Proof of concept to integrate the WordPress PWA plugin with the Content Indexing API. Depends on https://github.com/xwp/pwa-wp/pull/234

wp-content-indexing-api.js

if ( ! wp.hooks ) {
	console.warn( 'The WP Content Indexing API depends on this PR from the PWA plugin: https://github.com/xwp/pwa-wp/pull/234' );
} else {
	wp.hooks.addAction( 'navigation_response_success', 'basic_site_caching.navigation_response_success', async ( { event, response, navigationCacheStrategy } ) => {

		if ( navigationCacheStrategy instanceof wp.serviceWorker.strategies.NetworkOnly ) {
			console.warn( 'Unable to use Content Indexing API because NetworkOnly navigation caching strategy is being used.' );
			return;
		}
		if ( !registration.index ) {
			console.warn( 'Unable to use Content Indexing API because it is not currently enabled (e.g. via origin trial). See: https://web.dev/content-indexing-api/' );
			return;
		}

		console.info( "navigation_response_success", { event, response, navigationCacheStrategy } );

		console.info( { registration } )


		// @todo Scrape the title, URL, ID, description, and icons
		return;

		await registration.index.add( {
			// Required; set to something unique within your web app.
			id: 'article-123',

			// Required; this URL needs to be an offline-capable HTML page.
			launchUrl: '/articles/123',

			// Required; used in user-visible lists of content.
			title: 'Article title',

			// Required; used in user-visible lists of content.
			description: 'Amazing article about things!',

			// Required; used in user-visible lists of content.
			icons: [ {
				src: '/img/article-123.png',
				sizes: '64x64',
				type: 'image/png',
			} ],

			// Optional; valid categories are currently:
			// 'homepage', 'article', 'video', 'audio', or '' (default).
			category: 'article',
		} );

		// @todo what about removal?
	} );
}

wp-content-indexing-api.php

<?php
/**
 * WP Content Indexing API
 *
 * @package   WP_Content_Indexing_API
 * @author    Weston Ruter, Google
 * @license   GPL-2.0-or-later
 * @copyright 2019 Google Inc.
 *
 * @wordpress-plugin
 * Plugin Name: WP Content Indexing API
 * Description: Proof of concept for how to integrate the Content Indexing API into the WordPress service worker generated by the PWA plugin.
 * Plugin URI:  https://gist.github.com/westonruter/ce3b5a568f10f4ca76f8bc5d5e8e4ab3
 * Version:     0.1.0
 * Author:      Weston Ruter, Google
 * Author URI:  https://weston.ruter.net/
 * License:     GNU General Public License v2 (or later)
 * License URI: http://www.gnu.org/licenses/gpl-2.0.html
 */

namespace WP_Content_Indexing_API;

// Add caching for uploaded images.
add_action(
	'wp_front_service_worker',
	function ( \WP_Service_Worker_Scripts $scripts ) {
		$scripts->register(
			'content_indexing_api',
			array(
				'src' => plugin_dir_url( __FILE__ ) . 'wp-content-indexing-api.js',
			)
		);
	}
);