Run JSON Request in the background Swift 4 - ios

Question

I need to run this code in the background if possible. Im getting a JSON Request that sometimes takes a while to load(lag is on the server side of the URL, not the code itself.).
I want to run the code below in the background if possible. Any ideas?
var stockData: Data!
var concatTickersString = ""
for object in dataArray.reversed() {
concatTickersString = concatTickersString + "," + object.symbol
}
let url = URL(string: "https://www.alphavantage.co/query?function=BATCH_STOCK_QUOTES&symbols=" + concatTickersString + "&apikey=IX58FUCXKD695JY0")
do {
stockData = try Data(contentsOf: url!)
let json = try JSON(data: stockData)
if let jsonArray = json["Stock Quotes"].array {
for ticker in jsonArray.reversed() {
if(jsonArray.count != 0){
let stockTicker = ticker["1. symbol"].string!
let stockPrice = ticker["2. price"].string!
self.watchListArray.append(WatchlistData(tickerName: stockTicker, tickerPrice: Double(stockPrice)?.currency))
}
}
tableView.isHidden = false
}
} catch {
print(error)
}
Its the server of the JSON that takes long I dont think its necessarily the Data(contents of)
I tried using dispatch_async but im getting no luck.

Answer 1

The lag is caused by the fact that Data(contentsOf:) is a synchronous method. As the documentation says,
Important
Don't use this synchronous method to request network-based URLs. For network-based URLs, this method can block the current thread for tens of seconds on a slow network, resulting in a poor user experience, and in iOS, may cause your app to be terminated.
Instead, for non-file URLs, consider using the dataTask(with:completionHandler:) method of the URLSession class. See Fetching Website Data into Memory for an example.
As you discovered through experimentation, placing this method in DispatchQueue.main.async does not make it asynchronous. Instead, follow the documentation's instruction.
This is the slightly modified example found at Fetching Website Data into Memory:
func startLoad() {
let url = URL(string: "https://www.example.com/")!
let task = URLSession.shared.dataTask(with: url) { data, response, error in
if let error = error {
self.handleClientError(error)
return
}
guard let httpResponse = response as? HTTPURLResponse,
(200...299).contains(httpResponse.statusCode) else {
self.handleServerError(response)
return
}
if let data = data,
let string = String(data: data, encoding: .utf8) {
DispatchQueue.main.async {
doSomething(with: string)
}
}
}
task.resume()
}