HtmlResponseAttachmentsProcessor::processAttachments

public HtmlResponseAttachmentsProcessor::processAttachments(AttachmentsInterface $response)

$build['#attached']['library'][] = [
  'library' => ['core/jquery']
];
$build['#attached']['http_header'] = [
  ['Content-Type', 'application/rss+xml; charset=utf-8'],
];

public function processAttachments(AttachmentsInterface $response) {
  // @todo Convert to assertion once https://www.drupal.org/node/2408013 lands
  if (!$response instanceof HtmlResponse) {
    throw new \InvalidArgumentException('\Drupal\Core\Render\HtmlResponse instance expected.');
  }

  // First, render the actual placeholders; this may cause additional
  // attachments to be added to the response, which the attachment
  // placeholders rendered by renderHtmlResponseAttachmentPlaceholders() will
  // need to include.
  //
  // @todo Exceptions should not be used for code flow control. However, the
  //   Form API does not integrate with the HTTP Kernel based architecture of
  //   Drupal 8. In order to resolve this issue properly it is necessary to
  //   completely separate form submission from rendering.
  //   @see https://www.drupal.org/node/2367555
  try {
    $response = $this->renderPlaceholders($response);
  }
  catch (EnforcedResponseException $e) {
    return $e->getResponse();
  }

  // Get a reference to the attachments.
  $attached = $response->getAttachments();

  // Send a message back if the render array has unsupported #attached types.
  $unsupported_types = array_diff(
  array_keys($attached), 
  ['html_head', 'feed', 'html_head_link', 'http_header', 'library', 'html_response_attachment_placeholders', 'placeholders', 'drupalSettings']
  );
  if (!empty($unsupported_types)) {
    throw new \LogicException(sprintf('You are not allowed to use %s in #attached.', implode(', ', $unsupported_types)));
  }

  // If we don't have any placeholders, there is no need to proceed.
  if (!empty($attached['html_response_attachment_placeholders'])) {
    // Get the placeholders from attached and then remove them.
    $attachment_placeholders = $attached['html_response_attachment_placeholders'];
    unset($attached['html_response_attachment_placeholders']);

    $assets = AttachedAssets::createFromRenderArray(['#attached' => $attached]);
    // Take Ajax page state into account, to allow for something like
    // Turbolinks to be implemented without altering core.
    // @see https://github.com/rails/turbolinks/
    $ajax_page_state = $this->requestStack->getCurrentRequest()->get('ajax_page_state');
    $assets->setAlreadyLoadedLibraries(isset($ajax_page_state) ? explode(',', $ajax_page_state['libraries']) : []);
    $variables = $this->processAssetLibraries($assets, $attachment_placeholders);
    // $variables now contains the markup to load the asset libraries. Update
    // $attached with the final list of libraries and JavaScript settings, so
    // that $response can be updated with those. Then the response object will
    // list the final, processed attachments.
    $attached['library'] = $assets->getLibraries();
    $attached['drupalSettings'] = $assets->getSettings();

    // Since we can only replace content in the HTML head section if there's a
    // placeholder for it, we can safely avoid processing the render array if
    // it's not present.
    if (!empty($attachment_placeholders['head'])) {
      // 'feed' is a special case of 'html_head_link'. We process them into
      // 'html_head_link' entries and merge them.
      if (!empty($attached['feed'])) {
        $attached = BubbleableMetadata::mergeAttachments(
        $attached, 
        $this->processFeed($attached['feed'])
        );
        unset($attached['feed']);
      }
      // 'html_head_link' is a special case of 'html_head' which can be present
      // as a head element, but also as a Link: HTTP header depending on
      // settings in the render array. Processing it can add to both the
      // 'html_head' and 'http_header' keys of '#attached', so we must address
      // it before 'html_head'.
      if (!empty($attached['html_head_link'])) {
        // Merge the processed 'html_head_link' into $attached so that its
        // 'html_head' and 'http_header' values are present for further
        // processing.
        $attached = BubbleableMetadata::mergeAttachments(
        $attached, 
        $this->processHtmlHeadLink($attached['html_head_link'])
        );
        unset($attached['html_head_link']);
      }

      // Now we can process 'html_head', which contains both 'feed' and
      // 'html_head_link'.
      if (!empty($attached['html_head'])) {
        $variables['head'] = $this->processHtmlHead($attached['html_head']);
      }
    }

    // Now replace the attachment placeholders.
    $this->renderHtmlResponseAttachmentPlaceholders($response, $attachment_placeholders, $variables);
  }

  // Set the HTTP headers and status code on the response if any bubbled.
  if (!empty($attached['http_header'])) {
    $this->setHeaders($response, $attached['http_header']);
  }

  // AttachmentsResponseProcessorInterface mandates that the response it
  // processes contains the final attachment values.
  $response->setAttachments($attached);

  return $response;
}