CssAggregator.php

  public function aggregate(array $css_assets) {
    // Group the assets.
    $css_groups = $this->grouper->group($css_assets);

    // Now optimize (concatenate + minify) and dump each asset group, unless
    // that was already done, in which case it should appear in
    // drupal_css_cache_files.
    // Drupal contrib can override this default CSS aggregator to keep the same
    // grouping, optimizing and dumping, but change the strategy that is used to
    // determine when the aggregate should be rebuilt (e.g. mtime, HTTPS …).
    $map = state()->get('drupal_css_cache_files') ?: array();
    $css_assets = array();
    foreach ($css_groups as $order => $css_group) {
      // We have to return a single asset, not a group of assets. It is now up
      // to one of the pieces of code in the switch statement below to set the
      // 'data' property to the appropriate value.
      $css_assets[$order] = $css_group;
      unset($css_assets[$order]['items']);

      switch ($css_group['type']) {
        case 'file':
          // No preprocessing, single CSS asset: just use the existing URI.
          if (!$css_group['preprocess']) {
            $uri = $css_group['items'][0]['data'];
            $css_assets[$order]['data'] = $uri;
          }
          // Preprocess (aggregate), unless the aggregate file already exists.
          else {
            $key = $this->generateHash($css_group);
            $uri = '';
            if (isset($map[$key])) {
              $uri = $map[$key];
            }
            if (empty($uri) || !file_exists($uri)) {
              $optimized_css_contents = $this->optimizer->optimize($css_group);
              $uri = $this->dumper->dump($optimized_css_contents, $css_group);
              $css_assets[$order]['data'] = $uri;

              // Persist the URI for this aggregate file.
              $map[$key] = $uri;
              state()->set('drupal_css_cache_files', $map);
            }
            else {
              // Use the persisted URI for the optimized CSS file.
              $css_assets[$order]['data'] = $uri;
            }
          }
          break;

        case 'inline':
          // We don't do any caching for inline CSS assets.
          $optimized_css_contents = $this->optimizer->optimize($css_group);
          unset($css_assets[$order]['data']['items']);
          $css_assets[$order]['data'] = $optimized_css_contents;
          break;

        case 'external':
          // We don't do any aggregation and hence also no caching for external
          // CSS assets.
          $css_assets[$order] = $css_group;
          break;
      }
    }

    return $css_assets;
  }

  /**
   * Generate a hash for a given group of CSS assets.
   */
  protected function generateHash($css_group) {
    $css_data = array();
    foreach ($css_group['items'] as $css_file) {
      $css_data[] = $css_file['data'];
    }
    return  hash('sha256', serialize($css_data));
  }