Download large number of files in background in iOS

Question

I have an app in which I have to download a large number of files, from 400 to 900 files that are about 1GB total.

Which is the best approach to accomplish this?

1. One NSURLSession and all task enqueued in it?
2. One NSURLSession and enqueue tasks by packages (10 by 10 for example)?
3. Multiple NSURLSession with different queues?

Actually I have a NSURLSession within all task (one per file) enqueued, but sometimes I get a Lost connection to background transfer service.

Here is my code:

if([[UIDevice currentDevice] isMultitaskingSupported])
{
    __block UIBackgroundTaskIdentifier bgTask;

    UIApplication *application = [UIApplication sharedApplication];

    bgTask = [application beginBackgroundTaskWithExpirationHandler:^{

        [application endBackgroundTask:bgTask];
        bgTask = UIBackgroundTaskInvalid;
    }];

    dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{

        NSString *uuidString;
        CFUUIDRef uuid = CFUUIDCreate(nil);
        uuidString = CFBridgingRelease(CFUUIDCreateString(nil, uuid));
        CFRelease(uuid);
        //            }

        NSURLSessionConfiguration *sessionConfiguration;

        if (SYSTEM_VERSION_GREATER_THAN_OR_EQUAL_TO(@"8.0"))
        {
            sessionConfiguration = [NSURLSessionConfiguration backgroundSessionConfigurationWithIdentifier:@"com.fiveflamesmobile.bakgroundDownload"];
        }
        else
        {
            sessionConfiguration = [NSURLSessionConfiguration backgroundSessionConfiguration:@"com.fiveflamesmobile.bakgroundDownload"];
        }
        sessionConfiguration.HTTPMaximumConnectionsPerHost = 5;
        sessionConfiguration.sessionSendsLaunchEvents = YES;
        sessionConfiguration.discretionary = YES;
        sessionConfiguration.timeoutIntervalForResource = 0; //NO timeout
        sessionConfiguration.timeoutIntervalForRequest = 0; //No timeout
        sessionConfiguration.networkServiceType = NSURLNetworkServiceTypeBackground;

        self.session = [NSURLSession sessionWithConfiguration:sessionConfiguration
                                                     delegate:self
                                                delegateQueue:nil];

        NSLog(@"##### ------- Sesion created succesfully");

        //    [self batchDownloading];

        for (id<FFDownloadFileProtocol> file in self.selectedCatalogProducto.downloadInfo.arrayFiles)
        {
            [self startDownloadFile:file];
        }

        NSLog(@"##### ------- Download tasks created successfully ------");

        [application endBackgroundTask:bgTask];
        bgTask = UIBackgroundTaskInvalid;
    });
}

}

Answer 1

One NSURLSession - because you only want to handle session based things just once (auth for example).

One NSOperationQueue - with multiple operations running at the same time. (See property operationCount). It might be a little tricky implementing a NSOperation for the first time, but I'm sure this would be your best bet. https://developer.apple.com/library/mac/documentation/Cocoa/Reference/NSOperationQueue_class/index.html

http://nshipster.com/nsoperation/

Oh and by the way, this is a highly object oriented approach, which is always nice =)