/**
* 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;
}
