CODEWITHSUNDEEP
iosnsurlsession
Dennis
Dennis

Reputation: 2221

NSURLSession download task failing after it already succeeded

I am experiencing a strange issue from time to with my NSURLSessionDownloadTasks (using a background download configuration).

It is always related to errors with NSURLDomain code NSURLErrorBackgroundSessionWasDisconnected (-997). The error itself usually seems to be coming from the app generically crashing or being forcefully closed during development from Xcode.

The relevant code boils down to

- (void)download:(NSURLRequest *)request
{
    NSURLSessionDownloadTask *downloadTask = [self.urlSession downloadTaskWithRequest:request];
    [downloadTask resume];
}

- (void)       URLSession:(NSURLSession *)session
             downloadTask:(NSURLSessionDownloadTask *)downloadTask
didFinishDownloadingToURL:(NSURL *)tempLocation
{
    NSLog(@"task %@ didFinishDownloadingToURL with status code %@", downloadTask, @([downloadTask.response statusCode]));
}

- (void)  URLSession:(NSURLSession *)session
                task:(NSURLSessionTask *)task
didCompleteWithError:(NSError *)error
{
    NSLog(@"task %@ didCompleteWithError %@", task, error);
}

The problem in these cases is, that the same NSURLSessionDownloadTask gets two (instead of only one expected) call in -[NSURLSessionTaskDelegate URLSession:task:didCompleteWithError:]. Once without an error (which aligns to a call with a 200 OK response code to -[NSURLSessionDownloadDelegate URLSession:downloadTask:didFinishDownloadingToURL:]) and then another time, usually a couple seconds later with an error (-997). Both times it's exactly the same memory address for the task being passed into these delegate methods.

Has anybody else experienced something similar? I am not expecting that second callback after I was already told that my task succeeded. Is there any obvious reason why the NSURLSession may still hold on to a task it reportedly finished already but then all of a sudden thinks it should inform me that it's lost connectivity to it background transfer service?

Upvotes: 0

Views: 1414

Answers (1)

CSawy
CSawy

Reputation: 924

FWIW, I had a similar issue because I was updating the UI from the NSURLSession delegate methods, which run in background thread.

So, the solution for me was to move the UI-related code to run on the main thread as below:

dispatch_async(dispatch_get_main_queue(), ^{
      //code updating the UI.
});

Upvotes: 1

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