code: public function run(\Traversable $items, callable $itemCallback)
      {
          $this->start();
          foreach ($items as $item) {
              call_user_func($itemCallback, $this, $item);
          }
          return $this->finish();
      }
docs: Iterate over items and track them
  
  @param \Traversable $items
  @param callable     $itemCallback  Callback accepts arguments: (Tracker $tracker, $item)
  @return Report      The final report