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Eric Gu
3,871 PointsSwift JSON Asynchronous Calls
I was wondering if anyone more experienced than I can shed light on why I can print a value for convertedAmount inside the function to the log but when I try to return convertedAmount, it shows as 0.
Am I missing something?
Thank you!!!!
func convertCurrency(#localAmount:Double, localCurrency:String, foreignCurrency: String)->Double{
let baseURL = NSURL(string: "http://www.freecurrencyconverterapi.com/api/v2/")
let query = String(localCurrency)+"_"+String(foreignCurrency)
let forecastURL = NSURL(string: "convert?q="+query, relativeToURL: baseURL)
var convertedAmount:Double = 0.0
//use a session object to manage.//like the sun
let sharedSession = NSURLSession.sharedSession()
let downloadTask:NSURLSessionDownloadTask = sharedSession.downloadTaskWithURL(forecastURL, completionHandler: { (location:NSURL!, response: NSURLResponse!, error: NSError!) -> Void in
if error == nil {
let dataObject = NSData(contentsOfURL: location)
let conversionDictionary:NSDictionary = NSJSONSerialization.JSONObjectWithData(dataObject, options: nil, error: nil) as NSDictionary
if let results = conversionDictionary["results"] as? NSDictionary{
if let queryResults = results[query] as? NSDictionary{
if let exchangeRate = queryResults["val"] as? Double{
convertedAmount = (localAmount * exchangeRate)
print(convertedAmount)
//prints here
}
}
}
}
})
downloadTask.resume()
//converted amount is 0 here
return convertedAmount
}
}
2 Answers
Stone Preston
42,016 Pointsmy guess is that you are actually assigning the value to the convertedAmount in the background thread, and that your function is returning before that background thread finishes. try putting your return in the completion block of the background thread instead. that might work
Eric Gu
3,871 PointsTo solve this in the short-term, I had to make a synchronous call. My network query is small.
Eric Gu
3,871 PointsEric Gu
3,871 PointsThanks for your answer. Am I forced to use a synchronous call? Can I wait for the download to finish and then return?
Stone Preston
42,016 PointsStone Preston
42,016 Pointstry moving the return into the completion closure. it might complain that it reached the end of a non void function though, you will probably just have to experiment a bit. one thing you may have to do is separate your background code into one function, and then the rest of the code into another that calls that background function. that way you can wait until the background thread completes, then return a value