wp_preload_resources

WordPress Version: 6.1

/**
 * Prints resource preloads directives to browsers.
 *
 * Gives directive to browsers to preload specific resources that website will
 * need very soon, this ensures that they are available earlier and are less
 * likely to block the page's render. Preload directives should not be used for
 * non-render-blocking elements, as then they would compete with the
 * render-blocking ones, slowing down the render.
 *
 * These performance improving indicators work by using `<link rel="preload">`.
 *
 * @link https://developer.mozilla.org/en-US/docs/Web/HTML/Link_types/preload
 * @link https://web.dev/preload-responsive-images/
 *
 * @since 6.1.0
 */
function wp_preload_resources()
{
    /**
     * Filters domains and URLs for resource preloads.
     *
     * @since 6.1.0
     *
     * @param array  $preload_resources {
     *     Array of resources and their attributes, or URLs to print for resource preloads.
     *
     *     @type array ...$0 {
     *         Array of resource attributes.
     *
     *         @type string $href        URL to include in resource preloads. Required.
     *         @type string $as          How the browser should treat the resource
     *                                   (`script`, `style`, `image`, `document`, etc).
     *         @type string $crossorigin Indicates the CORS policy of the specified resource.
     *         @type string $type        Type of the resource (`text/html`, `text/css`, etc).
     *         @type string $media       Accepts media types or media queries. Allows responsive preloading.
     *         @type string $imagesizes  Responsive source size to the source Set.
     *         @type string $imagesrcset Responsive image sources to the source set.
     *     }
     * }
     */
    $preload_resources = apply_filters('wp_preload_resources', array());
    if (!is_array($preload_resources)) {
        return;
    }
    $unique_resources = array();
    // Parse the complete resource list and extract unique resources.
    foreach ($preload_resources as $resource) {
        if (!is_array($resource)) {
            continue;
        }
        $attributes = $resource;
        if (isset($resource['href'])) {
            $href = $resource['href'];
            if (isset($unique_resources[$href])) {
                continue;
            }
            $unique_resources[$href] = $attributes;
            // Media can use imagesrcset and not href.
        } elseif ('image' === $resource['as'] && (isset($resource['imagesrcset']) || isset($resource['imagesizes']))) {
            if (isset($unique_resources[$resource['imagesrcset']])) {
                continue;
            }
            $unique_resources[$resource['imagesrcset']] = $attributes;
        } else {
            continue;
        }
    }
    // Build and output the HTML for each unique resource.
    foreach ($unique_resources as $unique_resource) {
        $html = '';
        foreach ($unique_resource as $resource_key => $resource_value) {
            if (!is_scalar($resource_value)) {
                continue;
            }
            // Ignore non-supported attributes.
            $non_supported_attributes = array('as', 'crossorigin', 'href', 'imagesrcset', 'imagesizes', 'type', 'media');
            if (!in_array($resource_key, $non_supported_attributes, true) && !is_numeric($resource_key)) {
                continue;
            }
            // imagesrcset only usable when preloading image, ignore otherwise.
            if ('imagesrcset' === $resource_key && (!isset($unique_resource['as']) || 'image' !== $unique_resource['as'])) {
                continue;
            }
            // imagesizes only usable when preloading image and imagesrcset present, ignore otherwise.
            if ('imagesizes' === $resource_key && (!isset($unique_resource['as']) || 'image' !== $unique_resource['as'] || !isset($unique_resource['imagesrcset']))) {
                continue;
            }
            $resource_value = ('href' === $resource_key) ? esc_url($resource_value, array('http', 'https')) : esc_attr($resource_value);
            if (!is_string($resource_key)) {
                $html .= " {$resource_value}";
            } else {
                $html .= " {$resource_key}='{$resource_value}'";
            }
        }
        $html = trim($html);
        printf("<link rel='preload' %s />\n", $html);
    }
}