Ios 我怎样才能解决这个问题；致命错误：在展开可选值“0”时意外发现零；？

我一直在做我的第一个iOS Swift项目，我遇到了一个致命的错误。此错误报告仅当从调用的API获取错误消息时，在展开可选值时意外发现nil

这是Swift代码

@IBAction func pressScan(sender: AnyObject) {

        let barcode = lastCapturedCode

    print("Received following barcode: \(barcode)")

    let url = "https://api.nutritionix.com/v1_1/item?upc="

    let urlWithUPC = url + barcode! + "&appId=[app ID goes here]&appKey=[app key goes here]"

    print("API Query: "+urlWithUPC)

    NSURLSession.sharedSession().dataTaskWithURL(NSURL(string: urlWithUPC)!) { data, response, error in
        // Handle result
        print("Checked the bar code")


        //  let JSONData = NSData()
        do {
           let JSON = try NSJSONSerialization.JSONObjectWithData(data!, options:NSJSONReadingOptions(rawValue: 0))


            guard let JSONDictionary :NSDictionary = JSON as? NSDictionary else {
                print("Not a Dictionary")
                // put in function
                return
            }
            print("JSONDictionary \(JSONDictionary)")


            let foodScreenName = (JSONDictionary as NSDictionary)["item_name"] as! String

            let foodBrandName = (JSONDictionary as NSDictionary)["brand_name"] as! String


            let fullFoodName = foodBrandName + " " + foodScreenName

            dispatch_async(dispatch_get_main_queue(),{

                self.foodName.text = fullFoodName
            });



        }
        catch let JSONError as NSError {
            print("\(JSONError)")
        }



        }.resume


}

以下是返回的JSON结果，其中包含一个错误

JSONDictionary {
"error_code" = "item_not_found";
"error_message" = "Item ID or UPC was invalid";
"status_code" = 404;
}

如果API找到该项，则显示JSON结果

    JSONDictionary {
   "item_id": "51c3d78797c3e6d8d3b546cf",

   "item_name": "Cola, Cherry",

   "brand_id": "51db3801176fe9790a89ae0b",

   "brand_name": "Coke",
   "item_description": "Cherry",
   "updated_at": "2013-07-09T00:00:46.000Z",
   "nf_ingredient_statement": "Carbonated Water, High Fructose Corn Syrup and/or Sucrose, Caramel Color, Phosphoric Acid, Natural Flavors, Caffeine.",
   "nf_calories": 100,
   "nf_calories_from_fat": 0,
   "nf_total_fat": 0,
   "nf_saturated_fat": null,
   "nf_cholesterol": null,
   "nf_sodium": 25,
   "nf_total_carbohydrate": 28,
   "nf_dietary_fiber": null,
   "nf_sugars": 28,
   "nf_protein": 0,
   "nf_vitamin_a_dv": 0,
   "nf_vitamin_c_dv": 0,
   "nf_calcium_dv": 0,
   "nf_iron_dv": 0,
   "nf_servings_per_container": 6,
   "nf_serving_size_qty": 8,
   "nf_serving_size_unit": "fl oz",
}

---

由于您的字典结构发生了变化，并且您不确定是否有几个键存在，因此您必须在使用它们之前进行安全检查，如下所示：

if let screenName = (JSONDictionary as NSDictionary)["item_name"] {
    foodScreenName = screenName as! String
}

if let brandName = (JSONDictionary as NSDictionary)["brand_name"] {
    foodBrandName = brandName as! String
}

---

在你的代码的哪一行“意外发现零”？修复它？永远不要使用

再次出现（除了
！=
）。将所有这些情况包装在if-lets或类似的构造中。错误响应也包含JSON，因此在尝试将JSON解析为已找到的项之前，应该检查响应statusCode==200。也是一个好主意吗？而不是在解析字段时，如果服务器没有发送它们，则会得到此断言，因为在出现错误的情况下，
（JSONDictionary as NSDictionary）[“item_name”]as没有键！字符串
或
（JSONDictionary作为NSDictionary）[“brand_name”]作为！字符串
。您首先需要检查它们是否存在。可能存在重复的