class BigPipeStrategy

class BigPipeStrategy implements PlaceholderStrategyInterface {
    
    /**
     * BigPipe no-JS cookie name.
     */
    const NOJS_COOKIE = 'big_pipe_nojs';
    
    /**
     * The session configuration.
     *
     * @var \Drupal\Core\Session\SessionConfigurationInterface
     */
    protected $sessionConfiguration;
    
    /**
     * The request stack.
     *
     * @var \Symfony\Component\HttpFoundation\RequestStack
     */
    protected $requestStack;
    
    /**
     * The current route match.
     *
     * @var \Drupal\Core\Routing\RouteMatchInterface
     */
    protected $routeMatch;
    
    /**
     * Constructs a new BigPipeStrategy class.
     *
     * @param \Drupal\Core\Session\SessionConfigurationInterface $session_configuration
     *   The session configuration.
     * @param \Symfony\Component\HttpFoundation\RequestStack $request_stack
     *   The request stack.
     * @param \Drupal\Core\Routing\RouteMatchInterface $route_match
     *   The current route match.
     */
    public function __construct(SessionConfigurationInterface $session_configuration, RequestStack $request_stack, RouteMatchInterface $route_match) {
        $this->sessionConfiguration = $session_configuration;
        $this->requestStack = $request_stack;
        $this->routeMatch = $route_match;
    }
    
    /**
     * {@inheritdoc}
     */
    public function processPlaceholders(array $placeholders) {
        $request = $this->requestStack
            ->getCurrentRequest();
        // @todo remove this check when https://www.drupal.org/node/2367555 lands.
        if (!$request->isMethodCacheable()) {
            return [];
        }
        // Routes can opt out from using the BigPipe HTML delivery technique.
        if ($this->routeMatch
            ->getRouteObject()
            ->getOption('_no_big_pipe')) {
            return [];
        }
        if (!$this->sessionConfiguration
            ->hasSession($request)) {
            return [];
        }
        return $this->doProcessPlaceholders($placeholders);
    }
    
    /**
     * Transforms placeholders to BigPipe placeholders, either no-JS or JS.
     *
     * @param array $placeholders
     *   The placeholders to process.
     *
     * @return array
     *   The BigPipe placeholders.
     */
    protected function doProcessPlaceholders(array $placeholders) {
        $overridden_placeholders = [];
        foreach ($placeholders as $placeholder => $placeholder_elements) {
            // BigPipe uses JavaScript and the DOM to find the placeholder to replace.
            // This means finding the placeholder to replace must be efficient. Most
            // placeholders are HTML, which we can find efficiently thanks to the
            // querySelector API. But some placeholders are HTML attribute values or
            // parts thereof, and potentially even plain text in DOM text nodes. For
            // BigPipe's JavaScript to find those placeholders, it would need to
            // iterate over all DOM text nodes. This is highly inefficient. Therefore,
            // the BigPipe placeholder strategy only converts HTML placeholders into
            // BigPipe placeholders. The other placeholders need to be replaced on the
            // server, not via BigPipe.
            // @see \Drupal\Core\Access\RouteProcessorCsrf::renderPlaceholderCsrfToken()
            // @see \Drupal\Core\Form\FormBuilder::renderFormTokenPlaceholder()
            // @see \Drupal\Core\Form\FormBuilder::renderPlaceholderFormAction()
            if (static::placeholderIsAttributeSafe($placeholder)) {
                $overridden_placeholders[$placeholder] = static::createBigPipeNoJsPlaceholder($placeholder, $placeholder_elements, TRUE);
            }
            else {
                // If the current request/session doesn't have JavaScript, fall back to
                // no-JS BigPipe.
                if ($this->requestStack
                    ->getCurrentRequest()->cookies
                    ->has(static::NOJS_COOKIE)) {
                    $overridden_placeholders[$placeholder] = static::createBigPipeNoJsPlaceholder($placeholder, $placeholder_elements, FALSE);
                }
                else {
                    $overridden_placeholders[$placeholder] = static::createBigPipeJsPlaceholder($placeholder, $placeholder_elements);
                }
                $overridden_placeholders[$placeholder]['#cache']['contexts'][] = 'cookies:' . static::NOJS_COOKIE;
            }
        }
        return $overridden_placeholders;
    }
    
    /**
     * Determines whether the given placeholder is attribute-safe or not.
     *
     * @param string $placeholder
     *   A placeholder.
     *
     * @return bool
     *   Whether the placeholder is safe for use in a HTML attribute (in case it's
     *   a placeholder for a HTML attribute value or a subset of it).
     */
    protected static function placeholderIsAttributeSafe($placeholder) {
        assert(is_string($placeholder));
        return $placeholder[0] !== '<' || $placeholder !== Html::normalize($placeholder);
    }
    
    /**
     * Creates a BigPipe JS placeholder.
     *
     * @param string $original_placeholder
     *   The original placeholder.
     * @param array $placeholder_render_array
     *   The render array for a placeholder.
     *
     * @return array
     *   The resulting BigPipe JS placeholder render array.
     */
    protected static function createBigPipeJsPlaceholder($original_placeholder, array $placeholder_render_array) {
        $big_pipe_placeholder_id = static::generateBigPipePlaceholderId($original_placeholder, $placeholder_render_array);
        return [
            '#markup' => '<span data-big-pipe-placeholder-id="' . Html::escape($big_pipe_placeholder_id) . '"></span>',
            '#cache' => [
                'max-age' => 0,
                'contexts' => [
                    'session.exists',
                ],
            ],
            '#attached' => [
                'library' => [
                    'big_pipe/big_pipe',
                ],
                // Inform BigPipe' JavaScript known BigPipe placeholder IDs.
'drupalSettings' => [
                    'bigPipePlaceholderIds' => [
                        $big_pipe_placeholder_id => TRUE,
                    ],
                ],
                'big_pipe_placeholders' => [
                    Html::escape($big_pipe_placeholder_id) => $placeholder_render_array,
                ],
            ],
        ];
    }
    
    /**
     * Creates a BigPipe no-JS placeholder.
     *
     * @param string $original_placeholder
     *   The original placeholder.
     * @param array $placeholder_render_array
     *   The render array for a placeholder.
     * @param bool $placeholder_must_be_attribute_safe
     *   Whether the placeholder must be safe for use in a HTML attribute (in case
     *   it's a placeholder for a HTML attribute value or a subset of it).
     *
     * @return array
     *   The resulting BigPipe no-JS placeholder render array.
     */
    protected static function createBigPipeNoJsPlaceholder($original_placeholder, array $placeholder_render_array, $placeholder_must_be_attribute_safe = FALSE) {
        if (!$placeholder_must_be_attribute_safe) {
            $big_pipe_placeholder = '<span data-big-pipe-nojs-placeholder-id="' . Html::escape(static::generateBigPipePlaceholderId($original_placeholder, $placeholder_render_array)) . '"></span>';
        }
        else {
            $big_pipe_placeholder = 'big_pipe_nojs_placeholder_attribute_safe:' . Html::escape($original_placeholder);
        }
        return [
            '#markup' => $big_pipe_placeholder,
            '#cache' => [
                'max-age' => 0,
                'contexts' => [
                    'session.exists',
                ],
            ],
            '#attached' => [
                'big_pipe_nojs_placeholders' => [
                    $big_pipe_placeholder => $placeholder_render_array,
                ],
            ],
        ];
    }
    
    /**
     * Generates a BigPipe placeholder ID.
     *
     * @param string $original_placeholder
     *   The original placeholder.
     * @param array $placeholder_render_array
     *   The render array for a placeholder.
     *
     * @return string
     *   The generated BigPipe placeholder ID.
     */
    protected static function generateBigPipePlaceholderId($original_placeholder, array $placeholder_render_array) {
        // Generate a BigPipe placeholder ID (to be used by BigPipe's JavaScript).
        // @see \Drupal\Core\Render\PlaceholderGenerator::createPlaceholder()
        if (isset($placeholder_render_array['#lazy_builder'])) {
            $callback = $placeholder_render_array['#lazy_builder'][0];
            $arguments = $placeholder_render_array['#lazy_builder'][1];
            $token = Crypt::hashBase64(serialize($placeholder_render_array));
            return UrlHelper::buildQuery([
                'callback' => $callback,
                'args' => $arguments,
                'token' => $token,
            ]);
        }
        else {
            return Html::getId($original_placeholder);
        }
    }

}