HeadersCacheContext.php

<?php

namespace Drupal\Core\Cache\Context;

use Drupal\Core\Cache\CacheableMetadata;

/**
 * Defines the HeadersCacheContext service, for "per header" caching.
 *
 * Cache context ID: 'headers' (to vary by all headers).
 * Calculated cache context ID: 'headers:%name', e.g. 'headers:X-Something' (to
 * vary by the 'X-Something' header).
 */
class HeadersCacheContext extends RequestStackCacheContextBase implements CalculatedCacheContextInterface {
    
    /**
     * {@inheritdoc}
     */
    public static function getLabel() {
        return t('HTTP headers');
    }
    
    /**
     * {@inheritdoc}
     */
    public function getContext($header = NULL) {
        if ($header === NULL) {
            $headers = $this->requestStack
                ->getCurrentRequest()->headers
                ->all();
            // Order headers by name to have less cache variations.
            ksort($headers);
            $result = '';
            foreach ($headers as $name => $value) {
                if ($result) {
                    $result .= '&';
                }
                // Sort values to minimize cache variations.
                sort($value);
                $result .= $name . '=' . implode(',', $value);
            }
            return $result;
        }
        elseif ($this->requestStack
            ->getCurrentRequest()->headers
            ->has($header)) {
            $value = $this->requestStack
                ->getCurrentRequest()->headers
                ->get($header);
            if ($value !== '') {
                return $value;
            }
            return '?valueless?';
        }
        return '';
    }
    
    /**
     * {@inheritdoc}
     */
    public function getCacheableMetadata($header = NULL) {
        return new CacheableMetadata();
    }

}