AFNetworking and background transfers

You have to make sure you do the necessary coding outlined in the Handling iOS Background Activity section of the URL Loading System Programming Guide says:

If you are using NSURLSession in iOS, your app is automatically relaunched when a download completes. Your app's application:handleEventsForBackgroundURLSession:completionHandler: app delegate method is responsible for recreating the appropriate session, storing a completion handler, and calling that handler when the session calls your session delegate's URLSessionDidFinishEventsForBackgroundURLSession: method.

That guide shows some examples of what you can do. Frankly, I think the code samples discussed in the latter part of the WWDC 2013 video What's New in Foundation Networking are even more clear.

The basic implementation of AFURLSessionManager will work in conjunction with background sessions if the app is merely suspended (you'll see your blocks called when the network tasks are done, assuming you've done the above). But as you guessed, any task-specific block parameters that are passed to the AFURLSessionManager method where you create the NSURLSessionTask for uploads and downloads are lost "if the app terminated or crashes."

For background uploads, this is an annoyance (as your task-level informational progress and completion blocks you specified when creating the task will not get called). But if you employ the session-level renditions (eg setTaskDidCompleteBlock and setTaskDidSendBodyDataBlock ), that will get called properly (assuming you always set these blocks when you re-instantiate the session manager).

As it turns out, this issue of losing the blocks is actually more problematic for background downloads, but the solution there is very similar (do not use task-based block parameters, but rather use session-based blocks, such as setDownloadTaskDidFinishDownloadingBlock ).

An alternative, you could stick with default (non-background) NSURLSession , but make sure your app requests a little time to finish the upload if the user leaves the app while the task is in progress. For example, before you create your NSURLSessionTask , you can create a UIBackgroundTaskIdentifier :

UIBackgroundTaskIdentifier __block taskId = [[UIApplication sharedApplication] beginBackgroundTaskWithExpirationHandler:^(void) {
    // handle timeout gracefully if you can

    [[UIApplication sharedApplication] endBackgroundTask:taskId];
    taskId = UIBackgroundTaskInvalid;
}];

But make sure that the completion block of the network task correctly informs iOS that it is complete:

if (taskId != UIBackgroundTaskInvalid) {
    [[UIApplication sharedApplication] endBackgroundTask:taskId];
    taskId = UIBackgroundTaskInvalid;
}

This is not as powerful as a background NSURLSession (eg, you have a limited amount of time available), but in some cases this can be useful.

First define your background manager.

//
//  BackgroundSessionManager.h
//
//  Created by Robert Ryan on 10/11/14.
//  Copyright (c) 2014 Robert Ryan. All rights reserved.
//

#import "AFHTTPSessionManager.h"

@interface BackgroundSessionManager : AFHTTPSessionManager

+ (instancetype)sharedManager;

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

@end

and

//
//  BackgroundSessionManager.m
//
//  Created by Robert Ryan on 10/11/14.
//  Copyright (c) 2014 Robert Ryan. All rights reserved.
//

#import "BackgroundSessionManager.h"

static NSString * const kBackgroundSessionIdentifier = @"com.domain.backgroundsession";

@implementation BackgroundSessionManager

+ (instancetype)sharedManager {
    static id sharedMyManager = nil;
    static dispatch_once_t onceToken;
    dispatch_once(&onceToken, ^{
        sharedMyManager = [[self alloc] init];
    });
    return sharedMyManager;
}

- (instancetype)init {
    NSURLSessionConfiguration *configuration = [NSURLSessionConfiguration backgroundSessionConfigurationWithIdentifier:kBackgroundSessionIdentifier];
    self = [super initWithSessionConfiguration:configuration];
    if (self) {
        [self configureDownloadFinished];            // when download done, save file
        [self configureBackgroundSessionFinished];   // when entire background session done, call completion handler
        [self configureAuthentication];              // my server uses authentication, so let's handle that; if you don't use authentication challenges, you can remove this
    }
    return self;
}

- (void)configureDownloadFinished {
    // just save the downloaded file to documents folder using filename from URL

    [self setDownloadTaskDidFinishDownloadingBlock:^NSURL *(NSURLSession *session, NSURLSessionDownloadTask *downloadTask, NSURL *location) {
        if ([downloadTask.response isKindOfClass:[NSHTTPURLResponse class]]) {
            NSInteger statusCode = [(NSHTTPURLResponse *)downloadTask.response statusCode];
            if (statusCode != 200) {
                // handle error here, e.g.

                NSLog(@"%@ failed (statusCode = %ld)", [downloadTask.originalRequest.URL lastPathComponent], statusCode);
                return nil;
            }
        }

        NSString *filename      = [downloadTask.originalRequest.URL lastPathComponent];
        NSString *documentsPath = NSSearchPathForDirectoriesInDomains(NSDocumentDirectory, NSUserDomainMask, YES)[0];
        NSString *path          = [documentsPath stringByAppendingPathComponent:filename];
        return [NSURL fileURLWithPath:path];
    }];

    [self setTaskDidCompleteBlock:^(NSURLSession *session, NSURLSessionTask *task, NSError *error) {
        if (error) {
            // handle error here, e.g.,

            NSLog(@"%@: %@", [task.originalRequest.URL lastPathComponent], error);
        }
    }];
}

- (void)configureBackgroundSessionFinished {
    typeof(self) __weak weakSelf = self;

    [self setDidFinishEventsForBackgroundURLSessionBlock:^(NSURLSession *session) {
        if (weakSelf.savedCompletionHandler) {
            weakSelf.savedCompletionHandler();
            weakSelf.savedCompletionHandler = nil;
        }
    }];
}

- (void)configureAuthentication {
    NSURLCredential *myCredential = [NSURLCredential credentialWithUser:@"userid" password:@"password" persistence:NSURLCredentialPersistenceForSession];

    [self setTaskDidReceiveAuthenticationChallengeBlock:^NSURLSessionAuthChallengeDisposition(NSURLSession *session, NSURLSessionTask *task, NSURLAuthenticationChallenge *challenge, NSURLCredential *__autoreleasing *credential) {
        if (challenge.previousFailureCount == 0) {
            *credential = myCredential;
            return NSURLSessionAuthChallengeUseCredential;
        } else {
            return NSURLSessionAuthChallengePerformDefaultHandling;
        }
    }];
}

@end

Make sure app delegate saves completion handler (instantiating the background session as necessary):

- (void)application:(UIApplication *)application handleEventsForBackgroundURLSession:(NSString *)identifier completionHandler:(void (^)())completionHandler {
    NSAssert([[BackgroundSessionManager sharedManager].session.configuration.identifier isEqualToString:identifier], @"Identifiers didn't match");
    [BackgroundSessionManager sharedManager].savedCompletionHandler = completionHandler;
}

Then start your downloads:

for (NSString *filename in filenames) {
    NSURL *url = [baseURL URLByAppendingPathComponent:filename];
    NSURLRequest *request = [NSURLRequest requestWithURL:url];
    [[[BackgroundSessionManager sharedManager] downloadTaskWithRequest:request progress:nil destination:nil completionHandler:nil] resume];
}

Note, I don't supply any of those task related blocks, because those aren't reliable with background sessions. (Background downloads proceed even after the app is terminated and these blocks have long disappeared.) One must rely upon the session-level, easily recreated setDownloadTaskDidFinishDownloadingBlock only.