NSURLSessionUploadTask "finishes" but no network traffic - ios

Question

I have an application that uploads several pictures held in an array of TSPhoto objects.
When I start the upload, the delegate method eventually fires to say that it's done, but the app never hits the server and Instruments is showing no network traffic:
_session = [self backgroundSession];
for(TSPhoto *photo in _picsetPhotos) {
NSURL *uploadURL = [NSURL URLWithString:[NSString stringWithFormat:#"%#upload", #"https://example.com/"]];
NSMutableURLRequest *req = [NSMutableURLRequest requestWithURL:uploadURL];
[req setHTTPMethod:#"POST"];
NSLog(#"req dump: %#", req);
// Add it to the queue
NSURLSessionUploadTask *uploadTask = [_session uploadTaskWithRequest:req fromFile:[NSURL fileURLWithPath:photo.fullImagePath]];
// "Start" it
NSLog(#"Enqueueing %#", uploadTask);
[uploadTask resume];
NSLog(#"Post-resume");
}
-backgroundSession:
-(NSURLSession *)backgroundSession {
static NSURLSession *session = nil;
static dispatch_once_t onceToken;
dispatch_once(&onceToken, ^{
NSURLSessionConfiguration *configuration = [NSURLSessionConfiguration backgroundSessionConfigurationWithIdentifier:#"TSBackgroundUploader"];
configuration.HTTPMaximumConnectionsPerHost = 1;
session = [NSURLSession sessionWithConfiguration:configuration delegate:self delegateQueue:nil];
});
return session;
}
I have implemented all of the appropriate delegate methods:
-(void)URLSessionDidFinishEventsForBackgroundURLSession:(NSURLSession *)session {
NSLog(#"Finished all uploads!");
[[[UIAlertView alloc] initWithTitle:#"Upload Completed" message:#"Your album has been created." delegate:nil cancelButtonTitle:#"OK" otherButtonTitles:nil, nil] show];
}
-(void)URLSession:(NSURLSession *)session task:(NSURLSessionTask *)task didSendBodyData:(int64_t)bytesSent totalBytesSent:(int64_t)totalBytesSent totalBytesExpectedToSend:(int64_t)totalBytesExpectedToSend {
dispatch_async(dispatch_get_main_queue(), ^{
[_uploadProgress setProgress:
(double)totalBytesSent /
(double)totalBytesExpectedToSend animated:YES];
});
}
-(void)URLSession:(NSURLSession *)session task:(NSURLSessionTask *)task didCompleteWithError:(NSError *)error {
dispatch_async(dispatch_get_main_queue(), ^{
[[UIApplication sharedApplication] setNetworkActivityIndicatorVisible:NO];
[_uploadProgress setProgress:0.5];
// Did we succeed?
if(!error) {
NSLog(#"Upload done");
} else {
NSLog(#"%#", error.description);
}
});
The progress bar also acts really wonky during all of this, rising and falling until eventually reaching 100.
Any clue what might be going on?
edit:
Here's the log after I added some more NSLog statements. The tasks call the delegate in the reverse order in which they were started:
2015-05-04 09:55:42.492 TS[348:95052] Starting background upload
2015-05-04 09:55:42.493 TS[348:95052] req dump: <NSMutableURLRequest: 0x1740138b0> { URL: https://example.com/upload }
2015-05-04 09:55:42.512 TS[348:95052] Enqueueing <__NSCFBackgroundUploadTask: 0x155d804c0>{ taskIdentifier: 3 }
2015-05-04 09:55:42.513 TS[348:95052] Post-resume
2015-05-04 09:55:42.513 TS[348:95052] req dump: <NSMutableURLRequest: 0x17400de00> { URL: https://example.com/upload }
2015-05-04 09:55:42.524 TS[348:95052] Enqueueing <__NSCFBackgroundUploadTask: 0x155e68db0>{ taskIdentifier: 4 }
2015-05-04 09:55:42.525 TS[348:95052] Post-resume
2015-05-04 09:56:01.726 TS[348:95052] ID: 4
2015-05-04 09:56:01.726 TS[348:95052] Upload done
2015-05-04 09:56:02.920 TS[348:95052] ID: 3
2015-05-04 09:56:02.920 TS[348:95052] Upload done

Answer 1

You don't see any network activity because the background session transfers data on a separate daemon that is managed by the system.
This is an excerpt from Apple Documentation:
"Once configured, your NSURLSession object seamlessly hands off upload and download tasks to the system at appropriate times. If tasks finish while your app is still running (either in the foreground or the background), the session object notifies its delegate in the usual way. If tasks have not yet finished and the system terminates your app, the system automatically continues managing the tasks in the background. If the user terminates your app, the system cancels any pending tasks."
You can read more here:
https://developer.apple.com/library/content/documentation/iPhone/Conceptual/iPhoneOSProgrammingGuide/BackgroundExecution/BackgroundExecution.html

Answer 2

Part of me feels really foolish and part of me is really confused.
It turns out that data was being sent. I was working with an incomplete upload endpoint, and when I finished implementing the endpoint, the pictures went through fine!
But for the entire transaction - uploading 10MB worth of pictures - the only network traffic that's showing in the debugger is a few KB for a smaller request I'm making before starting the tasks:
I guess I'll just file this one under "weird shit that Xcode does" and move on.