wp_preload_resources()

wp_preload_resources()

function wp_preload_resources() {
	/**
	 * Filters domains and URLs for resource preloads.
	 *
	 * @since 6.1.0
	 * @since 6.6.0 Added the `$fetchpriority` attribute.
	 *
	 * @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.
	 *         @type string $fetchpriority Fetchpriority value for the resource.
	 *     }
	 * }
	 */
	$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', 'fetchpriority' );
			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 );
	}
}