如果我使用以下代码，则会向我显示错误“类型’HttpConnection’不符合协议’NSObjectProtocol’”

class HttpConnection : NSURLConnectionDataDelegate {
        var urlConnection       : NSURLConnection?
        weak var delegate       : HttpConnecting?

        init(delegate:HttpConnecting){
            self.delegate = delegate;
        }

        func startAsynRequestWithUrlString(url:NSString, withMethod:NSString){
        }
    }

如果我将HttpConnection子类化为NSObject，则可以正常工作。所以我的问题是，当我需要迅速采用NSURLConnectionDataDelegate协议时，为什么需要从NSObject继承该类？