I have this block:

public function build() {   $build = [     '#attached' => [       'library' => [         'mymodule/mylibrary',       ],     ],     '#markup' => '<!-- see template -->',   ];    return $build; } 

Somewhere I am rendering this block via custom text filter:

{{ '[embed:block:mymodulemyblock]' | text_format }} 

This is my text filter that replaces the “token” with the rendered block:

public function process($text, $langcode) {   if (preg_match_all('/[embed:.+]/isU', $text, $matchCodes)) {     foreach ($matchCodes[0] as $index => $token) {       $parts = explode(':', $cleaned);       array_splice($parts, 0, 1);        $entityType = trim($parts[0]);       $entityId = trim($parts[1]);       $viewMode = isset($parts[2]) ? trim($parts[2]) : 'full';        if ('block' == $entityType) {         $rendered = '';          if ($block = entity_load('block', $entityId)) {           $replacement = entity_view($block, 'block');           $rendered = drupal_render($replacement);         }       }       $text = str_replace($token, $rendered, $text);      }   }   return new FilterProcessResult($text); } 

It works fine, except the block’s library isnt attached. Not even when I add this inside the block template:

{{ attach_library('mymodule/mylibrary') }} 

When I place the block in the block layout it works fine.

Why isnt the library attached?

Edit

This is how I register the template for the block itself:

function mmodule_theme($existing, $type, $theme, $path) {   return array(     'block__mymodulemyblock' => array(       'render element' => 'elements',       'template' => 'block--foobar',       'base hook' => 'block',     ),   ); }