/**
  * @param ICalendar &$calendar
  * @param callable $doesExist
  * @param boolean $isPublicUri
  */
 public static function generateURI(ICalendar &$calendar, \closure $doesExist, $isPublicUri = true)
 {
     if ($isPublicUri === true) {
         $uri = $calendar->getPublicUri();
     } else {
         $uri = $calendar->getPrivateUri();
     }
     if ($uri === null) {
         $uri = mb_strtolower($calendar->getDisplayname());
     }
     $uri = CalendarUtility::slugify($uri);
     while ($doesExist($uri)) {
         $newUri = self::suggestUri($uri);
         if ($newUri === $uri) {
             break;
         }
         $uri = $newUri;
     }
     if ($isPublicUri) {
         $calendar->setPublicUri($uri);
     } else {
         $calendar->setPrivateUri($uri);
     }
 }