How do I get the attachments out a twig file and add them to a build array?

If there is a twig file with

{{ attach_library('mytheme/pullquote') }} 

And because it’s in an ajax response, I need to get the attachments out without rendering the page, how do I get a new build array with that library attached?

In my controller:

   $build['pullquote'] = [       '#theme' => 'pullquote',       '#quote_text' => $quote_text,       '#attribution_name' => $attribution_name,       '#attribution_title' => $attribution_title,       '#size' => $size,     ];      $response = new AjaxResponse();     $response->addCommand(new EmbedInsertCommand($build));     return $response; 

Because the library is added in twig and not in the build array, it’s not coming through in the response.

The EmbedInsertCommand is from the embed module, which entity_embed inherits from, at embed/src/Ajax/EmbedInsertCommand.php

which uses the CommandWithAttachedAssetsTrait, with this function:

 /**    * Processes the content for output.    *    * If content is a render array, it may contain attached assets to be    * processed.    *    * @return string|DrupalComponentRenderMarkupInterface    *   HTML rendered content.    */   protected function getRenderedContent() {     $this->attachedAssets = new AttachedAssets();     if (is_array($this->content)) {       $html = Drupal::service('renderer')->renderRoot($this->content);       $this->attachedAssets = AttachedAssets::createFromRenderArray($this->content);       return $html;     }     else {       return $this->content;     }   } 

Unfortunately, AttachedAssets::createFromRenderArray misses libraries added within twig.