Renderer::replacePlaceholders

protected Renderer::replacePlaceholders(array &$elements)

protected function replacePlaceholders(array &$elements) {
  if (!isset($elements['#attached']['placeholders']) || empty($elements['#attached']['placeholders'])) {
    return FALSE;
  }

  // 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 https://www.drupal.org/node/2712935#comment-11368923

  // First render all placeholders except 'status messages' placeholders.
  $message_placeholders = [];
  foreach ($elements['#attached']['placeholders'] as $placeholder => $placeholder_element) {
    if (isset($placeholder_element['#lazy_builder']) && $placeholder_element['#lazy_builder'][0] === 'Drupal\Core\Render\Element\StatusMessages::renderMessages') {
      $message_placeholders[] = $placeholder;
    }
    else {
      $elements = $this->renderPlaceholder($placeholder, $elements);
    }
  }

  // Then render 'status messages' placeholders.
  foreach ($message_placeholders as $message_placeholder) {
    $elements = $this->renderPlaceholder($message_placeholder, $elements);
  }

  return TRUE;
}