在Swift中访问JSON对象出错

Question

我有这个Swift代码:

func getData(completion: (array: Array<AnyObject>)->Void) {
    UIApplication.sharedApplication().networkActivityIndicatorVisible = true
    let session: NSURLSession = NSURLSession(configuration: sessionConfiguration)
    var request: NSURLRequest = NSURLRequest(URL: NSURL(string: "http://coinabul.com/api.php"))
    let task: NSURLSessionDataTask = session.dataTaskWithRequest(request, completionHandler: {(data, response, error) in
        if !error {
            let httpResp: NSHTTPURLResponse = response as NSHTTPURLResponse
            if httpResp.statusCode == 200 {
                var json: NSDictionary = self.convertDataToJSON(data)
                var myArray: Array = Array<Currency>()
                for (key: AnyObject, object: AnyObject) in json {
                    let info: NSDictionary = object as NSDictionary
                    let grams: CGFloat = info["Grams"] as CGFloat
                    let ounces: CGFloat = info["Ounces"] as CGFloat
                    let usd: CGFloat = info["USD"] as CGFloat
                    // Do something with those values
                }
            }
        }
        })
    task.resume()
}
func convertDataToJSON(data: NSData) -> NSDictionary {
    var error: NSError?
    var jsonDictionary: NSDictionary = NSJSONSerialization.JSONObjectWithData(data, options: .MutableContainers, error: &error) as NSDictionary
    return jsonDictionary
}

json NSDictionary是正确的。如果我打印出来:

{
    BTC =     {
        Grams = "15.398340412292";
        Ounces = "0.49506814017307";
        SilverGrams = "1010.0959168583";
        SilverOunces = "32.475337832916";
        USD = "624.825500000000";
    };
    Gold =     {
        Grams = "0.06494206341493";
        Ounces = "2.0199239627704";
        USD = "1262.100000000000";
    };
    Silver =     {
        Grams = "0.0009900049917623401";
        Ounces = "0.030792597293164";
        USD = "19.240000000000";
    };
}

我可以做json["Silver"]，得到相应的对象。但是如果我尝试json["Silver"]["USD"]，我的应用程序崩溃了。

我在这里错过了什么?

Answer 1

表达式:

json["Silver"]

返回AnyObject?，它不允许[]，因为它是未知的，所以你必须对它进行类型转换以供进一步参考。注意，如果你尝试在工作空间

中执行此操作，Xcode将会崩溃。

json["Silver"] as? Dictionary

可以使用可选的链接将它们串在一起，如:

(json["Silver"] as? NSDictionary)?["USD"]

请注意，索引操作的结果几乎总是可选的，而可选链接意味着这个操作肯定是可选的，因此您必须处理检查和展开。