code: void release() {
      /* release current download request */
      cancelAll();
  
      /* release download queue */
      if (downloadQueue != null) {
        downloadQueue = null;
      }
  
      /* release dispathcers */
      if (dispatchers != null) {
        stop();
  
        for (int i = 0; i < dispatchers.length; i++) {
          dispatchers[i] = null;
        }
  
        dispatchers = null;
      }
    }
docs: Release all the resource.