Question

我正在尝试从URL获取JSON文件并使用Swift返回内容。但是，代码在以下代码中的let httpResponse = response as! NSHTTPURLResponse行失败。我在这一行得到一个异常，Xcode进入调试模式。

class func downloadJSONFile()->AnyObject
    {
        let requestURL: NSURL = NSURL(string: "http://www.learnswiftonline.com/Samples/subway.json")!
        let urlRequest: NSMutableURLRequest = NSMutableURLRequest(URL: requestURL)
        let session = NSURLSession.sharedSession()
        var json:AnyObject = ""
        let task = session.dataTaskWithRequest(urlRequest) {
            (data, response, error) -> Void in

            let httpResponse = response as! NSHTTPURLResponse
            let statusCode = httpResponse.statusCode

            if (statusCode == 200) {

                do{
                    json = try NSJSONSerialization.JSONObjectWithData(data!, options:.AllowFragments)

                }catch {
                    print("Error with Json: \(error)")

                }

            }

        }

        task.resume()

        return json
    }

我该如何解决这个问题？

Answer 1

有一些问题：

如果请求中有任何错误，response将为nil，因此您强制转换它的尝试将导致致命错误。处理网络响应时，请勿使用强制解包/强制转换。

此处存在一个更深层次的问题，即您尝试从异步运行的方法返回数据。你应该改变你的方法本身不返回任何东西，而是提供一个完成处理程序，通过它可以异步传回相关数据：

class func downloadJSONFile(completionHandler:(AnyObject?) -> ()) {
    let requestURL: NSURL = NSURL(string: "http://www.learnswiftonline.com/Samples/subway.json")!
    let urlRequest: NSMutableURLRequest = NSMutableURLRequest(URL: requestURL)
    let session = NSURLSession.sharedSession()
    let task = session.dataTaskWithRequest(urlRequest) { data, response, error in

        // check for fundamental networking errors

        guard error == nil && data != nil else {
            print(error)
            completionHandler(nil)
            return
        }

        // check to see if status code found, and if so, that it's 200

        guard let httpResponse = response as? NSHTTPURLResponse where httpResponse.statusCode == 200 else {
            completionHandler(nil)
            return
        }

        do {
            let json = try NSJSONSerialization.JSONObjectWithData(data!, options:[])
            completionHandler(json)
        } catch let parseError as NSError {
            print("Error with Json: \(parseError)")
            completionHandler(nil)
        }
    }

    task.resume()
}

然后使用完成处理程序（或使用尾随闭包语法，如下所示）调用它：

APIClass.downloadJSONFile() { json in
    guard json != nil else {
        print("there was some problem")
        return
    }

    // now you can use `json` here

    dispatch_async(dispatch_get_main_queue()) {
        // and if you're doing any model or UI updates, dispatch that back to the main queue
    }
}

// but do not use `json` here, as the above runs asynchronously

注意，如果您想将错误信息提供回调用例程，您可以将其更改为还返回错误信息，例如：

enum DownloadError : ErrorType {
    case NetworkError(NSError?)
    case NotHTTPResponse
    case InvalidHTTPResponse(Int)
    case JSONError(NSError)
}

class func downloadJSONFile(completionHandler:(AnyObject?, ErrorType?) -> ()) {
    let requestURL: NSURL = NSURL(string: "http://www.learnswiftonline.com/Samples/subway.json")!
    let urlRequest: NSMutableURLRequest = NSMutableURLRequest(URL: requestURL)
    let session = NSURLSession.sharedSession()
    let task = session.dataTaskWithRequest(urlRequest) { data, response, error in

        guard error == nil && data != nil else {
            completionHandler(nil, DownloadError.NetworkError(error))
            return
        }

        guard let httpResponse = response as? NSHTTPURLResponse else {
            completionHandler(nil, DownloadError.NotHTTPResponse)
            return
        }

        guard httpResponse.statusCode == 200 else {
            completionHandler(nil, DownloadError.InvalidHTTPResponse(httpResponse.statusCode))
            return
        }

        do {
            let json = try NSJSONSerialization.JSONObjectWithData(data!, options:[])
            completionHandler(json, nil)
        } catch let parseError as NSError {
            completionHandler(nil, DownloadError.JSONError(parseError))
        }
    }

    task.resume()
}

而且，显然，调用将改为采用两个参数：

APIClass.downloadJSONFile() { json, error in
    guard json != nil && error == nil else {
        print("there was some problem \(error)")
        return
    }

    // and then it would be like before ...
}

在iOS 9及更高版本中使用NSURLSession时，它不会允许明文请求（即＆＃34; http＆＃34;不允许，只有＆＃34; https＆＃34;是，默认情况下）。您可以通过将以下内容添加到info.plist来强制该应用允许非https请求。有关详细信息，请参阅https://stackoverflow.com/a/31254874/1271826

<key>NSAppTransportSecurity</key>
<dict>
    <key>NSExceptionDomains</key>
    <dict>
        <key>learnswiftonline.com</key>
        <dict>
            <!--Include to allow subdomains-->
            <key>NSIncludesSubdomains</key>
            <true/>
            <!--Include to allow HTTP requests-->
            <key>NSTemporaryExceptionAllowsInsecureHTTPLoads</key>
            <true/>
            <!--Include to specify minimum TLS version-->
            <key>NSTemporaryExceptionMinimumTLSVersion</key>
            <string>TLSv1.1</string>
        </dict>
    </dict>
</dict>

在NSHTTPURLResponse下载JSON数据失败

1 个答案: