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();

    // Prevent placeholders from being processed by BigPipe on uncacheable
    // request methods. For example, a form rendered inside a placeholder will
    // be rendered as soon as possible before any headers are sent, so that it
    // can be detected, submitted, and redirected immediately.
    // @todo https://www.drupal.org/node/2367555
    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 $placeholder
   *   A placeholder.
   *
   * @return bool
   *   Whether the placeholder is safe for use in an HTML attribute (in case
   *   it's a placeholder for an HTML attribute value or a subset of it).
   */
  protected static function placeholderIsAttributeSafe($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);
    $interface_preview = [];
    if (isset($placeholder_render_array['#lazy_builder'])) {
      $interface_preview = [
        '#theme' => 'big_pipe_interface_preview',
        '#callback' => $placeholder_render_array['#lazy_builder'][0],
        '#arguments' => $placeholder_render_array['#lazy_builder'][1],
      ];
      if (isset($placeholder_render_array['#preview'])) {
        $interface_preview['#preview'] = $placeholder_render_array['#preview'];
        unset($placeholder_render_array['#preview']);
      }
    }
    return [
      '#prefix' => '<span data-big-pipe-placeholder-id="' . Html::escape($big_pipe_placeholder_id) . '">',
      'interface_preview' => $interface_preview,
      '#suffix' => '</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 $placeholder_must_be_attribute_safe
   *   Whether the placeholder must be safe for use in an HTML attribute (in
   *   case it's a placeholder for an HTML attribute value or a subset of it).
   *
   * @return array
   *   The resulting BigPipe no-JS placeholder render array.
   */
  protected static function createBigPipeNoJsPlaceholder($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'])) {

      // Be sure cache contexts and tags are sorted before serializing them and
      // making hash. Issue #3225328 removes sort from contexts and tags arrays
      // for performances reasons.
      if (isset($placeholder_render_array['#cache']['contexts'])) {
        sort($placeholder_render_array['#cache']['contexts']);
      }
      if (isset($placeholder_render_array['#cache']['tags'])) {
        sort($placeholder_render_array['#cache']['tags']);
      }
      $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);
    }
  }

}