功能不等待加载数据

Question

我可能做错了什么，但我似乎无法让我的函数在继续下一部分之前等待数据加载。在这种情况下，我需要 requestProductInfo（）在下一步显示警报之前提供数据。从我所读到的，dispatch_async将是正确的方法，但它似乎没有工作。我做错了吗？

func purchaseRequest() {
    requestProductInfo()
    //NEED TO HOLD UNTIL PRODUCTS REQUEST IS COMPLETED
    dispatch_async(dispatch_get_main_queue()) {
        print("Product1: \(self.productsArray)")
        let aSC = UIAlertController(title: "Premium App Required", message: "Premium App is Required for this feature. Would you like to purchase it for $0.99?", preferredStyle: UIAlertControllerStyle.ActionSheet)
        let buyAction = UIAlertAction(title: "Purchase", style: UIAlertActionStyle.Default) { (action) -> Void in
            let payment = SKPayment(product: self.productsArray)
            SKPaymentQueue.defaultQueue().addPayment(payment)  
        }
        let cancelAction = UIAlertAction(title: "Cancel", style: UIAlertActionStyle.Cancel) { (action) -> Void in
        }
        aSC.addAction(buyAction)
        aSC.addAction(cancelAction)
        self.presentViewController(aSC, animated: true, completion:  nil)
    }
}

谢谢！