xcode 为什么我会收到错误“Domain=NSCocoaErrorDomain Code=3840”“无值”。UserInfo={NSDebugDescription=没有值。}'?

Why do I get the error 'Domain=NSCocoaErrorDomain Code=3840 "No value." UserInfo={NSDebugDescription=No value.}'?

提问by MHDev

I am new to Swift and iOS development but I am trying to download and parse data that is stored in a MySQL Database.

我是 Swift 和 iOS 开发的新手，但我正在尝试下载和解析存储在 MySQL 数据库中的数据。

I keep getting the error:

我不断收到错误：

Domain=NSCocoaErrorDomain Code=3840 "No value." UserInfo={NSDebugDescription=No value.}

域=NSCocoaErrorDomain 代码=3840 “无值。” UserInfo={NSDebugDescription=无值。}

I have posted my code below but I don't think the problem is in the parseJSON function but instead in the actual download of the data as when I print the 'data' out it returns '<>'.

我已经在下面发布了我的代码，但我不认为问题出在 parseJSON 函数中，而是在数据的实际下载中，因为当我打印“数据”时它返回“<>”。

Here is my code:

这是我的代码：

//properties

weak var delegate: HomeModelProtocal!

var data : NSMutableData = NSMutableData()

let urlPath: String = "http://localhost/service.php" //this will be changed to the path where service.php lives

// Function to download the incoming JSON data
func downloadItems(){
    let url: URL = URL(string: urlPath)!
    var session: URLSession!
    let configuration = URLSessionConfiguration.default


    session = URLSession(configuration: configuration, delegate: self, delegateQueue: nil)

    let task = session.dataTask(with: url)

    task.resume()
}

func urlSession(_ session: URLSession, task: URLSessionDataTask, didCompleteWithError error: Error?) {
    self.data.append(data as Data)
}

func urlSession(_ session: URLSession, task: URLSessionTask, didCompleteWithError error: Error?) {
    if error != nil{
        print("Failed to download data")
    }else{
        print("Data downloaded")
        print(data)
        self.parseJSON()
    }
}

func parseJSON(){

    var jsonResult: NSMutableArray = NSMutableArray()

    do{
        jsonResult = try JSONSerialization.jsonObject(with: self.data as Data, options: []) as! NSMutableArray
    } catch let error as NSError {
        print("**** sake its happened again \(error)")
    }

    var jsonElement: NSDictionary = NSDictionary()
    let locations: NSMutableArray = NSMutableArray()

    for i in 0 ..< jsonResult.count{
        jsonElement = jsonResult[i] as! NSDictionary

        let location = LocationModel()

        //the following insures none of the JsonElement values are nil through optional binding
        if let exerciseName = jsonElement["stationName"] as? String,
            let bodyPart = jsonElement["buildYear"] as? String
        {
            print(exerciseName, bodyPart)
            location.exerciseName = exerciseName
            location.bodyPart = bodyPart

        }

        locations.add(location)

    }

    DispatchQueue.main.async(execute: { () -> Void in

        self.delegate.itemsDownloaded(items:locations)

    })
}

采纳答案by OOPer

Especially bad thing in your code:

在您的代码中尤其糟糕的是：

//This method is not being called...
func urlSession(_ session: URLSession, task: URLSessionDataTask, didCompleteWithError error: Error?) {
    self.data.append(data as Data) //<-This line adding self.data to self.data
}

There is no urlSession(_:task:didCompleteWithError:)method which takes URLSessionDataTaskas its second parameter. So, this method would never be called.

没有urlSession(_:task:didCompleteWithError:)方法将URLSessionDataTask其作为第二个参数。所以，这个方法永远不会被调用。

And inside the method, self.datais appended to self.data, so even if the method is called, self.datais still being empty...

并且在方法内部，self.data被附加到self.data，所以即使调用了方法，self.data仍然是空的......

You need to implement this method instead:

您需要改为实现此方法：

func urlSession(_ session: URLSession, dataTask: URLSessionDataTask, didReceive data: Data) {
    self.data.append(data)
}

But if you want to do nothing other than accumulating received data, you have no need to use delegates.

但是如果你只想累积接收到的数据，你就没有必要使用委托。

And you are using forced casting in your parseJSON()method:

你在你的parseJSON()方法中使用强制转换：

    jsonResult = try JSONSerialization.jsonObject(with: self.data as Data, options: []) as! NSMutableArray

without specifying .mutableContainersoption. This also would crash your app.

不指定.mutableContainers选项。这也会使您的应用程序崩溃。

And your code uses too much of NSSomethings.

而且您的代码使用了太多的NSSomethings.

---

---

With all such things fixed, you can get something like this:

解决所有这些问题后，您可以获得如下内容：

//properties

weak var delegate: HomeModelProtocal!

let urlPath: String =  "http://localhost/service.php" //this will be changed to the path where service.php lives

// Function to download the incoming JSON data
func downloadItems() {
    let url: URL = URL(string: urlPath)!
    let session = URLSession.shared

    let task = session.dataTask(with: url) {data, response, error in
        if let error = error {
            print("Failed to download data: \(error)")
        } else if let data = data {
            print("Data downloaded")
            print(data as NSData)
            //print(String(data: data, encoding: .utf8))
            self.parseJSON(data: data)
        } else {
            print("Something is wrong...")
        }
    }

    task.resume()
}

func parseJSON(data: Data){

    do {
        if let jsonResult = try JSONSerialization.jsonObject(with: data) as? [[String: AnyObject]] {

            var locations: [LocationModel] = []

            for jsonElement in jsonResult {
                let location = LocationModel()

                //the following insures none of the JsonElement values are nil through optional binding
                if let exerciseName = jsonElement["stationName"] as? String,
                    let bodyPart = jsonElement["buildYear"] as? String
                {
                    print(exerciseName, bodyPart)
                    location.exerciseName = exerciseName
                    location.bodyPart = bodyPart

                }

                locations.append(location)

                DispatchQueue.main.async {
                    self.delegate.itemsDownloaded(items: locations)
                }
            }
        } else {
            print("bad JSON")
        }
    } catch let error as NSError {
        print("**** sake its happened again \(error)")
    }
}