Can't update user interface in NSURLSession delegate methods

Question

- (void)URLSession:(NSURLSession *)session downloadTask:(NSURLSessionDownloadTask
 *)downloadTask didWriteData:(int64_t)bytesWritten totalBytesWritten:(int64_t)totalBytesWritten totalBytesExpectedToWrite:(int64_t)totalBytesExpectedToWrite
{
    float progress = (float)((float)totalBytesWritten /(float)totalBytesExpectedToWrite);
    self.progressView.progress = progress;
    self.progressLabel.text = [NSString stringWithFormat:@"%.2f%%", progress*100];
}

I'm downloading a file via NSURLSession added in iOS 7. Everything works so far, but for some reason the user interface doesn't update. I've checked that the method is being called and I can also NSLog the progress.

Maybe because this method is called so often? But how do you update your user interface, e.g. if you have a progress bar then? Thanks in advance :)

Answer 1

NSURLSession runs in background thread by default, so you need to call your UI update on main thread.

dispatch_async(dispatch_get_main_queue(), ^{
    // perform on main
    float progress = (float)((float)totalBytesWritten /(float)totalBytesExpectedToWrite);
    self.progressView.progress = progress;
    self.progressLabel.text = [NSString stringWithFormat:@"%.2f%%", progress*100];
});