Notify when a background request has finished swift - ios

Question

I'm sending a request to my backend server and I need to know the right way to notify my main thread the response. I'm using the NSNotificationCenter for this task but this not work sometimes and has some delay when it works because when I'm debugging I can see when the console prints the result but then a few secs later the corresponding alert to appear.
Here is my request:
let request1 = NSMutableURLRequest(URL: NSURL(string: serverID)!)
request1.HTTPMethod = "GET"
for key in parameters.keys{
request1.addValue(parameters[key] as String!, forHTTPHeaderField: key)
}
let task = NSURLSession.sharedSession().dataTaskWithRequest(request1) {
data, response1, error in
if error != nil {
print("error=\(error)")
NSNotificationCenter.defaultCenter().postNotificationName("request1Error", object: self)
return
}
else {
let httpResponse = response1 as! NSHTTPURLResponse
let headers = httpResponse.allHeaderFields as NSDictionary
let sucess: AnyObject? = headers.objectForKey("UserId")
if (sucess != nil){
let value = headers.valueForKey("info") as! String
print("info: \(value)")
NSNotificationCenter.defaultCenter().postNotificationName("sucessRequest", object: self)
}
else{
NSNotificationCenter.defaultCenter().postNotificationName("requestError2", object: self)
}
}
}
task.resume()
How should I notify my main thread the result of the request?

Answer 1

I don't know what exactly do you mean with main thread and if that is just a main thread (and not another controller or something similar) I think you should use closures that will get executed on main thread by using dispatch async.A function that described what I wrote would look similar like this :
func request(onSuccess : (value:String) -> Void, onError: ()->Void){
let request1 = NSMutableURLRequest(URL: NSURL(string: serverID)!)
request1.HTTPMethod = "GET"
for key in parameters.keys{
request1.addValue(parameters[key] as String!, forHTTPHeaderField: key)
}
let task = NSURLSession.sharedSession().dataTaskWithRequest(request1) {
data, response1, error in
if error != nil {
print("error=\(error)")
NSNotificationCenter.defaultCenter().postNotificationName("request1Error", object: self)
return
}
else {
let httpResponse = response1 as! NSHTTPURLResponse
let headers = httpResponse.allHeaderFields as NSDictionary
let sucess: AnyObject? = headers.objectForKey("UserId")
if (sucess != nil){
let value = headers.valueForKey("info") as! String
print("info: \(value)")
dispatch_async(dispatch_get_main_queue(),{
onSuccess(value)
})
}
else{
dispatch_async(dispatch_get_main_queue(),{
onError()
})
}
}
}
task.resume()
}

Answer 2

The delay is due to the fact that you're trying use the NSNotificationCenter on the same thread that you're using NSURLSession on. Try updating your calls to the NSNotificationCenter with something along the lines of:
dispatch_async(dispatch_get_main_queue()) {
NSNotificationCenter.defaultCenter().postNotificationName("sucessRequest", object: self)
}