在 NSHTTPURLResponse 处下载 JSON 数据失败

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let httpResponse = response as! NSHTTPURLResponse 在以下代码中的行失败。我在这一行遇到异常，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
    }

我怎样才能解决这个问题？

Answer 1

有几个问题：

1. 如果请求中有任何错误，responsewill nil，因此您尝试强制转换它会导致致命错误。处理网络响应时不要使用强制解包/强制转换。

2. 这里有一个更深层次的问题，即您试图从异步运行的方法中返回数据。您应该更改您的方法，使其本身不返回任何内容，而是提供一个完成处理程序，您可以通过它异步传回相关数据：

    class func downloadJSONFile(completionHandler: @escaping (Any?) -> Void) {
        let requestURL = URL(string: "http://www.learnswiftonline.com/Samples/subway.json")!
        let urlRequest = URLRequest(url: requestURL)
        let session = URLSession.shared
        let task = session.dataTask(with: urlRequest) { data, response, error in
            // check for fundamental networking errors
   
            guard
                error == nil,
                let data = data
            else {
                print(error ?? "Other error")
                completionHandler(nil)
                return
            }
   
            guard
                let httpResponse = response as? HTTPURLResponse,
                (200 ..< 300) ~= httpResponse.statusCode
            else {
                print("Invalid status code")
                completionHandler(nil)
                return
            }
   
            do {
                let json = try JSONSerialization.jsonObject(with: data)
                completionHandler(json)
            } catch let parseError {
                print("Error parsing: \(parseError)")
                completionHandler(nil)
            }
        }
   
        task.resume()
    }
   

   然后使用完成处理程序调用它（或使用尾随闭包语法，如下所示）：

    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
   

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

   enum DownloadError: Error {
       case networkError(Error)
       case notHTTPResponse
       case invalidHTTPResponse(Int)
       case parseError(Error)
   }
   
   class func downloadJSONFile(completionHandler: @escaping (Result<Any, Error>) -> Void) {
       let requestURL = URL(string: "http://www.learnswiftonline.com/Samples/subway.json")!
       let urlRequest = URLRequest(url: requestURL)
       let session = URLSession.shared
       let task = session.dataTask(with: urlRequest) { data, response, error in
           if let error = error {
               completionHandler(.failure(DownloadError.networkError(error)))
               return
           }
   
           guard let httpResponse = response as? HTTPURLResponse, let data = data else {
               completionHandler(.failure(DownloadError.notHTTPResponse))
               return
           }
   
           guard 200 ..< 300 ~= httpResponse.statusCode else {
               completionHandler(.failure(DownloadError.invalidHTTPResponse(httpResponse.statusCode)))
               return
           }
   
           do {
               let json = try JSONSerialization.jsonObject(with: data)
               completionHandler(.success(json))
           } catch let parseError {
               completionHandler(.failure(DownloadError.parseError(parseError)))
           }
       }
   
       task.resume()
   }
   

   而且，显然，调用将更改为采用两个参数：

   APIClass.downloadJSONFile() { result in
       switch result {
       case .failure(let error):
           print(error)
   
       case .success(let value):
           // and then it would be like before ...
       }
   }
   

4. 当使用URLSession在IOS 9和后，它不会允许明文请求（即“HTTP”是不允许的，只有“HTTPS”是，默认情况下）。您可以通过将以下内容添加到您的info.plist. 有关更多信息，请参阅/sf/answers/2187841211/

    <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>