big_pipe_theme_suggestions_big_pipe_interface_preview example

    $block = Block::load('test_block1');
    // Using the BlockViewBuilder we will be able to get a lovely     // #lazy_builder callback assigned.     $build = $this->blockViewBuilder->view($block);

    $variables = [];
    // In turn this is what createBigPipeJsPlaceholder() uses to     // build the BigPipe JS placeholder render array which is used as input     // for big_pipe_theme_suggestions_big_pipe_interface_preview().     $variables['callback'] = $build['#lazy_builder'][0];
    $variables['arguments'] = $build['#lazy_builder'][1];
    $suggestions = big_pipe_theme_suggestions_big_pipe_interface_preview($variables);
    $suggested_id = preg_replace('/[^a-zA-Z0-9]/', '_', $block->id());
    $this->assertSame([
      'big_pipe_interface_preview__block',
      'big_pipe_interface_preview__block__' . $suggested_id,
      'big_pipe_interface_preview__block__full',
    ], $suggestions);
  }

}