Asynchronous thread in Swift - How to handle? - swift

Question

I am trying to recover a data set from a URL (after parsing a JSON through the parseJSON function which works correctly - I'm not attaching it in the snippet below).
The outcome returns nil - I believe it's because the closure in retrieveData function is processed asynchronously. I can't manage to have the outcome saved into targetData.
Thanks in advance for your help.
class MyClass {
var targetData:Download?
func triggerEvaluation() {
retrieveData(url: "myurl.com") { downloadedData in
self.targetData = downloadedData
}
print(targetData) // <---- Here is where I get "nil"!
}
func retrieveData(url: String, completion: #escaping (Download) -> ()) {
let myURL = URL(url)!
let mySession = URLSession(configuration: .default)
let task = mySession.dataTask(with: myURL) { [self] (data, response, error) in
if error == nil {
if let fetchedData = data {
let safeData = parseJSON(data: fetchedData)
completion(safeData)
}
} else {
//
}
}
task.resume()
}
}

Answer 1

Yes, it’s nil because retrieveData runs asynchronously, i.e. the data hasn’t been retrieved by the time you hit the print statement. Move the print statement (and, presumably, all of the updating of your UI) inside the closure, right where you set self.targetData).
E.g.
func retrieveData(from urlString: String, completion: #escaping (Result<Download, Error>) -> Void) {
let url = URL(urlString)!
let mySession = URLSession.shared
let task = mySession.dataTask(with: url) { [self] data, response, error in
guard
let responseData = data,
error == nil,
let httpResponse = response as? HTTPURLResponse,
200 ..< 300 ~= httpResponse.statusCode
else {
DispatchQueue.main.async {
completion(.failure(error ?? NetworkError.unknown(response, data))
}
return
}
let safeData = parseJSON(data: responseData)
DispatchQueue.main.async {
completion(.success(safeData))
}
}
task.resume()
}
Where
enum NetworkError: Error {
case unknown(URLResponse?, Data?)
}
Then the caller would:
func triggerEvaluation() {
retrieveData(from: "https://myurl.com") { result in
switch result {
case .failure(let error):
print(error)
// handle error here
case .success(let download):
self.targetData = download
// update the UI here
print(download)
}
}
// but not here
}
A few unrelated observations:
You don't want to create a new URLSession for every request. Create only one and use it for all requests, or just use shared like I did above.
Make sure every path of execution in retrieveData calls the closure. It might not be critical yet, but when we write asynchronous code, we always want to make sure that we call the closure.
To detect errors, I'd suggest the Result pattern, shown above, where it is .success or .failure, but either way you know the closure will be called.
Make sure that model updates and UI updates happen on the main queue. Often, we would have retrieveData dispatch the calling of the closure to the main queue, that way the caller is not encumbered with that. (E.g. this is what libraries like Alamofire do.)