CssCollectionRenderer.php

<?php

namespace Drupal\Core\Asset;

use Drupal\Core\State\StateInterface;

/**
 * Renders CSS assets.
 */
class CssCollectionRenderer implements AssetCollectionRendererInterface {
    
    /**
     * The state key/value store.
     *
     * @var \Drupal\Core\State\StateInterface
     */
    protected $state;
    
    /**
     * Constructs a CssCollectionRenderer.
     *
     * @param \Drupal\Core\State\StateInterface $state
     *   The state key/value store.
     */
    public function __construct(StateInterface $state) {
        $this->state = $state;
    }
    
    /**
     * {@inheritdoc}
     */
    public function render(array $css_assets) {
        $elements = [];
        // A dummy query-string is added to filenames, to gain control over
        // browser-caching. The string changes on every update or full cache
        // flush, forcing browsers to load a new copy of the files, as the
        // URL changed.
        $query_string = $this->state
            ->get('system.css_js_query_string', '0');
        // Defaults for LINK and STYLE elements.
        $link_element_defaults = [
            '#type' => 'html_tag',
            '#tag' => 'link',
            '#attributes' => [
                'rel' => 'stylesheet',
            ],
        ];
        foreach ($css_assets as $css_asset) {
            $element = $link_element_defaults;
            $element['#attributes']['media'] = $css_asset['media'];
            $element['#browsers'] = $css_asset['browsers'];
            switch ($css_asset['type']) {
                // For file items, output a LINK tag for file CSS assets.
                case 'file':
                    $element['#attributes']['href'] = file_url_transform_relative(file_create_url($css_asset['data']));
                    // Only add the cache-busting query string if this isn't an aggregate
                    // file.
                    if (!isset($css_asset['preprocessed'])) {
                        $query_string_separator = strpos($css_asset['data'], '?') !== FALSE ? '&' : '?';
                        $element['#attributes']['href'] .= $query_string_separator . $query_string;
                    }
                    break;
                case 'external':
                    $element['#attributes']['href'] = $css_asset['data'];
                    break;
                default:
                    throw new \Exception('Invalid CSS asset type.');
            }
            // Merge any additional attributes.
            if (!empty($css_asset['attributes'])) {
                $element['#attributes'] += $css_asset['attributes'];
            }
            $elements[] = $element;
        }
        return $elements;
    }

}