ThemeCacheContext.php

<?php

namespace Drupal\Core\Cache\Context;

use Drupal\Core\Cache\CacheableMetadata;
use Drupal\Core\Theme\ThemeManagerInterface;

/**
 * Defines the ThemeCacheContext service, for "per theme" caching.
 *
 * Cache context ID: 'theme'.
 */
class ThemeCacheContext implements CacheContextInterface {
    
    /**
     * The theme manager.
     *
     * @var \Drupal\Core\Theme\ThemeManagerInterface
     */
    protected $themeManager;
    
    /**
     * Constructs a new ThemeCacheContext service.
     *
     * @param \Drupal\Core\Theme\ThemeManagerInterface $theme_manager
     *   The theme manager.
     */
    public function __construct(ThemeManagerInterface $theme_manager) {
        $this->themeManager = $theme_manager;
    }
    
    /**
     * {@inheritdoc}
     */
    public static function getLabel() {
        return t('Theme');
    }
    
    /**
     * {@inheritdoc}
     */
    public function getContext() {
        return $this->themeManager
            ->getActiveTheme()
            ->getName() ?: 'stark';
    }
    
    /**
     * {@inheritdoc}
     */
    public function getCacheableMetadata() {
        return new CacheableMetadata();
    }

}