public function addItem(Item $item)
 {
     if (null === $this->items) {
         $this->items = array();
     }
     $sku = $item->getSku();
     if (array_key_exists($sku, $this->items)) {
         $quantity = $this->items[$sku]->getQuantity() + $item->getQuantity();
         $this->items[$sku]->setQuantity($quantity);
     } else {
         $this->items[$sku] = $item;
     }
 }
 /**
  * @link http://code.google.com/apis/analytics/docs/gaJS/gaJSApiEcommerce.html#_gat.GA_Tracker_._addItem
  * @param \UnitedPrototype\GoogleAnalytics\Item $item
  */
 public function addItem(Item $item)
 {
     // Associated items inherit the transaction's order ID
     $item->setOrderId($this->orderId);
     $sku = $item->getSku();
     $this->items[$sku] = $item;
 }