How to get server response data in NSURLSession without completion block

A couple of thoughts:

First, instantiate your session with a delegate, because background sessions must have a delegate:

NSURLSessionConfiguration *configuration = [NSURLSessionConfiguration backgroundSessionConfigurationWithIdentifier:kSessionIdentifier];self.session = [NSURLSession sessionWithConfiguration:configuration delegate:self delegateQueue:nil];

Second, instantiate your NSURLSessionUploadTask without a completion handler, because tasks added to a background session cannot use completion blocks. Also note, I'm using a file URL rather than a NSData:

NSURLSessionTask *task = [self.session uploadTaskWithRequest:request fromFile:fileURL];[task resume];

Third, implement the relevant delegate methods. At a minimum, that might look like:

- (void)URLSession:(NSURLSession *)session dataTask:(NSURLSessionDataTask *)dataTask didReceiveData:(NSData *)data {    NSMutableData *responseData = self.responsesData[@(dataTask.taskIdentifier)];    if (!responseData) {        responseData = [NSMutableData dataWithData:data];        self.responsesData[@(dataTask.taskIdentifier)] = responseData;    } else {        [responseData appendData:data];    }}- (void)URLSession:(NSURLSession *)session task:(NSURLSessionTask *)task didCompleteWithError:(NSError *)error {    if (error) {        NSLog(@"%@ failed: %@", task.originalRequest.URL, error);    }    NSMutableData *responseData = self.responsesData[@(task.taskIdentifier)];    if (responseData) {        // my response is JSON; I don't know what yours is, though this handles both        NSDictionary *response = [NSJSONSerialization JSONObjectWithData:responseData options:0 error:nil];        if (response) {            NSLog(@"response = %@", response);        } else {            NSLog(@"responseData = %@", [[NSString alloc] initWithData:responseData encoding:NSUTF8StringEncoding]);        }        [self.responsesData removeObjectForKey:@(task.taskIdentifier)];    } else {        NSLog(@"responseData is nil");    }}

Note, the above is taking advantage of a previously instantiated NSMutableDictionary called responsesData (because, much to my chagrin, these "task" delegate methods are done at the "session" level).

Finally, you want to make sure to define a property to store the completionHandler provided by handleEventsForBackgroundURLSession:

@property (nonatomic, copy) void (^backgroundSessionCompletionHandler)(void);

And obviously, have your app delegate respond to handleEventsForBackgroundURLSession, saving the completionHandler, which will be used below in the URLSessionDidFinishEventsForBackgroundURLSession method.

- (void)application:(UIApplication *)application handleEventsForBackgroundURLSession:(NSString *)identifier completionHandler:(void (^)())completionHandler {    // This instantiates the `NSURLSession` and saves the completionHandler.     // I happen to be doing this in my session manager, but you can do this any    // way you want.    [SessionManager sharedManager].backgroundSessionCompletionHandler = completionHandler;}

And then make sure your NSURLSessionDelegate calls this handler on the main thread when the background session is done:

- (void)URLSessionDidFinishEventsForBackgroundURLSession:(NSURLSession *)session {    if (self.backgroundSessionCompletionHandler) {        dispatch_async(dispatch_get_main_queue(), ^{            self.backgroundSessionCompletionHandler();            self.backgroundSessionCompletionHandler = nil;        });    }}

This is only called if some of the uploads finished in the background.

There are a few moving parts, as you can see, but that's basically what's entailed.