URLSession.dataTask return the data with 0

Question

I have a web page for giving me the server data in JSON format and I am trying to to reach the site and get these objects. But the data will return with 0; empty variable. My code like this.

    func downloadItems() {

    let url: URL = URL(string: urlPath)!

    let defaultSession = URLSession(configuration: URLSessionConfiguration.default)

    let task = defaultSession.dataTask(with: url) {
        (data, response, error) in

        if error != nil {
            print("Failed to download data")
        }else {
            let httpResponse = response as? HTTPURLResponse
            print("data is like: \(data as! NSData)")
            print(httpResponse)
            self.data = data!
            print("Data downloaded")
            self.parseJSON(self.data)
        }

    }

    task.resume()
}

func parseJSON(_ data: Data){
    var jsonResult = NSArray()

    do{
        jsonResult = try JSONSerialization.jsonObject(with: data, options: []) as! NSArray
        print(jsonResult)
    } catch let error as NSError {
        print(error)
    }

    var jsonElement = NSDictionary()
    let forest = NSMutableArray()

    for tree in 0 ..< jsonResult.count {

        jsonElement = jsonResult[tree] as! NSDictionary

        let tempTree = trees()

        if let latinName = jsonElement["latin_name"] as? String,
        let turkishName = jsonElement["turkish_name"] as? String,
        let seedType = jsonElement["seed_type"] as? Int,
        let leafType = jsonElement["leaf_type"] as? Int,
        let spreadingArea = jsonElement["spreading_area"] as? String,
        let bothanicalProp = jsonElement["bothanical_prop"] as? String {
            tempTree.latin_name = latinName
            tempTree.turkish_name = turkishName
            tempTree.seed_type = seedType
            tempTree.leaf_type = leafType
            tempTree.spreading_area = spreadingArea
            tempTree.botanical_prop = bothanicalProp
        }

        forest.add(tempTree)

    }

    DispatchQueue.main.async {
        self.delegate.itemsDownloaded(items: forest)
    }

}

and my main view controller is like that

    override func viewDidLoad() {
    super.viewDidLoad()

    //table view delegats and datasource declaration
    treeTableView.delegate = self
    treeTableView.dataSource = self

    let treeModel = trees()
    treeModel.delegate = self
    treeModel.downloadItems(completion: <#T##(Data?) -> Void#>)

}

func itemsDownloaded(items: NSArray) {
    feedTrees = items
    self.treeTableView.reloadData()
}

When I enter the site the JSON object is here. But when I tried to retrieve the object with dataTask. Data will be 0. Also HTTP Response is 200. Is there any knowledge about this situation ? Thanks and good coding.

print(jsonResult) is

{
    "bothanical_prop" = "T\U00fcrkiyede park ve bah\U00e7elerde dekoratif ama\U00e7la kullan\U0131l\U0131r. Do\U011fal yay\U0131l\U0131\U015f alan\U0131 \U015eili'nin g\U00fcneyi ve G\U00fcnaybat\U0131 Arjantin'deki And Da\U011flar\U0131d\U0131r.";
    "latin_name" = "Araucaria araucana (Molina) K. Koch.";
    "leaf_type" = 1;
    "seed_type" = 1;
    "spreading_area" = "Do\U011fal alanlarda erkekler 15-18 m, di\U015filer ise 30-50 m ye kadar boylanabilir. Park ve bah\U00e7elerde ise 10 m'yi nadiren ge\U00e7en herdemye\U015fil a\U011fa\U00e7lard\U0131r. G\U00f6zvdesi d\U00fczg\U00fcnd\U00fcr; \U00f6zellikle gen\U00e7 bireyleri simetriktir ve konik formludur. Yapraklar\U0131, deri gibi sert, u\U0308\U00e7gen \U015feklinde, u\U00e7lar\U0131 sivri, bat\U0131c\U0131 ve her iki yu\U0308zu\U0308 koyu ye\U015fildir.";
    "turkis_name" = "Maynum \U00c7\U0131kmaz A\U011fac\U0131";
},
    {
    "bothanical_prop" = "Norfolk adalar\U0131na (Avustralya) \U00f6zgu\U0308 endemik bir tu\U0308rdu\U0308r. Dekoratif ama\U00e7l\U0131 \U00fclkemizde Ege, Akdeniz ve Marmara \U00e7evresinde d\U0131\U00e7 mekan bitkisi olarak kullan\U0131l\U0131r.";
    "latin_name" = "Araucaria heterophylla (Salisb.) Franco";
    "leaf_type" = 1;
    "seed_type" = 1;
    "spreading_area" = "60-70 m\U2019ye kadar boylanabilen bir orman a\U011fac\U0131d\U0131r. Dallar g\U00f6vdeden 4-7\U2019li \U00e7evrel olarak \U00e7\U0131kar. G\U00f6vde kabu\U011fu ince levhalar halinde \U00e7atlakl\U0131d\U0131r. Yapraklar\U0131 2 farkl\U0131 \U015fekildedir: Gen\U00e7 ve yan su\U0308rgu\U0308nlerde a\U00e7\U0131k ye\U015fil, biz gibi uzun, k\U00f6\U015feli ve yumu\U015fak; ya\U015fl\U0131 su\U0308rgu\U0308nlerde ise daha k\U0131sa, daha sert, s\U0131k, birbiri u\U0308zerine binmi\U015f ve u\U00e7 k\U0131s\U0131mlar\U0131 boynuz gibi \U00f6ne do\U011fru k\U0131vr\U0131lm\U0131\U015f olan yapraklar vard\U0131r";
    "turkis_name" = "Salon arokaryas\U0131";
}

Since you get a 200 response, but cannot retrieve data, maybe there is a mistake in your parseJSON method? I feel like there is something wrong with the way you try to parse data. — emrepun, Oct 01 '18 at 23:16
Many API servers return unexpected results with status code 200, so that cannot be a useful info. What do you get when you put `print(data! as NSData)` before your `print("Data downloaded")` ? — OOPer, Oct 02 '18 at 00:56
"Data will be 0." How are you calling this function? Post the code that calls this function and tell us why you think your "data is 0." — Duncan C, Oct 02 '18 at 03:36
@OOPer when I wrote your suggestion I faced it that hell `<5b7b226c 6174696e 5f6e616d 65223a22 41726175 63617269 61206172 61756361 6e612028 4d6f6c69 6e612920...` its goes like that I do not know what is this. — Mertalp Tasdelen, Oct 02 '18 at 07:23
That represents a String `[{"latin_name":"Araucaria araucana (Molina) ...`. Facts, **#1** the API server returns non-nil, non-empty `data`. **#2** the content of the `data` seems to be a valid(?) JSON. Hypothese: **#1** you are accessing the `data` before it is ready. **#2** your `parseJSON(_:)` silently ignoring some errors, so you cannot distinguish between empty `data` and invalid JSON. Anyway, how did you check your `data` other than the way I have shown? — OOPer, Oct 02 '18 at 07:42
Your updated code is not consistent. The method `downloadItems()` has no arguments, but you call it as `downloadItems(completion: <#T##(Data?) -> Void#>)`. You say you have a code like `DispatchQueue.main.async { self.delegate.itemsDownloaded(items: forest) }`, but I cannot find it anywhere. If you really want your issue to be solved soon, show your current code as is, and tell us how you checked that _data will return with 0; empty variable_. — OOPer, Oct 02 '18 at 13:34
Some parts are still missing or mismatching, but I have found one point to check. What do you get when you put `print(jsonElement)` before the line `let tempTree = trees()` ? (If it's long, please add the result into your question. Comments are hard to read.) — OOPer, Oct 02 '18 at 14:06

score 1 · Answer 1 · answered Oct 02 '18 at 03:36

1

If you call it and then expect data to be non-empty you will be disappointed. A URLSession dataTask is asynchronous. It won't have read your data when the downloadItems() function returns.

answered Oct 02 '18 at 03:36

Duncan C

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I know that the dataTask İs asynchronous. In my code I have the line for that like this `DispatchQueue.main.async { self.delegate.itemsDownloaded(items: forest) }` – Mertalp Tasdelen Oct 02 '18 at 07:31
@MertalpTaşdelen that code is intended for asynchronous thread, not for asynchronous request, check my answer. – Andrew21111 Oct 02 '18 at 07:33
Mertalp, what you said about `DispatchQueue.main.async` makes no sense. That is not some magic incantation that makes async code synchronous. See Andrew's answer. You need to pass a completion handler to downloadItems, and use the result in the completion handler. – Duncan C Oct 02 '18 at 10:18

score 0 · Answer 2 · edited Dec 03 '19 at 21:38

URLSession dataTask is asynchronous, so you have to make a callback to get your data.

Change you code this way:

func downloadItems(completion: @escaping (_ result: Data?)->Void) {

  let url: URL = URL(string: urlPath)!
  let defaultSession = URLSession(configuration: URLSessionConfiguration.default)
  let task = defaultSession.dataTask(with: url) { (data, response, error) in
    if error != nil {
      print("Failed to download data")
      completion(nil)
    } else {
      let httpResponse = response as? HTTPURLResponse
      print(httpResponse)
      // remove this
      //self.data = data!
      //print("Data downloaded")
      //self.parseJSON(self.data)

      //add this
      guard let data = data else { return }
      completion(data)
    }
  }
  task.resume()
}

Then you can call your method anywhere this way:

downloadItems{ data in
 self.data = self.parseJSON(data)
}

EDIT: So in your case you have to change this:

treeModel.downloadItems(completion: <#T##(Data?) -> Void#>)

into this:

treeModel.downloadItems{ data in
    feedTrees = self.parseJSON(data)
}

I say this because you're parseJSON function does return nothing, so I assume you're parseJSON is wrong

I would do it this way:

NOTE: this function assume that your feedTrees is NSMutableArray

//Make your function return NSMutable array
func parseJSON(_ data: Data) -> NSMutableArray{
   var jsonResult = NSArray()
   //Put all of your code into do-catch
do{
    jsonResult = try JSONSerialization.jsonObject(with: data, options: JSONSerialization.ReadingOptions.allowFragments) as! NSArray

var jsonElement = NSDictionary()
let forest = NSMutableArray()

for tree in 0 ..< jsonResult.count {

    jsonElement = jsonResult[tree] as! NSDictionary

    let tempTree = trees()

    if let latinName = jsonElement["latin_name"] as? String,
    let turkishName = jsonElement["turkish_name"] as? String,
    let seedType = jsonElement["seed_type"] as? Int,
    let leafType = jsonElement["leaf_type"] as? Int,
    let spreadingArea = jsonElement["spreading_area"] as? String,
    let bothanicalProp = jsonElement["bothanical_prop"] as? String {
        tempTree.latin_name = latinName
        tempTree.turkish_name = turkishName
        tempTree.seed_type = seedType
        tempTree.leaf_type = leafType
        tempTree.spreading_area = spreadingArea
        tempTree.botanical_prop = bothanicalProp

        //Add here your tree or it can be empty
        forest.add(tempTree)
    }

}

 //AFTER THE LOOP
 return forest

} catch let error as NSError {
    print(error)
}

}

there is a error in guard let line like this "'guard' body must not fall through, consider using a 'return' or 'throw' to exit the scope" — Mertalp Tasdelen, Oct 02 '18 at 07:55
My bad, change completion(nil) with return and add the completion(nil) in the error if, check again, I updated the answer — Andrew21111, Oct 02 '18 at 08:05
how can I use the downloadItems(completion: ) in my main view controller ? and also my I am using the parseJSON method. I will edit the my question now — Mertalp Tasdelen, Oct 02 '18 at 10:40

score 0 · Accepted Answer · answered Oct 02 '18 at 22:30

Seems your API returns turkis_name instead of turkish_name:

"turkis_name" = "Maynum \U00c7\U0131kmaz A\U011fac\U0131";

Tell your server side engineers fix it to follow the API spec. Or, is that you who mis-read the API spec?

Anyway, when you need to adapt your code to the actual response, you may need to fix only one line.

    let turkishName = jsonElement["turkis_name"] as? String,

But I give you a practically useful advice:

Do not ignore errors or invalid inputs silently.

With some modifications to make your code more Swifty, your parseJSON(_:) would be something like below:

func parseJSON(_ data: Data) -> NSArray {
    do{
        guard let jsonResult = try JSONSerialization.jsonObject(with: data) as? [[String: Any]] else {
            print("Invalid JSON structure")
            return []
        }

        let forest = NSMutableArray()

        for jsonElement in jsonResult {
            if
                let latinName = jsonElement["latin_name"] as? String,
                let turkishName = jsonElement["turkis_name"] as? String, //###Use the right key here
                let seedType = jsonElement["seed_type"] as? Int,
                let leafType = jsonElement["leaf_type"] as? Int,
                let spreadingArea = jsonElement["spreading_area"] as? String,
                let bothanicalProp = jsonElement["bothanical_prop"] as? String
            {
                let tempTree = trees()

                tempTree.latin_name = latinName
                tempTree.turkish_name = turkishName
                tempTree.seed_type = seedType
                tempTree.leaf_type = leafType
                tempTree.spreading_area = spreadingArea
                tempTree.botanical_prop = bothanicalProp

                forest.add(tempTree)
            } else {
                //### Do not ignore errors or invalid inputs silently.
                print("Missing any of 'latin_name', 'turkis_name', 'seed_type', 'leaf_type', 'spreading_area', 'bothanical_prop'", jsonElement)
            }
        }
        return forest

    } catch let error {
        print(error)
        return []
    }
}

(There are some more parts I want to fix, but those needs modification of the hidden parts of your code, so I have given up. And consider adopting Codable, when working with JSON.)

You may need some more fixes, but when you write something about unexpected result, please remember to include:

Concrete output of the actual result
In addition to where and when you get the output
And the expected result

Putting print statement and showing the output with the code including it would be a good way.

score 0 · Answer 4 · answered Oct 03 '18 at 22:41

I fix the issue... My code before the fix

override func viewDidLoad() {
    super.viewDidLoad()

    //table view delegats and datasource declaration
    self.treeTableView.delegate = self
    self.treeTableView.dataSource = self

    let treeModel = TreeModel()
    treeModel.delegate = self
    treeModel.downloadItems()

}
    func itemsDownloaded(items: NSArray) {
    feedTrees = items
    self.treeTableView.reloadData()
}

After fix the issue like that

    func itemsDownloaded(items: NSArray) {
    feedTrees = items
    self.treeTableView.reloadData()
}
override func viewDidLoad() {
    super.viewDidLoad()

    //table view delegats and datasource declaration
    self.treeTableView.delegate = self
    self.treeTableView.dataSource = self

    let treeModel = TreeModel()
    treeModel.delegate = self
    treeModel.downloadItems()

}

Just move the protocol method top of the viewDidLoad().

URLSession.dataTask return the data with 0

4 Answers4

Do not ignore errors or invalid inputs silently.