HtmlRenderer::prepare

protected HtmlRenderer::prepare(array $main_content, Request $request, RouteMatchInterface $route_match)

protected function prepare(array $main_content, Request $request, RouteMatchInterface $route_match) {
  // Determine the title: use the title provided by the main content if any,
  // otherwise get it from the routing information.
  $get_title = function(array $main_content) use ($request, $route_match) {
    return isset($main_content['#title']) ? $main_content['#title'] : $this->titleResolver->getTitle($request, $route_match->getRouteObject());
  };

  // If the _controller result already is #type => page,
  // we have no work to do: The "main content" already is an entire "page"
  // (see html.html.twig).
  if (isset($main_content['#type']) && $main_content['#type'] === 'page') {
    $page = $main_content;
    $title = $get_title($page);
  }
  // Otherwise, render it as the main content of a #type => page, by selecting
  // page display variant to do that and building that page display variant.
  else {
    // Select the page display variant to be used to render this main content,
    // default to the built-in "simple page".
    $event = new PageDisplayVariantSelectionEvent('simple_page', $route_match);
    $this->eventDispatcher->dispatch(RenderEvents::SELECT_PAGE_DISPLAY_VARIANT, $event);
    $variant_id = $event->getPluginId();

    // We must render the main content now already, because it might provide a
    // title. We set its $is_root_call parameter to FALSE, to ensure
    // placeholders are not yet replaced. This is essentially "pre-rendering"
    // the main content, the "full rendering" will happen in
    // ::renderResponse().
    // @todo Remove this once https://www.drupal.org/node/2359901 lands.
    if (!empty($main_content)) {
      $this->renderer->executeInRenderContext(new RenderContext(), function() use (&$main_content) {
        if (isset($main_content['#cache']['keys'])) {
          // Retain #title, otherwise, dynamically generated titles would be
          // missing for controllers whose entire returned render array is
          // render cached.
          $main_content['#cache_properties'][] = '#title';
        }
        return $this->renderer->render($main_content, FALSE);
      });
      $main_content = $this->renderCache->getCacheableRenderArray($main_content) + [
        '#title' => isset($main_content['#title']) ? $main_content['#title'] : NULL
      ];
    }

    $title = $get_title($main_content);

    // Instantiate the page display, and give it the main content.
    $page_display = $this->displayVariantManager->createInstance($variant_id);
    if (!$page_display instanceof PageVariantInterface) {
      throw new \LogicException('Cannot render the main content for this page because the provided display variant does not implement PageVariantInterface.');
    }
    $page_display
    ->setMainContent($main_content)
      ->setTitle($title)
      ->addCacheableDependency($event)
      ->setConfiguration($event->getPluginConfiguration());
    // Some display variants need to be passed an array of contexts with
    // values because they can't get all their contexts globally. For example,
    // in Page Manager, you can create a Page which has a specific static
    // context (e.g. a context that refers to the Node with nid 6), if any
    // such contexts were added to the $event, pass them to the $page_display.
    if ($page_display instanceof ContextAwareVariantInterface) {
      $page_display->setContexts($event->getContexts());
    }

    // Generate a #type => page render array using the page display variant,
    // the page display will build the content for the various page regions.
    $page = array(
      '#type' => 'page',
    );
    $page += $page_display->build();
  }

  // $page is now fully built. Find all non-empty page regions, and add a
  // theme wrapper function that allows them to be consistently themed.
  $regions = \Drupal::theme()->getActiveTheme()->getRegions();
  foreach ($regions as $region) {
    if (!empty($page[$region])) {
      $page[$region]['#theme_wrappers'][] = 'region';
      $page[$region]['#region'] = $region;
    }
  }

  // Allow hooks to add attachments to $page['#attached'].
  $this->invokePageAttachmentHooks($page);

  return [$page, $title];
}