amp-comments-count-in-excerpts.php

<?php
/**
 * Plugin Name: AMP Comment Counts in Excerpts
 *
 * @author    Weston Ruter, Google
 * @license   GPL-2.0-or-later
 * @copyright 2019 Google Inc.
 *
 * @wordpress-plugin
 * Plugin Name: AMP Comment Counts in Excerpts
 * Description: Demonstration for how to add comment counts to post excerpts (in post list) in a way they can dynamically update on an AMP Cache.
 * Plugin URI: https://gist.github.com/westonruter/acb6220de53c16db552c0ece0e709bd1
 * 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
 * Gist Plugin URI: https://gist.github.com/westonruter/acb6220de53c16db552c0ece0e709bd1
 */

add_action(
	'rest_api_init',
	function () {
		register_rest_route(
			'example/v1',
			'comment-counts',
			[
				'methods' => 'GET',
				'args'    => [
					'ids' => [
						// @todo More validation needed, e.g. put a maximum on the number.
						// @todo Add a proper schema.
						'sanitize_callback' => function ( $value ) {
							return wp_parse_id_list( $value );
						},
						'required' => true,
					],
				],
				'callback' => function( WP_REST_Request $request ) {
					$comment_counts = [];
					foreach ( $request->get_param( 'ids' ) as $id ) {
						$comment_counts[ $id ] = get_comment_count( $id )['approved'];
					}
					return $comment_counts;
				},
			]
		);
	}
);

// Demonstrate injecting comment count into Excerpt.
add_filter(
	'the_excerpt',
	function ( $content ) {
		global $wp_query;
		if ( is_archive() && $wp_query instanceof WP_Query && in_array( get_post(), $wp_query->get_posts() ) ) {
			$rest_api_url = add_query_arg(
				[ 'ids' => implode( ',', array_values( wp_list_pluck( $wp_query->get_posts(), 'ID' ) ) ) ],
				rest_url( '/example/v1/comment-counts' )
			);

			ob_start();
			?>
			<div style="display:flex">
				<span style="padding-right:1ex;">Comments:</span>
				<amp-list noloading single-item items="." layout="flex-item" src="<?php echo esc_url( $rest_api_url ); ?>">
					<template type="amp-mustache">{{<?php the_ID(); ?>}}</template>
					<span placeholder>?</span>
				</amp-list>
			</div>
			<?php
			$amp_list = preg_replace( '/\s+/', ' ', ob_get_clean() );

			$content = $amp_list . $content;
		}

		return $content;
	},
	PHP_INT_MAX
);

Installation instructions: https://gist.github.com/westonruter/6110fbc4bef0c4b8c021a112012f7e9c