Add native block-editor support

• Add wp-scripts for build
• Enqueue script, excluding it from widget block editor
• Localize script with configurable/filterable options
• Localize script with titles for post types to match current functionality
• Register meta and set sanitization callbacks
• Allow meta to be accessed via endpoint
• Create sidebar plugin to show the two fields used by the plugin

inc/class-external-permalinks-redux-block-editor.php

+<?php
+/**
+ * Block editor support.
+ *
+ * @package External_Permalinks_Redux
+ */
+
+/**
+ * Class Block_Editor.
+ */
+class External_Permalinks_Redux_Block_Editor {
+	/**
+	 * Singleton!
+	 *
+	 * @var self
+	 */
+	protected static $instance;
+
+	/**
+	 * Instantiate class as a singleton.
+	 *
+	 * @return object
+	 */
+	public static function get_instance() {
+		if ( empty( self::$instance ) ) {
+			self::$instance = new self();
+			self::$instance->_setup();
+		}
+
+		return self::$instance;
+	}
+
+	/**
+	 * Unused constructor.
+	 */
+	final private function __construct() {}
+
+	/**
+	 * Set up class.
+	 *
+	 * @return void
+	 */
+	protected function _setup() {
+		add_action( 'rest_api_init', array( $this, 'register_meta' ) );
+		add_filter( 'is_protected_meta', array( $this, 'allow_meta_updates' ), 10, 3 );
+		add_action( 'enqueue_block_editor_assets', array( $this, 'enqueue' ) );
+	}
+
+	/**
+	 * Register meta for access in block editor.
+	 *
+	 * @return void
+	 */
+	public function register_meta() {
+		global $wp_version;
+
+		if (
+			! function_exists( 'register_meta' )
+			|| version_compare( $wp_version, '4.6.0', '<' )
+		) {
+			return;
+		}
+
+		register_meta(
+			'post',
+			external_permalinks_redux::get_instance()->meta_key_target,
+			array(
+				'default'           => '',
+				'description'       => __(
+					'Redirect destination',
+					'external-permalinks-redux'
+				),
+				'type'              => 'string',
+				'sanitize_callback' => 'esc_url_raw',
+				'show_in_rest'      => true,
+				'single'            => true,
+			)
+		);
+
+		register_meta(
+			'post',
+			external_permalinks_redux::get_instance()->meta_key_type,
+			array(
+				'default'           => 0,
+				'description'       => __(
+					'Redirect status code',
+					'external-permalinks-redux'
+				),
+				'type'              => 'integer',
+				'sanitize_callback' => 'absint',
+				'show_in_rest'      => true,
+				'single'            => true,
+			)
+		);
+	}
+
+	/**
+	 * Allow meta updates from REST API.
+	 *
+	 * @param bool   $protected Whether meta is protected or not.
+	 * @param string $meta_key  Meta key.
+	 * @param string $meta_type Meta key type.
+	 * @return bool
+	 */
+	public function allow_meta_updates( $protected, $meta_key, $meta_type ) {
+		if ( 'post' !== $meta_type ) {
+			return $protected;
+		}
+
+		if (
+			external_permalinks_redux::get_instance()->meta_key_target === $meta_key
+			|| external_permalinks_redux::get_instance()->meta_key_type === $meta_key
+		) {
+			return false;
+		}
+
+		return $protected;
+	}
+
+	/**
+	 * Enqueue block-editor script.
+	 *
+	 * @return void
+	 */
+	public function enqueue() {
+		global $pagenow;
+
+		if ( 'widgets.php' === $pagenow ) {
+			return;
+		}
+
+		$asset_data   = require_once dirname( dirname( __FILE__ ) ) . '/assets/build/index.asset.php';
+		$asset_handle = 'external-permalinks-redux';
+
+		wp_enqueue_script(
+			$asset_handle,
+			plugins_url( 'assets/build/index.js', dirname( __FILE__ ) ),
+			$asset_data['dependencies'],
+			$asset_data['version'],
+			true
+		);
+
+		wp_localize_script(
+			$asset_handle,
+			'externalPermalinksReduxConfig',
+			array(
+				'metaKeys'    => array(
+					'target' => external_permalinks_redux::get_instance()->meta_key_target,
+					'type'   => external_permalinks_redux::get_instance()->meta_key_type,
+				),
+				'postTypes'   => external_permalinks_redux::get_instance()->post_types,
+				'statusCodes' => $this->_get_status_codes(),
+			)
+		);
+	}
+
+	/**
+	 * Format status codes for use with `SelectControl` component.
+	 *
+	 * @return array
+	 */
+	protected function _get_status_codes() {
+		$codes = external_permalinks_redux::get_instance()->status_codes;
+		$formatted = array(
+			array(
+				'label'    => __( '-- Select --', 'external-permalinks-redux' ),
+				'value'    => 0,
+			),
+		);
+
+		foreach ( $codes as $code => $label ) {
+			$formatted[] = array(
+				'label' => $label,
+				'value' => $code,
+			);
+		}
+
+		return $formatted;
+	}
+}