UIKeyboardTaskQueue may only be called from the main thread

Question

I have a problem while trying to make a function that automatically create POST request, send them, get the response, and handle them then show UIAlertView to tell the user which problem it is.

Here is my code :

let task = session.dataTaskWithRequest(request, completionHandler:{ (data, response, error) -> Void in      dispatch_async(dispatch_get_main_queue(),{         var alert = UIAlertController(title: "Chargement", message: "Envoi des informations...", preferredStyle: UIAlertControllerStyle.Alert)         viewController.presentViewController(alert, animated: true, completion: nil)           answer = NSString(data: data!, encoding: NSUTF8StringEncoding)!         print(answer)     var complete = false     alert.dismissViewControllerAnimated(true, completion: { () -> Void in         complete = true     })      while(!complete)     {      }     var textmsg: String     if(answer == "#400")     {         textmsg = "Il manque une information !"     }     else if(answer == "#50")     {         textmsg = "Le compte fourni ne correspond pas."     }     else if(answer == "#100")     {         textmsg = "Impossible d'identifier l'application."     }     else if(answer == "#1")     {         textmsg = "Transfert terminé avec succès !"     }     else {         textmsg = "Echec du transfert."     }     let alertComplete = UIAlertController(title: "Chargement", message: textmsg, preferredStyle: UIAlertControllerStyle.Alert)     alertComplete.addAction(UIAlertAction(title: "Ok", style: UIAlertActionStyle.Default, handler: nil))     viewController.presentViewController(alertComplete, animated: true, completion: nil)     }) })  task.resume(); 

The error code is the following :

Terminating app due to uncaught exception 'NSInternalInconsistencyException', reason: '-[UIKeyboardTaskQueue waitUntilAllTasksAreFinished] may only be called from the main thread.' 

When my request return a bad response (the #400, #50, #100), the code works and show the UIAlertView but if the response is good, it gives me the error code as above.

Answer 1

You should call your UIAlertController on the main thread because you're dealing with the ui.

Swift 2.X

dispatch_async(dispatch_get_main_queue(),{                 var alert = UIAlertController(title: "Chargement", message: "Envoi des informations...", preferredStyle: UIAlertControllerStyle.Alert)                 viewController.presentViewController(alert, animated: true, completion: nil)                  answer = NSString(data: data!, encoding: NSUTF8StringEncoding)!                 print(answer)              var complete = false     alert.dismissViewControllerAnimated(true, completion: { () -> Void in         complete = true     })     while(!complete)     {      } } 

Swift 4.2

DispatchQueue.main.async {     var alert = UIAlertController(title: "Chargement", message: "Envoi des informations...", preferredStyle: UIAlertControllerStyle.Alert)     viewController.presentViewController(alert, animated: true, completion: nil)      let answer = String(data: data!, encoding: .utf8)!     print(answer)     var complete = false     alert.dismissViewControllerAnimated(true, completion: { () -> Void in         complete = true     })     while(!complete)     {      } } 

Answer 2

For swift 3.x and 4.x

DispatchQueue.main.async(execute: { // work Needs to be done  })