smush-preload-image-with-cdn.php

<?php

if (!function_exists('wp_smush_cdn')) {
    function wp_smush_cdn()
    {
        static $smush_cdn;
        if (is_null($smush_cdn) && class_exists('\WP_Smush')) {
            $smush_cdn = \WP_Smush::get_instance()->core()->mod->cdn;
        }
        return $smush_cdn;
    }
}

if (!function_exists('wp_smush_generate_cdn_image_size')) {
    function wp_smush_generate_cdn_image_size($image_id, $image_size)
    {
        $smush_cdn = wp_smush_cdn();
        static $image;
        if (isset($image[$image_id])) {
            $image_url = $image[$image_id];
        } else {
            $image              = array();
            $image_url          = wp_get_attachment_url($image_id);
            $image[$image_id] = $image_url;
        }
        if (!$smush_cdn) {
            return $image_url;
        }

        $image_size   = (array) $image_size;
        $image_size[] = 0;
        list($width, $height) = $image_size;

        $args = array();
        // $args['webp'] = 1; // Already set it via Smush CDN admin.
        if (!empty($width)) {
            $args['size'] = "{$width}x{$height}";
        }

        $cdn_resized_url = $smush_cdn->generate_cdn_url($image_url, $args);

        return $cdn_resized_url;
    }
}
if (!function_exists('wp_smush_preload_featured_image_with_cdn')) {
    function wp_smush_preload_featured_image_with_cdn($image_id)
    {

        $popular_device_sizes = array(0, 1440, 921, 769, 390); // Add 0 to generate full size.
        foreach ($popular_device_sizes as $img_size) {
            $srcst[] = wp_smush_generate_cdn_image_size($image_id, $img_size);
        }

        // Enter content width.
        $content_width = !empty($GLOBALS['content_width']) ? (int) $GLOBALS['content_width'] : 1920;
        // Avoid situations, when themes misuse the global.
        if (0 === $content_width) {
            $content_width = 1920;
        }
        $sizes = sprintf('(max-width: %1$dpx) 100vw, %1$dpx', $content_width);

        echo '
		<link rel="preload" as="image" href="' . $srcst[0] . '" 
        imagesrcset="' . implode(', ', $srcst) . '" imagesizes="' . $sizes . '" >';
    }
}

add_action('wp_head', function () {


    // page_id => image_id
    $image_mapping = array(
        2720 => 53915,
    );

    //get current page id
    $page_id = get_the_ID();

    if (isset($image_mapping[$page_id])) {
        $image_id = $image_mapping[$page_id];
    } else {
        // get featured image id
        $image_id = get_post_thumbnail_id();
    }

    if (!$image_id) {
        return;
    }

    // if everything is fine then preload the image
    wp_smush_preload_featured_image_with_cdn($image_id);
}, 0);