sendPreBody example

$cumulative_assets->setAlreadyLoadedLibraries($attachments['library']);

    $this->performPreSendTasks();

    // Find the closing </body> tag and get the strings before and after. But be     // careful to use the latest occurrence of the string "</body>", to ensure     // that strings in inline JavaScript or CDATA sections aren't used instead.     $parts = explode('</body>', $content);
    $post_body = array_pop($parts);
    $pre_body = implode('', $parts);

    $this->sendPreBody($pre_body, $nojs_placeholders, $cumulative_assets);
    $this->sendPlaceholders($placeholders, $this->getPlaceholderOrder($pre_body, $placeholders), $cumulative_assets);
    $this->sendPostBody($post_body);

    $this->performPostSendTasks();
  }

  /** * Sends everything until just before </body>. * * @param string $pre_body * The HTML response's content until the closing </body> tag. * @param array $no_js_placeholders * The no-JS BigPipe placeholders. * @param \Drupal\Core\Asset\AttachedAssetsInterface $cumulative_assets * The cumulative assets sent so far; to be updated while rendering no-JS * BigPipe placeholders. */