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.


git-svn-id: https://develop.svn.wordpress.org/trunk@48273 602fd350-edb4-49c9-b593-d223f7449a82

src/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;
+	}
 }