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Description

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"…">`.

Usage

wp_resource_hints();

Parameters

Returns

void

Source

File name: wordpress/wp-includes/general-template.php
Lines:

function wp_resource_hints() {
  $hints = array(
    'dns-prefetch' => wp_dependencies_unique_hosts(),
    'preconnect'   => array(),
    'prefetch'     => array(),
    'prerender'    => array(),
  );

  /*
	 * Add DNS prefetch for the Emoji CDN.
	 * The path is removed in the foreach loop below.
	 */
  
/** This filter is documented in wp-includes/formatting.php */
  $hints['dns-prefetch'][] = apply_filters( 'emoji_svg_url', 'https://s.w.org/images/core/emoji/13.0.0/svg/' );

  foreach ( $hints as $relation_type => $urls ) {
    $unique_urls = array();

    
/**
 * Filters domains and URLs for resource hints of relation type.
 *
 * @since 4.6.0
 * @since 4.7.0 The `$urls` parameter accepts arrays of specific HTML attributes
 *              as its child elements.
 *
 * @param array  $urls {
 *     Array of resources and their attributes, or URLs to print for resource hints.
 *
 *     @type array|string ...$0 {
 *         Array of resource attributes, or a URL string.
 *
 *         @type string $href        URL to include in resource hints. 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 float  $pr          Expected probability that the resource hint will be used.
 *         @type string $type        Type of the resource (`text/html`, `text/css`, etc).
 *     }
 * }
 * @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 );

    foreach ( $urls as $key => $url ) {
      $atts = array();

      if ( is_array( $url ) ) {
        if ( isset( $url['href'] ) ) {
          $atts = $url;
          $url  = $url['href'];
        } else {
          continue;
        }
      }

      $url = esc_url( $url, array( 'http', 'https' ) );

      if ( ! $url ) {
        continue;
      }

      if ( isset( $unique_urls[ $url ] ) ) {
        continue;
      }

      if ( in_array( $relation_type, array( 'preconnect', 'dns-prefetch' ), true ) ) {
        $parsed = wp_parse_url( $url );

        if ( empty( $parsed['host'] ) ) {
          continue;
        }

        if ( 'preconnect' === $relation_type && ! empty( $parsed['scheme'] ) ) {
          $url = $parsed['scheme'] . '://' . $parsed['host'];
        } else {
          // Use protocol-relative URLs for dns-prefetch or if scheme is missing.
          $url = '//' . $parsed['host'];
        }
      }

      $atts['rel']  = $relation_type;
      $atts['href'] = $url;

      $unique_urls[ $url ] = $atts;
    }

    foreach ( $unique_urls as $atts ) {
      $html = '';

      foreach ( $atts as $attr => $value ) {
        if ( ! is_scalar( $value )
          || ( ! in_array( $attr, array( 'as', 'crossorigin', 'href', 'pr', 'rel', 'type' ), true ) && ! is_numeric( $attr ) )
        ) {

          continue;
        }

        $value = ( 'href' === $attr ) ? esc_url( $value ) : esc_attr( $value );

 

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