Alertview not called when no Internet access

Question

Use the following standard calls to get data from a server - and standard Alert if there's an error - like no internet access. If I turn off the network the app crashes, never hitting the NSLog calls for *response or *error, never entering the alert.

dispatch_queue_t concurrentQueue =  dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0);

//this will start the URL call and download in bg

dispatch_async(concurrentQueue, ^{

   NSURLSession *session = [NSURLSession sharedSession];
   NSURLSessionDataTask *dataTask = [session dataTaskWithURL:[NSURL URLWithString:@"https://www.websitethatdownloadsdata.com"] completionHandler:^(NSData *myData, NSURLResponse *response, NSError *error) {

       NSLog(@"Resp value from NSURL task: %@", response);
       NSLog(@"Error value from NSURL task: %@", error);

       if (error == nil) {

       NSLog(@"Downloading data...");

       }

       if (error != nil) {

       UIAlertController * alert = [UIAlertController alertControllerWithTitle:@"Network Problem" message:@"Cannot download data" preferredStyle:UIAlertControllerStyleAlert];
         UIAlertAction * actionOK = [UIAlertAction actionWithTitle:@"OK" style:UIAlertActionStyleDefault handler:^(UIAlertAction * _Nonnull action) {
                            //Here Add Your Action
             abort();
                        }];

                    [alert addAction:actionOK];

       [self presentViewController:alert animated:YES completion:nil];

       }

"If I turn off the network the app crashes" Do you have a crash log in console when this happens? — Larme, Jan 08 '20 at 15:28
Also you are presenting a `UIAlertViewController` from a a thread that is not the main thread, this will cause issues. — rckoenes, Jan 08 '20 at 15:40
@rckoenes - using an async queue is the only way to get data then update UI - I fought that for days.... — user3741598, Jan 08 '20 at 15:44
So it was just missing a dispatch in main thread for the UI? I think that would have been stated in the console. And that's why I often ask the question's author do give that message, pointing out to read them. It's an important peace of info to debug by yourself. — Larme, Jan 08 '20 at 15:47
There are many issue with the posted code. You should not perform any UI off the main thread. — rckoenes, Jan 08 '20 at 15:47

score 2 · Answer 1 · answered Jan 08 '20 at 15:49

The background queue is redundant because NSURLSession dispatches its tasks on a background thread anyway.

But you have to present the alert controller on the main thread

NSURLSession *session = [NSURLSession sharedSession];
NSURLSessionDataTask *dataTask = [session dataTaskWithURL:[NSURL URLWithString:@"https://www.websitethatdownloadsdata.com"] completionHandler:^(NSData *myData, NSURLResponse *response, NSError *error) {

   NSLog(@"Resp value from NSURL task: %@", response);
   NSLog(@"Error value from NSURL task: %@", error);

   if (error == nil) {

      NSLog(@"Downloading data...");

   } else {
      UIAlertController * alert = [UIAlertController alertControllerWithTitle:@"Network Problem" message:@"Cannot download data" preferredStyle:UIAlertControllerStyleAlert];
      UIAlertAction * actionOK = [UIAlertAction actionWithTitle:@"OK" style:UIAlertActionStyleDefault handler:^(UIAlertAction * _Nonnull action) {
         //Here Add Your Action
         abort();
      }];

      [alert addAction:actionOK];
      dispatch_async(dispatch_get_main_queue()) {
          [self presentViewController:alert animated:YES completion:nil];
      }
   }

}

And you should display also the reason of the error in the NSError instance

Alert flashes but doesn't stop for the user to press OK. So it continues and crashes when the parser is invoked in the code below. — user3741598, Jan 09 '20 at 01:15

Alertview not called when no Internet access

1 Answers1