AFNetworking and background transfers

I'm a bit confuse of how to take advantage of the new iOS 7 NSURLSession background transfers features and AFNetworking (versions 2 and 3).

I saw the WWDC 705 - What’s New in Foundation Networking session, and they demonstrated background download that continues after the app terminated or even crashes.

This is done using the new API application:handleEventsForBackgroundURLSession:completionHandler: and the fact that the session's delegate will eventually get the callbacks and can complete its task.

So I'm wondering how to use it with AFNetworking (if possible) to continue downloading in background.

The problem is, AFNetworking conveniently uses block based API to do all the requests, but if the app terminated or crashes those block are also gone. So how can I complete the task?

Or maybe I'm missing something here...

Let me explain what I mean:

For example my app is a photo messaging app, lets say that I have a PhotoMessage object that represent one message and this object has properties like

state - describe the state of the photo download.
resourcePath - the path to the final downloaded photo file.

So when I get a new message from the server, I create a new PhotoMessage object, and start downloading its photo resource.

PhotoMessage *newPhotoMsg = [[PhotoMessage alloc] initWithInfoFromServer:info];
newPhotoMsg.state = kStateDownloading;

self.photoDownloadTask = [[BGSessionManager sharedManager] downloadTaskWithRequest:request progress:nil destination:^NSURL *(NSURL *targetPath, NSURLResponse *response) {
    NSURL *filePath = // some file url
    return filePath;
} completionHandler:^(NSURLResponse *response, NSURL *filePath, NSError *error) {
    if (!error) {
        // update the PhotoMessage Object
        newPhotoMsg.state = kStateDownloadFinished;
        newPhotoMsg.resourcePath = filePath;
    }
}];

[self.photoDownloadTask resume];

As you can see, I use the completion block to update that PhotoMessage object according to the response I get.

How can I accomplish that with a background transfer? This completion block won't be called and as a result, I can't update the newPhotoMsg.

A couple of thoughts:

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 (e.g. 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 (e.g., you have a limited amount of time available), but in some cases this can be useful.

Update:

I thought I'd add a practical example of how to do background downloads using AFNetworking.

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.

Clearly this is a simple example (only one background session object; just saving files to the docs folder using last component of URL as the filename; etc.), but hopefully it illustrates the pattern.

It shouldn't make any difference whether or not the callbacks are blocks or not. When you instantiate an AFURLSessionManager, make sure to instantiate it with NSURLSessionConfiguration backgroundSessionConfiguration:. Also, make sure to call the manager's setDidFinishEventsForBackgroundURLSessionBlock with your callback block - this is where you should write the code typically defined in NSURLSessionDelegate's method: URLSessionDidFinishEventsForBackgroundURLSession:(NSURLSession *)session. This code should invoke your app delegate's background download completion handler.

One word of advice regarding background download tasks - even when running in the foreground, their timeouts are ignored, meaning you could get "stuck" on a download that's not responding. This is not documented anywhere and drove me crazy for some time. The first suspect was AFNetworking but even after calling NSURLSession directly, the behaviour remained the same.

Good luck!

AFNetworking and background transfers

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