swift - 在 NSHTTPURLResponse 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，代码在 let httpResponse = response as! 行失败。以下代码中的 NSHTTPURLResponse。我在这一行遇到异常，Xcode 进入 Debug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:

1. 如果请求中有任何错误，response 将为nil，因此您尝试强制转换它会导致 fatal error 。处理网络响应时不要使用强制展开/转换。

2. 这里有一个更深层次的问题，您正试图从异步运行的方法返回数据。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，您可以通过该处理程序异步传回相关数据:

    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，调用会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. 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您的 info.plist 来强制应用程序允许非 https 请求。参见 https://stackoverflow.com/a/31254874/1271826了解更多信息

    <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>