Add storage for URL metrics to optimize image loading

admin/server-timing.php

@@ -43,7 +43,7 @@ function perflab_add_server_timing_page() {
  * @since 2.6.0
  */
 function perflab_load_server_timing_page() {
-	if ( ! has_filter( 'template_include', 'image_loading_optimization_buffer_output' ) ) {
+	if ( ! has_filter( 'template_include', 'ilo_buffer_output' ) ) {
 		/*
 		 * This settings section technically includes a field, however it is directly rendered as part of the section
 		 * callback due to requiring custom markup.
@@ -95,7 +95,7 @@ static function () {
 				);
 				?>
 			</p>
-			<?php if ( ! perflab_server_timing_use_output_buffer() && ! has_filter( 'template_include', 'image_loading_optimization_buffer_output' ) ) : ?>
+			<?php if ( ! perflab_server_timing_use_output_buffer() && ! has_filter( 'template_include', 'ilo_buffer_output' ) ) : ?>
 				<p>
 					<?php
 					echo wp_kses(

composer.json

@@ -27,7 +27,8 @@
   },
   "require": {
     "composer/installers": "~1.0",
-    "php": ">=7|^8"
+    "php": ">=7|^8",
+    "ext-json": "*"
   },
   "scripts": {
     "phpstan": "phpstan analyze --ansi --memory-limit=2048M",

modules/images/image-loading-optimization/detection.php

@@ -0,0 +1,69 @@
+<?php
+/**
+ * Detection for image loading optimization.
+ *
+ * @package performance-lab
+ * @since n.e.x.t
+ */
+
+if ( ! defined( 'ABSPATH' ) ) {
+	exit; // Exit if accessed directly.
+}
+
+/**
+ * Prints the script for detecting loaded images and the LCP element.
+ *
+ * @since n.e.x.t
+ * @access private
+ */
+function ilo_print_detection_script() /*: void (in PHP 7.1) */ {
+	if ( ! ilo_can_optimize_response() ) {
+		return;
+	}
+
+	$query_vars = ilo_get_normalized_query_vars();
+	$slug       = ilo_get_page_metrics_slug( $query_vars );
+	$microtime  = microtime( true );
+
+	// Abort if we already have all the sample size we need for all breakpoints.
+	$needed_minimum_viewport_widths = ilo_get_needed_minimum_viewport_widths_now_for_slug( $slug );
+	if ( ! ilo_needs_page_metric_for_breakpoint( $needed_minimum_viewport_widths ) ) {
+		return;
+	}
+
+	/**
+	 * Filters the time window between serve time and run time in which loading detection is allowed to run.
+	 *
+	 * Allow this amount of milliseconds between when the page was first generated (and perhaps cached) and when the
+	 * detect function on the page is allowed to perform its detection logic and submit the request to store the results.
+	 * This avoids situations in which there is missing detection metrics in which case a site with page caching which
+	 * also has a lot of traffic could result in a cache stampede.
+	 *
+	 * @since n.e.x.t
+	 * @todo The value should probably be something like the 99th percentile of Time To Last Byte (TTLB) for WordPress sites in CrUX.
+	 *
+	 * @param int $detection_time_window Detection time window in milliseconds.
+	 */
+	$detection_time_window = apply_filters( 'perflab_image_loading_detection_time_window', 5000 );
+
+	$detect_args = array(
+		'serveTime'                   => $microtime * 1000, // In milliseconds for comparison with `Date.now()` in JavaScript.
+		'detectionTimeWindow'         => $detection_time_window,
+		'isDebug'                     => WP_DEBUG,
+		'restApiEndpoint'             => rest_url( ILO_REST_API_NAMESPACE . ILO_PAGE_METRICS_ROUTE ),
+		'restApiNonce'                => wp_create_nonce( 'wp_rest' ),
+		'pageMetricsSlug'             => $slug,
+		'pageMetricsNonce'            => ilo_get_page_metrics_storage_nonce( $slug ),
+		'neededMinimumViewportWidths' => $needed_minimum_viewport_widths,
+		'storageLockTTL'              => ilo_get_page_metric_storage_lock_ttl(),
+	);
+	wp_print_inline_script_tag(
+		sprintf(
+			'import detect from %s; detect( %s );',
+			wp_json_encode( add_query_arg( 'ver', IMAGE_LOADING_OPTIMIZATION_VERSION, plugin_dir_url( __FILE__ ) . 'detection/detect.js' ) ),
+			wp_json_encode( $detect_args )
+		),
+		array( 'type' => 'module' )
+	);
+}
+add_action( 'wp_print_footer_scripts', 'ilo_print_detection_script' );