Script Loader: Introduce an API to register resource hints.

Resource hints allow browsers to prefetch specific pages or render them in the background to perform DNS lookups or to begin the connection handshake (DNS, TCP, TLS) in the background.

By default, `wp_resource_hints()` prints hints for "s.w.org" (the WordPress.org CDN) and for all scripts and styles which are enqueued from external hosts.
Use the `wp_resource_hints` filter to add custom domains and URLs for `dns-prefetch`, `preconnect`, `prefetch` or `prerender`.

Props voldemortensen, swissspidy.
Fixes #34292.
Built from https://develop.svn.wordpress.org/trunk@37920


git-svn-id: http://core.svn.wordpress.org/trunk@37861 1a063a9b-81f0-0310-95a4-ce76da25c4cd

wp-admin/includes/admin-filters.php

@@ -43,6 +43,9 @@ add_action( 'admin_head', 'wp_color_scheme_settings' );
 add_action( 'admin_head', 'wp_site_icon'             );
 add_action( 'admin_head', '_ipad_meta'               );
 
+// Prerendering.
+add_filter( 'admin_head', 'wp_resource_hints' );
+
 add_action( 'admin_print_scripts-post.php',     'wp_page_reload_on_back_button_js' );
 add_action( 'admin_print_scripts-post-new.php', 'wp_page_reload_on_back_button_js' );
 

wp-includes/default-filters.php

@@ -239,6 +239,7 @@ add_action( 'wp_head',             'wp_print_styles',                  8    );
 add_action( 'wp_head',             'wp_print_head_scripts',            9    );
 add_action( 'wp_head',             'wp_generator'                           );
 add_action( 'wp_head',             'rel_canonical'                          );
+add_action( 'wp_head',             'wp_resource_hints'                      );
 add_action( 'wp_head',             'wp_shortlink_wp_head',            10, 0 );
 add_action( 'wp_head',             'wp_site_icon',                    99    );
 add_action( 'wp_footer',           'wp_print_footer_scripts',         20    );

wp-includes/general-template.php

@@ -2787,6 +2787,97 @@ function wp_site_icon() {
 	}
 }
 
+/**
+ * Prints resource hints to browsers for pre-fetching, pre-rendering and pre-connecting to web sites.
+ *
+ * Gives hints to browsers to prefetch specific pages or render them in the background,
+ * to perform DNS lookups or to begin the connection handshake (DNS, TCP, TLS) in the background.
+ *
+ * These performance improving indicators work by using `<link rel"…">`.
+ *
+ * @since 4.6.0
+ */
+function wp_resource_hints() {
+	$hints = array(
+		'dns-prefetch' => wp_resource_hints_scripts_styles(),
+		'preconnect'   => array( 's.w.org' ),
+		'prefetch'     => array(),
+		'prerender'    => array(),
+	);
+
+	foreach ( $hints as $relation_type => $urls ) {
+		/**
+		 * Filters domains and URLs for resource hints.
+		 *
+		 * @since 4.6.0
+		 *
+		 * @param array  $urls          URLs to print for resource hints.
+		 * @param string $relation_type The relation type the URLs are printed for, e.g. 'preconnect' or 'prerender'.
+		 */
+		$urls = apply_filters( 'wp_resource_hints', $urls, $relation_type );
+		$urls = array_unique( $urls );
+
+		foreach ( $urls as $url ) {
+			$url = esc_url( $url, array( 'http', 'https' ) );
+
+			if ( in_array( $relation_type, array( 'preconnect', 'dns-prefetch' ) ) ) {
+				$parsed = parse_url( $url );
+
+				if ( ! empty( $parsed['scheme'] ) ) {
+					$url = $parsed['scheme'] . '://' . $parsed['host'];
+				} else {
+					$url = $parsed['host'];
+				}
+			}
+
+			printf( "<link rel='%s' href='%s'>\r\n", $relation_type, $url );
+		}
+	}
+}
+
+/**
+ * Adds dns-prefetch for all scripts and styles enqueued from external hosts.
+ *
+ * @since 4.6.0
+ */
+function wp_resource_hints_scripts_styles() {
+	global $wp_scripts, $wp_styles;
+
+	$unique_hosts = array();
+
+	if ( is_object( $wp_scripts ) && ! empty( $wp_scripts->registered ) ) {
+		foreach ( $wp_scripts->registered as $registered_script ) {
+			$src = $registered_script->src;
+			// Make sure the URL has a scheme, otherwise parse_url() could fail to pass the host.
+			if ( '//' == substr( $src, 0, 2 ) ) {
+				$src = set_url_scheme( $src );
+			}
+
+			$this_host = parse_url( $src, PHP_URL_HOST );
+			if ( ! empty( $this_host ) && ! in_array( $this_host, $unique_hosts ) && $this_host !== $_SERVER['SERVER_NAME'] ) {
+				$unique_hosts[] = $this_host;
+			}
+		}
+	}
+
+	if ( is_object( $wp_styles ) && ! empty( $wp_styles->registered ) ) {
+		foreach ( $wp_styles->registered as $registered_style ) {
+			$src = $registered_style->src;
+			// Make sure the URL has a scheme, otherwise parse_url() could fail to pass the host.
+			if ( '//' == substr( $src, 0, 2 ) ) {
+				$src = set_url_scheme( $src );
+			}
+
+			$this_host = parse_url( $src, PHP_URL_HOST );
+			if ( ! empty( $this_host ) && ! in_array( $this_host, $unique_hosts ) && $this_host !== $_SERVER['SERVER_NAME'] ) {
+				$unique_hosts[] = $this_host;
+			}
+		}
+	}
+
+	return $unique_hosts;
+}
+
 /**
  * Whether the user should have a WYSIWIG editor.
  *

wp-includes/version.php

@@ -4,7 +4,7 @@
  *
  * @global string $wp_version
  */
-$wp_version = '4.6-alpha-37919';
+$wp_version = '4.6-alpha-37920';
 
 /**
  * Holds the WordPress DB revision, increments when changes are made to the WordPress DB schema.