Introduce native Gutenberg controls

• Register meta
• Enqueue script
• Switch to use_block_editor_for_post_type and remove the option
• Build component
• Add sanitize callback to handle special cases
• Add auth callback to allow "private" meta to be edited
• Add padding between modal buttons
• Don't hide UI if set to 0, as there's no way to update the setting
• Switch to @wordpress/scripts
• Consider moving Gutenberg bits, and/or REST API elements, to their own class.

inc/class-block-editor.php

+<?php
+/**
+ * Support block editor (Gutenberg).
+ *
+ * @package WP_Revisions_Control
+ */
+
+namespace WP_Revisions_Control;
+
+use WP_REST_Response;
+use WP_REST_Request;
+use WP_Revisions_Control;
+
+/**
+ * Class Block_Editor.
+ */
+class Block_Editor {
+	use Singleton;
+
+	/**
+	 * Name of action used to clean up post's revisions via cron.
+	 *
+	 * @var string
+	 */
+	private $cron_action = 'wp_revisions_control_cron_purge';
+
+	/**
+	 * Prepare class.
+	 *
+	 * This is called at `init`, so cannot use that hook unless priority is
+	 * greater than 10.
+	 */
+	private function setup() {
+		add_action( 'rest_api_init', array( $this, 'action_rest_api_init' ) );
+		add_filter( 'is_protected_meta', array( $this, 'filter_is_protected_meta' ), 10, 2 );
+		add_action( $this->cron_action, array( WP_Revisions_Control::get_instance(), 'do_purge_excess' ), 10, 2 );
+		add_action( 'enqueue_block_editor_assets', array( $this, 'action_enqueue_block_editor_assets' ) );
+	}
+
+	/**
+	 * Register REST API components for Gutenberg UI.
+	 */
+	public function action_rest_api_init() {
+		if ( ! function_exists( 'register_rest_route' ) ) {
+			return;
+		}
+
+		foreach ( array_keys( WP_Revisions_Control::get_instance()->get_post_types() ) as $post_type ) {
+			register_meta(
+				'post',
+				WP_REVISIONS_CONTROL_LIMIT_META_KEY,
+				array(
+					'object_subtype' => $post_type,
+					'type'           => 'string', // Can be empty, so must be string.
+					'default'        => '',
+					'single'         => true,
+					'show_in_rest'   => true,
+					'description'    => __(
+						'Number of revisions to retain.',
+						'wp_revisions_control'
+					),
+				)
+			);
+		}
+
+		register_rest_route(
+			'wp-revisions-control/v1',
+			'schedule/(?P<id>[\d]+)/(?P<limit_override>[\d]+)',
+			array(
+				'methods'             => 'PUT',
+				'callback'            => array( $this, 'rest_api_schedule_purge' ),
+				'permission_callback' => array( $this, 'rest_api_permission_callback' ),
+				'args'                => array(
+					'id'             => array(
+						'required'          => true,
+						'type'              => 'integer',
+						'validate_callback' => array( $this, 'rest_api_validate_id' ),
+					),
+					'limit_override' => array(
+						'required'          => false,
+						'type'              => 'integer',
+						'default'           => null,
+						'validate_callback' => array( $this, 'rest_api_validate_id' ),
+					),
+				),
+				'show_in_index'       => false,
+			)
+		);
+	}
+
+	/**
+	 * Permissions callback for REST endpoint.
+	 *
+	 * @param WP_REST_Request $request Request object.
+	 * @return bool
+	 */
+	public function rest_api_permission_callback( $request ) {
+		return current_user_can(
+			'edit_post',
+			$request->get_param( 'id' )
+		);
+	}
+
+	/**
+	 * Validate post ID.
+	 *
+	 * @param int $input Post ID.
+	 * @return bool
+	 */
+	public function rest_api_validate_id( $input ) {
+		return is_numeric( $input );
+	}
+
+	/**
+	 * Schedule cleanup of post's excess revisions.
+	 *
+	 * @param WP_REST_Request $request Request object.
+	 * @return WP_REST_Response
+	 */
+	public function rest_api_schedule_purge( $request ) {
+		$result = wp_schedule_single_event(
+			time() + 3,
+			$this->cron_action,
+			array(
+				$request->get_param( 'id' ),
+				$request->get_param( 'limit_override' ),
+			)
+		);
+
+		return rest_ensure_response( $result );
+	}
+
+	/**
+	 * Allow our meta to be edited from Gutenberg.
+	 *
+	 * @param bool   $protected If meta is protected.
+	 * @param string $meta_key  Meta key being checked.
+	 * @return false
+	 */
+	public function filter_is_protected_meta( $protected, $meta_key ) {
+		if ( WP_REVISIONS_CONTROL_LIMIT_META_KEY === $meta_key ) {
+			return false;
+		}
+
+		return $protected;
+	}
+
+	/**
+	 * Register Gutenberg script.
+	 */
+	public function action_enqueue_block_editor_assets() {
+		$handle     = 'wp-revisions-control-block-editor';
+		$asset_data = require_once dirname( __DIR__ ) . '/assets/build/gutenberg.asset.php';
+
+		wp_enqueue_script(
+			$handle,
+			plugins_url(
+				'assets/build/gutenberg.js',
+				__DIR__
+			),
+			$asset_data['dependencies'],
+			$asset_data['version']
+		);
+
+		wp_localize_script(
+			$handle,
+			'wpRevisionsControlBlockEditorSettings',
+			array(
+				'metaKey' => WP_REVISIONS_CONTROL_LIMIT_META_KEY,
+			)
+		);
+	}
+}