BigPipe::getPlaceholderOrder

protected BigPipe::getPlaceholderOrder($html, $placeholders)

protected function getPlaceholderOrder($html, $placeholders) {
  $fragments = explode('<div data-big-pipe-placeholder-id="', $html);
  array_shift($fragments);
  $placeholder_ids = [];

  foreach ($fragments as $fragment) {
    $t = explode('"></div>', $fragment, 2);
    $placeholder_id = $t[0];
    $placeholder_ids[] = $placeholder_id;
  }
  $placeholder_ids = array_unique($placeholder_ids);

  // The 'status messages' placeholder needs to be special cased, because it
  // depends on global state that can be modified when other placeholders are
  // being rendered: any code can add messages to render.
  // This violates the principle that each lazy builder must be able to render
  // itself in isolation, and therefore in any order. However, we cannot
  // change the way drupal_set_message() works in the Drupal 8 cycle. So we
  // have to accommodate its special needs.
  // Allowing placeholders to be rendered in a particular order (in this case:
  // last) would violate this isolation principle. Thus a monopoly is granted
  // to this one special case, with this hard-coded solution.
  // @see \Drupal\Core\Render\Element\StatusMessages
  // @see \Drupal\Core\Render\Renderer::replacePlaceholders()
  // @see https://www.drupal.org/node/2712935#comment-11368923
  $message_placeholder_ids = [];
  foreach ($placeholders as $placeholder_id => $placeholder_element) {
    if (isset($placeholder_element['#lazy_builder']) && $placeholder_element['#lazy_builder'][0] === 'Drupal\Core\Render\Element\StatusMessages::renderMessages') {
      $message_placeholder_ids[] = $placeholder_id;
    }
  }

  // Return placeholder IDs in DOM order, but with the 'status messages'
  // placeholders at the end, if they are present.
  $ordered_placeholder_ids = array_merge(
  array_diff($placeholder_ids, $message_placeholder_ids), 
  array_intersect($placeholder_ids, $message_placeholder_ids)
  );
  return $ordered_placeholder_ids;
}