Use the plugin classes in the REST endpoint

The endpoint landed in WordPress 5.9, so we need to make sure even
in this environment the plugin will call the WP_Theme_JSON_* classes.

lib/compat/wordpress-6.0/class-gutenberg-rest-global-styles-controller.php

@@ -60,6 +60,151 @@ public function get_theme_items_permissions_check( $request ) { // phpcs:ignore
 		return true;
 	}
 
+	/**
+	 * Prepares a single global styles config for update.
+	 *
+	 * @since 5.9.0
+	 *
+	 * @param WP_REST_Request $request Request object.
+	 * @return stdClass Changes to pass to wp_update_post.
+	 */
+	protected function prepare_item_for_database( $request ) {
+		$changes     = new stdClass();
+		$changes->ID = $request['id'];
+		$post            = get_post( $request['id'] );
+		$existing_config = array();
+		if ( $post ) {
+			$existing_config     = json_decode( $post->post_content, true );
+			$json_decoding_error = json_last_error();
+			if ( JSON_ERROR_NONE !== $json_decoding_error || ! isset( $existing_config['isGlobalStylesUserThemeJSON'] ) ||
+				! $existing_config['isGlobalStylesUserThemeJSON'] ) {
+				$existing_config = array();
+			}
+		}
+		if ( isset( $request['styles'] ) || isset( $request['settings'] ) ) {
+			$config = array();
+			if ( isset( $request['styles'] ) ) {
+				$config['styles'] = $request['styles'];
+			} elseif ( isset( $existing_config['styles'] ) ) {
+				$config['styles'] = $existing_config['styles'];
+			}
+			if ( isset( $request['settings'] ) ) {
+				$config['settings'] = $request['settings'];
+			} elseif ( isset( $existing_config['settings'] ) ) {
+				$config['settings'] = $existing_config['settings'];
+			}
+			$config['isGlobalStylesUserThemeJSON'] = true;
+			$config['version']                     = WP_Theme_JSON_Gutenberg::LATEST_SCHEMA;
+			$changes->post_content                 = wp_json_encode( $config );
+		}
+		// Post title.
+		if ( isset( $request['title'] ) ) {
+			if ( is_string( $request['title'] ) ) {
+				$changes->post_title = $request['title'];
+			} elseif ( ! empty( $request['title']['raw'] ) ) {
+				$changes->post_title = $request['title']['raw'];
+			}
+		}
+		return $changes;
+	}
+
+	/**
+	 * Prepare a global styles config output for response.
+	 *
+	 * @since 5.9.0
+	 *
+	 * @param WP_Post         $post    Global Styles post object.
+	 * @param WP_REST_Request $request Request object.
+	 * @return WP_REST_Response Response object.
+	 */
+	public function prepare_item_for_response( $post, $request ) { // phpcs:ignore VariableAnalysis.CodeAnalysis.VariableAnalysis.UnusedVariable
+		$raw_config                       = json_decode( $post->post_content, true );
+		$is_global_styles_user_theme_json = isset( $raw_config['isGlobalStylesUserThemeJSON'] ) && true === $raw_config['isGlobalStylesUserThemeJSON'];
+		$config                           = array();
+		if ( $is_global_styles_user_theme_json ) {
+			$config = ( new WP_Theme_JSON_Gutenberg( $raw_config, 'custom' ) )->get_raw_data();
+		}
+		// Base fields for every post.
+		$data   = array();
+		$fields = $this->get_fields_for_response( $request );
+		if ( rest_is_field_included( 'id', $fields ) ) {
+			$data['id'] = $post->ID;
+		}
+		if ( rest_is_field_included( 'title', $fields ) ) {
+			$data['title'] = array();
+		}
+		if ( rest_is_field_included( 'title.raw', $fields ) ) {
+			$data['title']['raw'] = $post->post_title;
+		}
+		if ( rest_is_field_included( 'title.rendered', $fields ) ) {
+			add_filter( 'protected_title_format', array( $this, 'protected_title_format' ) );
+			$data['title']['rendered'] = get_the_title( $post->ID );
+			remove_filter( 'protected_title_format', array( $this, 'protected_title_format' ) );
+		}
+		if ( rest_is_field_included( 'settings', $fields ) ) {
+			$data['settings'] = ! empty( $config['settings'] ) && $is_global_styles_user_theme_json ? $config['settings'] : new stdClass();
+		}
+		if ( rest_is_field_included( 'styles', $fields ) ) {
+			$data['styles'] = ! empty( $config['styles'] ) && $is_global_styles_user_theme_json ? $config['styles'] : new stdClass();
+		}
+		$context = ! empty( $request['context'] ) ? $request['context'] : 'view';
+		$data    = $this->add_additional_fields_to_object( $data, $request );
+		$data    = $this->filter_response_by_context( $data, $context );
+		// Wrap the data in a response object.
+		$response = rest_ensure_response( $data );
+		$links = $this->prepare_links( $post->ID );
+		$response->add_links( $links );
+		if ( ! empty( $links['self']['href'] ) ) {
+			$actions = $this->get_available_actions();
+			$self    = $links['self']['href'];
+			foreach ( $actions as $rel ) {
+				$response->add_link( $rel, $self );
+			}
+		}
+		return $response;
+	}
+
+	/**
+	 * Returns the given theme global styles config.
+	 *
+	 * @since 5.9.0
+	 *
+	 * @param WP_REST_Request $request The request instance.
+	 * @return WP_REST_Response|WP_Error
+	 */
+	public function get_theme_item( $request ) {
+		if ( wp_get_theme()->get_stylesheet() !== $request['stylesheet'] ) {
+			// This endpoint only supports the active theme for now.
+			return new WP_Error(
+				'rest_theme_not_found',
+				__( 'Theme not found.', 'gutenberg' ),
+				array( 'status' => 404 )
+			);
+		}
+		$theme  = WP_Theme_JSON_Resolver_Gutenberg::get_merged_data( 'theme' );
+		$data   = array();
+		$fields = $this->get_fields_for_response( $request );
+		if ( rest_is_field_included( 'settings', $fields ) ) {
+			$data['settings'] = $theme->get_settings();
+		}
+		if ( rest_is_field_included( 'styles', $fields ) ) {
+			$raw_data       = $theme->get_raw_data();
+			$data['styles'] = isset( $raw_data['styles'] ) ? $raw_data['styles'] : array();
+		}
+		$context = ! empty( $request['context'] ) ? $request['context'] : 'view';
+		$data    = $this->add_additional_fields_to_object( $data, $request );
+		$data    = $this->filter_response_by_context( $data, $context );
+		$response = rest_ensure_response( $data );
+		$links = array(
+			'self' => array(
+				'href' => rest_url( sprintf( '%s/%s/themes/%s', $this->namespace, $this->rest_base, $request['stylesheet'] ) ),
+			),
+		);
+		$response->add_links( $links );
+		return $response;
+	}
+
+
 	/**
 	 * Returns the given theme global styles variations.
 	 *
@@ -94,4 +239,5 @@ public function get_theme_items( $request ) {
 
 		return $response;
 	}
+
 }