下载 JSON 数据在 NSHTTPURLResponse 失败

Question

我正在尝试使用 Swift 从 URL 和 return 的内容中获取 JSON 文件。但是，代码在以下代码中的 let httpResponse = response as! NSHTTPURLResponse 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
    }

我该如何解决这个问题？

Answer 1

有几个问题：

如果请求中有任何错误，response 将是 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unwrapping/casting。

这里有一个更深层次的问题，您正试图从一个异步运行的方法中 return 数据。您应该将您的方法更改为不 return 任何东西，本身，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

注意，如果您想将错误信息返回给调用例程，您可以将其更改为也 return 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 ...
    }
}

在 iOS 9 及更高版本中使用 URLSession 时，它将不允许明文请求（即不允许“http”，默认情况下仅允许“https” ).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

 <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>

下载 JSON 数据在 NSHTTPURLResponse 失败

Downloading JSON data fails at NSHTTPURLResponse

nsjsonserialization

nsurlsession

swift