Swift URL Session and URL Request not working

Question

I am getting very similar problems to this post, but I don't fully understand the answer. I've created a completion handler, but it doesn't seem to be working as expected.

func updateTeam(teamID: Int) {
    startConnection {NSArray, Int in
        //Do things with NSArray
    }
}

func startConnection(completion: (NSArray, Int) -> Void) {
    let url = URL(string: "http://www.example.com/path")
    var request : URLRequest = URLRequest(url: url!)
    request.httpMethod = "POST"
    let postString = "a=\(Int(teamInput.text!)!)"
    request.httpBody = postString.data(using: .utf8)

    let dataTask = URLSession.shared.dataTask(with: request) {
        data,response,error in
        print("anything")
        do {
            if let jsonResult = try JSONSerialization.jsonObject(with: data!, options: []) as? NSDictionary {
                self.teamResult = jsonResult
                print(jsonResult)
            }
        } catch let error as NSError {
            print(error.localizedDescription)
        }

    }
    dataTask.resume()

    completion(NSArray(object: teamResult), Int(teamInput.text!)!)
}

Nothing within the dataTask statement seems to run, or at least it doesn't complete before I try to use the data resulted. What is wrong with this completion handler?

Thank you in advance!

Answer 1

Your code is structured incorrectly.

URLSession creates tasks that are run asynchronously. You set up a task, and either pass in a completion block, or set up a delegate.

The task.resume() call returns immediately, long before the network download is complete.

Once the task is complete, the system calls your completion handler (or your delegate, if you use the delegate style).

Beware that URLSessions' completion handlers and delegate calls are done on a background thread. If you do any UIKit calls in response to a task completing, you need to do it on the main thread.

As @keithbhunter says in his comment, you need to put the call to your completion handler inside the completion handler for your task. It's probably safest if you wrap that whole completion handler call in a call to the main thread:

func startConnection(completion: (NSArray, Int) -> Void) {
    let url = URL(string: "http://www.example.com/path")
    var request : URLRequest = URLRequest(url: url!)
    request.httpMethod = "POST"
    let postString = "a=\(Int(teamInput.text!)!)"
    request.httpBody = postString.data(using: .utf8)

    let dataTask = URLSession.shared.dataTask(with: request) {
        data,response,error in
        print("anything")
        do {
            if let jsonResult = try JSONSerialization.jsonObject(with: data!, options: []) as? NSDictionary {
                self.teamResult = jsonResult
                print(jsonResult)
                //Use GCD to invoke the completion handler on the main thread
                DispatchQueue.main.async() {
                  completion(NSArray(object: teamResult), Int(teamInput.text!)!)
                }
            }
        } catch let error as NSError {
            print(error.localizedDescription)
        }
    }
    dataTask.resume()
}

Note that your force-unwrapping of teamInput.text is very fragile, and will crash if teamInput.text is nil, or if it can't be converted to an Int. You'd be much better off to write your completion handler to take optionals for both the data and the int value you get back from teamInput.text:

func startConnection(completion: (NSArray?, Int?) -> Void) {

and call it passing in an optional:

let value: Int? = teamInput.text != nil ? Int(teamInput.text!) : nil
completion(NSArray(object: teamResult), value)