Set Body Class if Facet is active

I need to set a body class if a facet filter is active.

It should be done in yourtheme_preprocess_html

if Facet is active
then
$variables['attributes']['class'][] = 'facet-active';

for fulltext search terms it works like that

  $variables['attributes']['class'][] = 'search-' . \Drupal::request()->query->get('search_api_fulltext');

Anyone know how to dig out the information if a facet is active or not.

thanks in advance

Matthias

You don't know when the HTML template is rendered whether a facet is active or not. So in mytheme_preprocess_html() assume it is not:

$variables['attributes']['class'][] = 'facet-is-not-active-class';

In the facets block replace the placeholder class if the block is visible:

function mytheme_preprocess_block(&$variables) {
  if ($variables['configuration']['provider'] == 'facets') {
    if (!in_array('hidden', (array) $variables['attributes']['class'])) {
      $variables['#attached']['placeholders']['facet-is-not-active-class'] = [
        '#markup' => 'facet-active',
      ];
    }
  }
}

Important is that the placeholder string facet-is-not-active-class is unique, use a longer name or even a hash. Placeholders are replaced for all occurrences of the string on the page.

Issue with nested placeholders

This works only for blocks not auto-placeholdered. The facet block, unfortunately, is always placeholdered. It sets cache max-age 0 which meets the default condition for auto-placeholdering. See https://www.drupal.org/docs/drupal-apis/render-api/auto-placeholdering#s-what

To prevent that you can set #create_placeholder to FALSE:

/**
 * Implements hook_block_build_BASE_BLOCK_ID_alter().
 */
function mymodule_block_build_facet_block_alter(array &$build, \Drupal\Core\Block\BlockPluginInterface $block) {
  $build['#create_placeholder'] = FALSE;
}

Caveat: This is a real performance killer, it doubles the page loading times in my tests. So if performance is important you probably should look for a client-side solution, better CSS or a javascript library which adds the necessary classes.