WP_Post $post Post ID or post object.
+ * @return string The route path with a leading slash for the given post, or an empty string if there is not a route.
+ */
+function rest_get_route_for_post( $post ) {
+	$post = get_post( $post );
+
+	if ( ! $post instanceof WP_Post ) {
+		return '';
+	}
+
+	$post_type = get_post_type_object( $post->post_type );
+	if ( ! $post_type ) {
+		return '';
+	}
+
+	$controller = $post_type->get_rest_controller();
+	if ( ! $controller ) {
+		return '';
+	}
+
+	$route = '';
+
+	// The only two controllers that we can detect are the Attachments and Posts controllers.
+	if ( in_array( get_class( $controller ), array( 'WP_REST_Attachments_Controller', 'WP_REST_Posts_Controller' ), true ) ) {
+		$namespace = 'wp/v2';
+		$rest_base = ! empty( $post_type->rest_base ) ? $post_type->rest_base : $post_type->name;
+		$route     = sprintf( '/%s/%s/%d', $namespace, $rest_base, $post->ID );
+	}
+
+	/**
+	 * Filters the REST API route for a post.
+	 *
+	 * @since 5.5.0
+	 *
+	 * @param string  $route The route path.
+	 * @param WP_Post $post  The post object.
+	 */
+	return apply_filters( 'rest_route_for_post', $route, $post );
+}
+
+/**
+ * Gets the REST API route for a term.
+ *
+ * @since 5.5.0
+ *
+ * @param int|WP_Term $term Term ID or term object.
+ * @return string The route path with a leading slash for the given term, or an empty string if there is not a route.
+ */
+function rest_get_route_for_term( $term ) {
+	$term = get_term( $term );
+
+	if ( ! $term instanceof WP_Term ) {
+		return '';
+	}
+
+	$taxonomy = get_taxonomy( $term->taxonomy );
+	if ( ! $taxonomy ) {
+		return '';
+	}
+
+	$controller = $taxonomy->get_rest_controller();
+	if ( ! $controller ) {
+		return '';
+	}
+
+	$route = '';
+
+	// The only controller that works is the Terms controller.
+	if ( 'WP_REST_Terms_Controller' === get_class( $controller ) ) {
+		$namespace = 'wp/v2';
+		$rest_base = ! empty( $taxonomy->rest_base ) ? $taxonomy->rest_base : $taxonomy->name;
+		$route     = sprintf( '/%s/%s/%d', $namespace, $rest_base, $term->term_id );
+	}
+
+	/**
+	 * Filters the REST API route for a term.
+	 *
+	 * @since 5.5.0
+	 *
+	 * @param string  $route The route path.
+	 * @param WP_Term $term  The term object.
+	 */
+	return apply_filters( 'rest_route_for_term', $route, $term );
+}
+
+/**
+ * Gets the REST route for the currently queried object.
+ *
+ * @since 5.5.0
+ *
+ * @return string The REST route of the resource, or an empty string if no resource identified.
+ */
+function rest_get_queried_resource_route() {
+	if ( is_singular() ) {
+		$route = rest_get_route_for_post( get_queried_object() );
+	} elseif ( is_category() || is_tag() || is_tax() ) {
+		$route = rest_get_route_for_term( get_queried_object() );
+	} elseif ( is_author() ) {
+		$route = '/wp/v2/users/' . get_queried_object_id();
+	} else {
+		$route = '';
+	}
+
+	/**
+	 * Filters the REST route for the currently queried object.
+	 *
+	 * @since 5.5.0
+	 *
+	 * @param string $link The route with a leading slash, or an empty string.
+	 */
+	return apply_filters( 'rest_queried_resource_route', $route );
+}

wp-includes/version.php

@@ -13,7 +13,7 @@
  *
  * @global string $wp_version
  */
-$wp_version = '5.5-alpha-48272';
+$wp_version = '5.5-alpha-48273';
 
 /**
  * Holds the WordPress DB revision, increments when changes are made to the WordPress DB schema.