class HtmlResponseAttachmentsProcessor

class HtmlResponseAttachmentsProcessor implements AttachmentsResponseProcessorInterface {
    
    /**
     * A config object for the system performance configuration.
     *
     * @var \Drupal\Core\Config\Config
     */
    protected $config;
    
    /**
     * Constructs a HtmlResponseAttachmentsProcessor object.
     *
     * @param \Drupal\Core\Asset\AssetResolverInterface $assetResolver
     *   An asset resolver.
     * @param \Drupal\Core\Config\ConfigFactoryInterface $config_factory
     *   A config factory for retrieving required config objects.
     * @param \Drupal\Core\Asset\AssetCollectionRendererInterface $cssCollectionRenderer
     *   The CSS asset collection renderer.
     * @param \Drupal\Core\Asset\AssetCollectionRendererInterface $jsCollectionRenderer
     *   The JS asset collection renderer.
     * @param \Symfony\Component\HttpFoundation\RequestStack $requestStack
     *   The request stack.
     * @param \Drupal\Core\Render\RendererInterface $renderer
     *   The renderer.
     * @param \Drupal\Core\Extension\ModuleHandlerInterface $moduleHandler
     *   The module handler service.
     * @param \Drupal\Core\Language\LanguageManagerInterface $languageManager
     *   The language manager.
     */
    public function __construct(AssetResolverInterface $assetResolver, ConfigFactoryInterface $config_factory, AssetCollectionRendererInterface $cssCollectionRenderer, AssetCollectionRendererInterface $jsCollectionRenderer, RequestStack $requestStack, RendererInterface $renderer, ModuleHandlerInterface $moduleHandler, LanguageManagerInterface $languageManager) {
        $this->config = $config_factory->get('system.performance');
    }
    
    /**
     * {@inheritdoc}
     */
    public function processAttachments(AttachmentsInterface $response) {
        assert($response instanceof HtmlResponse);
        // 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;
    }
    
    /**
     * Formats an attribute string for an HTTP header.
     *
     * @param array $attributes
     *   An associative array of attributes such as 'rel'.
     *
     * @return string
     *   A ; separated string ready for insertion in a HTTP header. No escaping is
     *   performed for HTML entities, so this string is not safe to be printed.
     *
     * @internal
     *
     * @see https://www.drupal.org/node/3000051
     */
    public static function formatHttpHeaderAttributes(array $attributes = []) {
        foreach ($attributes as $attribute => &$data) {
            if (is_array($data)) {
                $data = implode(' ', $data);
            }
            $data = $attribute . '="' . $data . '"';
        }
        return $attributes ? ' ' . implode('; ', $attributes) : '';
    }
    
    /**
     * Renders placeholders (#attached['placeholders']).
     *
     * First, the HTML response object is converted to an equivalent render array,
     * with #markup being set to the response's content and #attached being set to
     * the response's attachments. Among these attachments, there may be
     * placeholders that need to be rendered (replaced).
     *
     * Next, RendererInterface::renderRoot() is called, which renders the
     * placeholders into their final markup.
     *
     * The markup that results from RendererInterface::renderRoot() is now the
     * original HTML response's content, but with the placeholders rendered. We
     * overwrite the existing content in the original HTML response object with
     * this markup. The markup that was rendered for the placeholders may also
     * have attachments (e.g. for CSS/JS assets) itself, and cacheability metadata
     * that indicates what that markup depends on. That metadata is also added to
     * the HTML response object.
     *
     * @param \Drupal\Core\Render\HtmlResponse $response
     *   The HTML response whose placeholders are being replaced.
     *
     * @return \Drupal\Core\Render\HtmlResponse
     *   The updated HTML response, with replaced placeholders.
     *
     * @see \Drupal\Core\Render\Renderer::replacePlaceholders()
     * @see \Drupal\Core\Render\Renderer::renderPlaceholder()
     */
    protected function renderPlaceholders(HtmlResponse $response) {
        $build = [
            '#markup' => Markup::create($response->getContent()),
            '#attached' => $response->getAttachments(),
        ];
        // RendererInterface::renderRoot() renders the $build render array and
        // updates it in place. We don't care about the return value (which is just
        // $build['#markup']), but about the resulting render array.
        // @todo Simplify this when https://www.drupal.org/node/2495001 lands.
        $this->renderer
            ->renderRoot($build);
        // Update the Response object now that the placeholders have been rendered.
        $placeholders_bubbleable_metadata = BubbleableMetadata::createFromRenderArray($build);
        $response->setContent($build['#markup'])
            ->addCacheableDependency($placeholders_bubbleable_metadata)
            ->setAttachments($placeholders_bubbleable_metadata->getAttachments());
        return $response;
    }
    
    /**
     * Processes asset libraries into render arrays.
     *
     * @param \Drupal\Core\Asset\AttachedAssetsInterface $assets
     *   The attached assets collection for the current response.
     * @param array $placeholders
     *   The placeholders that exist in the response.
     *
     * @return array
     *   An array keyed by asset type, with keys:
     *     - styles
     *     - scripts
     *     - scripts_bottom
     */
    protected function processAssetLibraries(AttachedAssetsInterface $assets, array $placeholders) {
        $variables = [];
        $maintenance_mode = defined('MAINTENANCE_MODE') || \Drupal::state()->get('system.maintenance_mode');
        // Print styles - if present.
        if (isset($placeholders['styles'])) {
            // Optimize CSS if necessary, but only during normal site operation.
            $optimize_css = !$maintenance_mode && $this->config
                ->get('css.preprocess');
            $variables['styles'] = $this->cssCollectionRenderer
                ->render($this->assetResolver
                ->getCssAssets($assets, $optimize_css, $this->languageManager
                ->getCurrentLanguage()));
        }
        // Print scripts - if any are present.
        if (isset($placeholders['scripts']) || isset($placeholders['scripts_bottom'])) {
            // Optimize JS if necessary, but only during normal site operation.
            $optimize_js = !$maintenance_mode && $this->config
                ->get('js.preprocess');
            [
                $js_assets_header,
                $js_assets_footer,
            ] = $this->assetResolver
                ->getJsAssets($assets, $optimize_js, $this->languageManager
                ->getCurrentLanguage());
            $variables['scripts'] = $this->jsCollectionRenderer
                ->render($js_assets_header);
            $variables['scripts_bottom'] = $this->jsCollectionRenderer
                ->render($js_assets_footer);
        }
        return $variables;
    }
    
    /**
     * Renders HTML response attachment placeholders.
     *
     * This is the last step where all of the attachments are placed into the
     * response object's contents.
     *
     * @param \Drupal\Core\Render\HtmlResponse $response
     *   The HTML response to update.
     * @param array $placeholders
     *   An array of placeholders, keyed by type with the placeholders
     *   present in the content of the response as values.
     * @param array $variables
     *   The variables to render and replace, keyed by type with renderable
     *   arrays as values.
     */
    protected function renderHtmlResponseAttachmentPlaceholders(HtmlResponse $response, array $placeholders, array $variables) {
        $content = $response->getContent();
        foreach ($placeholders as $type => $placeholder) {
            if (isset($variables[$type])) {
                $content = str_replace($placeholder, $this->renderer
                    ->renderInIsolation($variables[$type]), $content);
            }
        }
        $response->setContent($content);
    }
    
    /**
     * Sets headers on a response object.
     *
     * @param \Drupal\Core\Render\HtmlResponse $response
     *   The HTML response to update.
     * @param array $headers
     *   The headers to set, as an array. The items in this array should be as
     *   follows:
     *   - The header name.
     *   - The header value.
     *   - (optional) Whether to replace a current value with the new one, or add
     *     it to the others. If the value is not replaced, it will be appended,
     *     resulting in a header like this: 'Header: value1,value2'
     */
    protected function setHeaders(HtmlResponse $response, array $headers) {
        foreach ($headers as $values) {
            $name = $values[0];
            $value = $values[1];
            $replace = !empty($values[2]);
            // Drupal treats the HTTP response status code like a header, even though
            // it really is not.
            if (strtolower($name) === 'status') {
                $response->setStatusCode($value);
            }
            else {
                $response->headers
                    ->set($name, $value, $replace);
            }
        }
    }
    
    /**
     * Ensure proper key/data order and defaults for renderable head items.
     *
     * @param array $html_head
     *   The ['#attached']['html_head'] portion of a render array.
     *
     * @return array
     *   The ['#attached']['html_head'] portion of a render array with #type of
     *   html_tag added for items without a #type.
     */
    protected function processHtmlHead(array $html_head) {
        $head = [];
        foreach ($html_head as $item) {
            [
                $data,
                $key,
            ] = $item;
            if (!isset($data['#type'])) {
                $data['#type'] = 'html_tag';
            }
            $head[$key] = $data;
        }
        return $head;
    }
    
    /**
     * Transform a html_head_link array into html_head and http_header arrays.
     *
     * Variable html_head_link is a special case of html_head which can be present
     * as a link item in the HTML head section, and also as a Link: HTTP header,
     * depending on options in the render array. Processing it can add to both the
     * html_head and http_header sections.
     *
     * @param array $html_head_link
     *   The 'html_head_link' value of a render array. Each head link is specified
     *   by a two-element array:
     *   - An array specifying the attributes of the link. The 'href' and 'rel'
     *     attributes are required, and the 'href' attribute is expected to be a
     *     percent-encoded URI for proper serialization in the Link: HTTP header,
     *     as specified by RFC 8288.
     *   - A boolean specifying whether the link should also be a Link: HTTP
     *     header.
     *
     * @return array
     *   An ['#attached'] section of a render array. This allows us to easily
     *   merge the results with other render arrays. The array could contain the
     *   following keys:
     *   - http_header
     *   - html_head
     */
    protected function processHtmlHeadLink(array $html_head_link) {
        $attached = [];
        foreach ($html_head_link as $item) {
            $attributes = $item[0];
            $should_add_header = $item[1] ?? FALSE;
            $element = [
                '#tag' => 'link',
                '#attributes' => $attributes,
            ];
            $href = $attributes['href'];
            $rel = $attributes['rel'];
            // Allow multiple hreflang tags to use the same href.
            if (isset($attributes['hreflang'])) {
                $attached['html_head'][] = [
                    $element,
                    'html_head_link:' . $rel . ':' . $attributes['hreflang'] . ':' . $href,
                ];
            }
            else {
                $attached['html_head'][] = [
                    $element,
                    'html_head_link:' . $rel . ':' . $href,
                ];
            }
            if ($should_add_header) {
                // Also add a HTTP header "Link:".
                $href = '<' . $attributes['href'] . '>';
                unset($attributes['href']);
                if ($param = static::formatHttpHeaderAttributes($attributes)) {
                    $href .= ';' . $param;
                }
                $attached['http_header'][] = [
                    'Link',
                    $href,
                    FALSE,
                ];
            }
        }
        return $attached;
    }
    
    /**
     * Transform a 'feed' attachment into an 'html_head_link' attachment.
     *
     * The RSS feed is a special case of 'html_head_link', so we just turn it into
     * one.
     *
     * @param array $attached_feed
     *   The ['#attached']['feed'] portion of a render array.
     *
     * @return array
     *   An ['#attached']['html_head_link'] array, suitable for merging with
     *   another 'html_head_link' array.
     */
    protected function processFeed($attached_feed) {
        $html_head_link = [];
        foreach ($attached_feed as $item) {
            $feed_link = [
                'href' => $item[0],
                'rel' => 'alternate',
                'title' => empty($item[1]) ? '' : $item[1],
                'type' => 'application/rss+xml',
            ];
            $html_head_link[] = [
                $feed_link,
                FALSE,
            ];
        }
        return [
            'html_head_link' => $html_head_link,
        ];
    }

}