Asynchronous JSON request in Swift

Question

I would like to randomize an outlet with randomized questions from a received JSON. Therefore I have to "load" it asynchronously.

************Question edited************************

Why is the Array not "filled"? Also the .count = 0.

@IBAction func refreshBtnTapped(_ sender: UIBarButtonItem) {

    let queue = DispatchQueue(label: "Json loading", qos: .userInteractive)

    self.btnOutlet.isEnabled = false

    func jsonDataRequest () {
        let url = "https://redaktion.pflegonaut.de/service.php"
        let urlObj = URL(string: url)
        URLSession.shared.dataTask(with: urlObj!) { (data, response, error) in
            do {

                // Json to Array
                self.questionsJsonVar = try JSONDecoder().decode([Question].self, from: data!)
                let countOfQuestions = self.questionsJsonVar.count
                print(self.questionsJsonVar)

                // MARK:-- later in queue
                DispatchQueue.main.async {

                    print("main.async")

                    // -- Randomize Question Outlet
                    print("Anzahl der Fragen" , self.countOfQuestions)
// PROBLEM prints "Anzahl der Fragen 0"    
                    let randNumber = Int.random(in: 1 ... self.countOfQuestions)
// PROBLEM therefore upperbound < lowerbound
                    print(self.questionsJsonVar[0].Frage)
// PROBLEM not filled
                    self.questionTextOutlet.text = self.questionsJsonVar[self.randNumber].Frage
// PROBLEM does not work
                }

            } catch {
                print(error)
            }
            }.resume()
    }

    //MARK:-- first in queue
    queue.async {
        jsonDataRequest()
    }

}

Debug Area:

 [R***.Question(ID: "1", Frage: "1", Antwort1: "2", Antwort2: "3", Antwort3: "4", Antwort4: "5", Correct: "1", Notiz: Optional("  1234"), LernsektorID: "0", LerneinheitID: "1", LernbereichID: "1", SchwierigkeitID: "1"),
 R***.Question (ID: "51", Frage: " Welche der drei genannten Werte steuert den Atemantrieb?", Antwort1: "pO2", Antwort2: "pCO2", Antwort3: "pH", Antwort4: "K+", Correct: "2", Notiz: Optional("   Gesteuert wird die Atmung im wesentlichen durch das Gehirn beziehungsweise das Atemzentrum in der Medulla oblongata. Ausschlaggebend ist dabei die Reaktion von Chemorezeptoren auf den Kohlendioxid-Gehalt..."),
 ...
 ...]
 main.async
 Anzahl der Fragen 0

gkivanov · Accepted Answer · 2019-01-03 16:39:16Z

1

The reason is because the dataTask is asynchronous itself.

If you want to execute the print("main.async") after the request has returned you need to add the snippet at the end of the response handler. Something like this:

func jsonDataRequest () {
    let url = "https://redaktion.pflegonaut.de/service.php"
    let urlObj = URL(string: url)
    URLSession.shared.dataTask(with: urlObj!) { (data, response, error) in
        do {

            // Json to Array
            self.questionsJsonVar = try JSONDecoder().decode([Question].self, from: data!)
            let countOfQuestions = self.questionsJsonVar.count
            print(self.questionsJsonVar)

            // Logic after response has arrived
            DispatchQueue.main.async {
                print("main.async")
            }
        } catch {
                print(error)
        }
    }.resume()
)

answered Jan 3, 2019 at 16:39

gkivanov

2211 silver badge11 bronze badges

Sign up to request clarification or add additional context in comments.

2 Comments

Valonaut Over a year ago

works! Thanks. I wanted to update the UI on Dispatch.Queue.main.async (change Label.text to a randomized Json property), but that is not working. "Index out of range". Therefore I think the Array is not "filled".

Valonaut Over a year ago

the var questionJsonVar: [Question] = [] was declared in the superclass QuizVC: UIViewController

Collectives™ on Stack Overflow

Asynchronous JSON request in Swift

1 Answer 1

2 Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

1 Answer 1

2 Comments

Your Answer

Sign up or log in

Post as a guest

Related