Try Legacy widget block

gutenberg.php

@@ -55,6 +55,13 @@ function the_gutenberg_project() {
 		<div id="metaboxes" class="hidden">
 			<?php the_block_editor_meta_boxes(); ?>
 		</div>
+		<?php
+		/** This action is documented in wp-admin/admin-footer.php */
+		do_action( 'admin_print_footer_scripts-widgets.php' );
+
+		/** This action is documented in wp-admin/admin-footer.php */
+		do_action( 'admin_footer-widgets.php' );
+		?>
 	</div>
 	<?php
 }
@@ -228,6 +235,9 @@ function gutenberg_init( $return, $post ) {
 	 */
 	remove_action( 'admin_print_scripts', 'print_emoji_detection_script' );
 
+	do_action( 'admin_print_styles-widgets.php' );
+	do_action( 'admin_print_scripts-widgets.php' );
+
 	/*
 	 * Ensure meta box functions are available to third-party code;
 	 * includes/meta-boxes is typically loaded from edit-form-advanced.php.

lib/class-wp-rest-widget-updater-controller.php

@@ -0,0 +1,106 @@
+<?php
+/**
+ * Widget Updater REST API: WP_REST_Widget_Updater_Controller class
+ *
+ * @package gutenberg
+ * @since 4.9.0
+ */
+
+/**
+ * Controller which provides REST endpoint for updating a widget.
+ *
+ * @since 2.8.0
+ *
+ * @see WP_REST_Controller
+ */
+class WP_REST_Widget_Updater_Controller extends WP_REST_Controller {
+
+	/**
+	 * Constructs the controller.
+	 *
+	 * @access public
+	 */
+	public function __construct() {
+		$this->namespace = 'wp/v2';
+		$this->rest_base = 'widget-updater';
+	}
+
+	/**
+	 * Registers the necessary REST API route.
+	 *
+	 * @access public
+	 */
+	public function register_routes() {
+			register_rest_route(
+				$this->namespace,
+				'/' . $this->rest_base . '/',
+				array(
+					'args' => array(
+						'name' => array(
+							'description' => __( 'Unique registered name for the block.', 'gutenberg' ),
+							'type'        => 'string',
+						),
+					),
+					array(
+						'methods'  => WP_REST_Server::EDITABLE,
+						'callback' => array( $this, 'compute_new_widget' ),
+					),
+				)
+			);
+	}
+
+	/**
+	 * Returns the new widget instance and the form that represents it.
+	 *
+	 * @since 2.8.0
+	 * @access public
+	 *
+	 * @param WP_REST_Request $request Full details about the request.
+	 * @return WP_REST_Response|WP_Error Response object on success, or WP_Error object on failure.
+	 */
+	public function compute_new_widget( $request ) {
+		$json_request = $request->get_json_params();
+		if ( ! isset( $json_request['identifier'] ) ) {
+			return;
+		}
+		$widget = $json_request['identifier'];
+		global $wp_widget_factory;
+
+		if ( ! isset( $wp_widget_factory->widgets[ $widget ] ) ) {
+			return;
+		}
+
+		$widget_obj = $wp_widget_factory->widgets[ $widget ];
+		if ( ! ( $widget_obj instanceof WP_Widget ) ) {
+			return;
+		}
+
+		$instance = isset( $json_request['instance'] ) ? $json_request['instance'] : array();
+
+		$id_to_use = isset( $json_request['id_to_use'] ) ? $json_request['id_to_use'] : -1;
+
+		$widget_obj->_set( $id_to_use );
+		ob_start();
+
+		if ( isset( $json_request['instance_changes'] ) ) {
+			$instance = $widget_obj->update( $json_request['instance_changes'], $instance );
+			// TODO: apply required filters.
+		}
+
+		$widget_obj->form( $instance );
+		// TODO: apply required filters.
+
+		$id_base = $widget_obj->id_base;
+		$id = $widget_obj->id;
+		$form = ob_get_clean();
+
+		return rest_ensure_response(
+			array(
+				'instance' => $instance,
+				'form'     => $form,
+				'id_base'  => $id_base,
+				'id'       => $id,
+			)
+		);
+	}
+}

lib/client-assets.php

@@ -1112,9 +1112,20 @@ function gutenberg_editor_scripts_and_styles( $hook ) {
 		);
 	}
 
+	$available_legacy_widgets = array();
+	global $wp_widget_factory;
+
+	foreach ( $wp_widget_factory->widgets as $class => $widget_obj ) {
+		$available_legacy_widgets[ $class ] = array(
+			'name'        => html_entity_decode( $widget_obj->name ),
+			'description' => html_entity_decode( $widget_obj->widget_options['description'] ),
+		);
+	}
+
 	$editor_settings = array(
 		'alignWide'              => $align_wide || ! empty( $gutenberg_theme_support[0]['wide-images'] ), // Backcompat. Use `align-wide` outside of `gutenberg` array.
 		'availableTemplates'     => $available_templates,
+		'availableLegacyWidgets' => $available_legacy_widgets,
 		'allowedBlockTypes'      => $allowed_block_types,
 		'disableCustomColors'    => get_theme_support( 'disable-custom-colors' ),
 		'disableCustomFontSizes' => get_theme_support( 'disable-custom-font-sizes' ),

lib/load.php

@@ -12,6 +12,9 @@
 // These files only need to be loaded if within a rest server instance
 // which this class will exist if that is the case.
 if ( class_exists( 'WP_REST_Controller' ) ) {
+	if ( ! class_exists( 'WP_REST_Widget_Updater_Controller' ) ) {
+		require dirname( __FILE__ ) . '/class-wp-rest-widget-updater-controller.php';
+	}
 	require dirname( __FILE__ ) . '/rest-api.php';
 }
 
@@ -43,9 +46,15 @@
 if ( ! function_exists( 'render_block_core_latest_posts' ) ) {
 	require dirname( __FILE__ ) . '/../packages/block-library/src/latest-posts/index.php';
 }
+
+if ( ! function_exists( 'render_block_legacy_widget' ) ) {
+	require dirname( __FILE__ ) . '/../packages/block-library/src/legacy-widget/index.php';
+}
+
 if ( ! function_exists( 'render_block_core_rss' ) ) {
 	require dirname( __FILE__ ) . '/../packages/block-library/src/rss/index.php';
 }
 if ( ! function_exists( 'render_block_core_shortcode' ) ) {
 	require dirname( __FILE__ ) . '/../packages/block-library/src/shortcode/index.php';
 }
+

lib/rest-api.php

@@ -20,6 +20,17 @@ function gutenberg_register_rest_routes() {
 	_deprecated_function( __FUNCTION__, '5.0.0' );
 }
 
+/**
+ * Registers the REST API routes needed by the legacy widget block.
+ *
+ * @since 5.0.0
+ */
+function gutenberg_register_rest_widget_updater_routes() {
+	$widgets_controller = new WP_REST_Widget_Updater_Controller();
+	$widgets_controller->register_routes();
+}
+add_action( 'rest_api_init', 'gutenberg_register_rest_widget_updater_routes' );
+
 /**
  * Handle a failing oEmbed proxy request to try embedding as a shortcode.
  *

packages/block-library/src/editor.scss

@@ -14,6 +14,7 @@
 @import "./image/editor.scss";
 @import "./latest-comments/editor.scss";
 @import "./latest-posts/editor.scss";
+@import "./legacy-widget/editor.scss";
 @import "./media-text/editor.scss";
 @import "./list/editor.scss";
 @import "./more/editor.scss";

packages/block-library/src/index.js

@@ -31,6 +31,7 @@ import * as html from './html';
 import * as mediaText from './media-text';
 import * as latestComments from './latest-comments';
 import * as latestPosts from './latest-posts';
+import * as legacyWidget from './legacy-widget';
 import * as list from './list';
 import * as missing from './missing';
 import * as more from './more';
@@ -81,6 +82,7 @@ export const registerCoreBlocks = () => {
 		mediaText,
 		latestComments,
 		latestPosts,
+		legacyWidget,
 		missing,
 		more,
 		nextpage,

packages/block-library/src/legacy-widget/WidgetEditDomManager.js

@@ -0,0 +1,86 @@
+/**
+ * External dependencies
+ */
+import { includes } from 'lodash';
+
+/**
+ * WordPress dependencies
+ */
+import { Component, createRef } from '@wordpress/element';
+
+class WidgetEditDomManager extends Component {
+	constructor() {
+		super( ...arguments );
+
+		this.containerRef = createRef();
+		this.triggerWidgetEvent = this.triggerWidgetEvent.bind( this );
+	}
+
+	componentDidMount() {
+		this.triggerWidgetEvent( 'widget-added' );
+	}
+
+	shouldComponentUpdate( nextProps ) {
+		// We can not leverage react render otherwise we would destroy dom changes applied by the plugins.
+		// We manually update the required dom node replicating what the widget screen and the customizer do.
+		if ( nextProps.form !== this.props.form && this.containerRef.current ) {
+			const widgetContent = this.containerRef.current.querySelector( '.widget-content' );
+			widgetContent.innerHTML = nextProps.form;
+			this.triggerWidgetEvent( 'widget-updated' );
+		}
+		return false;
+	}
+
+	render() {
+		const { id, idBase, widgetNumber, form } = this.props;
+		return (
+			<div className="widget open" ref={ this.containerRef }>
+				<div className="widget-inside">
+					<form method="post">
+						<div className="widget-content" dangerouslySetInnerHTML={ { __html: form } }>
+						</div>
+						<input type="hidden" name="widget-id" className="widget-id" value={ id } />
+						<input type="hidden" name="id_base" className="id_base" value={ idBase } />
+						<input type="hidden" name="widget_number" className="widget_number" value={ widgetNumber } />
+						<input type="hidden" name="multi_number" className="multi_number" value="" />
+						<input type="hidden" name="add_new" className="add_new" value="" />
+					</form>
+				</div>
+			</div>
+		);
+	}
+
+	triggerWidgetEvent( event ) {
+		window.$( window.document ).trigger(
+			event,
+			[ window.$( this.containerRef.current ) ]
+		);
+	}
+
+	retrieveUpdatedInstance() {
+		if ( this.containerRef.current ) {
+			const { idBase, widgetNumber } = this.props;
+			const form = this.containerRef.current.querySelector( 'form' );
+			const formData = new window.FormData( form );
+			const updatedInstance = {};
+			const keyPrefixLength = `widget-${ idBase }[${ widgetNumber }][`.length;
+			const keySuffixLength = `]`.length;
+			for ( const [ rawKey, value ] of formData ) {
+				const keyParsed = rawKey.substring( keyPrefixLength, rawKey.length - keySuffixLength );
+				// This fields are added to the form because the widget JavaScript code may use this values.
+				// They are not relevant for the update mechanism.
+				if ( includes(
+					[ 'widget-id', 'id_base', 'widget_number', 'multi_number', 'add_new' ],
+					keyParsed,
+				) ) {
+					continue;
+				}
+				updatedInstance[ keyParsed ] = value;
+			}
+			return updatedInstance;
+		}
+	}
+}
+
+export default WidgetEditDomManager;
+