iOS 7 NSURLSession Download multiple files in Background

Question

I want to download a List of files using NSUrlSession.

I have a variable for counting the successful downloads @property (nonatomic) int downloadsSuccessfulCounter;. While the files are being downloaded I disable the Download Button. When the counter is equal to the download list size, I enable the button again and set the counter to 0. I do this in the method:

-(void)URLSession:(NSURLSession *)session downloadTask:(NSURLSessionDownloadTask *)downloadTask didFinishDownloadingToURL:(NSURL *)location {

...

    [[NSOperationQueue mainQueue] addOperationWithBlock:^ {

        downloadsSuccessfulCounter++;

        if(downloadsSuccessfulCounter == self.downloadList.count) {
            NSLog(@"All downloads finished");

            [self.syncButton setEnabled:YES];

             downloadsSuccessfulCounter = 0;
        }
    }];

}

Everything is working fine, but when I open again the ViewController I get the message A background URLSession with identifier com.myApp already exists!. The counter is not set to 0 and the UI elements (UIButtons, UILabels) are not responding.

I guess the problem is because the NSURLSession is still open but I'm not really sure about how it works.

I have tried all the tutorials, but 99% of them are only for downloading 1 file, not more than 1... Any ideas?

Here is my code:

...    
@property (nonatomic, strong) NSURLSession *session;
...

    - (void)viewDidLoad {
        [super viewDidLoad];

        appDelegate = (AppDelegate *)[[UIApplication sharedApplication] delegate];

        self.downloadList = [[NSMutableArray alloc] init];

        NSURLSessionConfiguration *sessionConfiguration = [NSURLSessionConfiguration backgroundSessionConfiguration:@"com.myApp"];
        sessionConfiguration.HTTPMaximumConnectionsPerHost = 5;
        self.session = [NSURLSession sessionWithConfiguration:sessionConfiguration delegate:self delegateQueue:nil];
}

When I press the Download ButtonI call this method ( I have a Downloadable object which contains a NSURLSessionDownloadTask):

-(void)startDownload {

    for (int i=0; i<[self.downloadList count]; i++) {
        Downloadable *d = [self.downloadList objectAtIndex:i];

        if (!d.isDownloading) {
            if (d.taskIdentifier == -1) {
                d.downloadTask = [self.session downloadTaskWithURL:[NSURL URLWithString:d.downloadSource]];

            }else {
                d.downloadTask = [self.session downloadTaskWithResumeData:fdi.taskResumeData];
            }

            d.taskIdentifier = d.downloadTask.taskIdentifier;
            [d.downloadTask resume];
            d.isDownloading = YES;
        }
    }
}

When the app is in Background:

-(void)URLSessionDidFinishEventsForBackgroundURLSession:(NSURLSession *)session{
    AppDelegate *appDelegate = [UIApplication sharedApplication].delegate;

    [self.session getTasksWithCompletionHandler:^(NSArray *dataTasks, NSArray *uploadTasks, NSArray *downloadTasks) {

        if ([downloadTasks count] == 0) {
            if (appDelegate.backgroundTransferCompletionHandler != nil) {

                void(^completionHandler)() = appDelegate.backgroundTransferCompletionHandler;

                appDelegate.backgroundTransferCompletionHandler = nil;

                [[NSOperationQueue mainQueue] addOperationWithBlock:^{               
                    completionHandler();

                    UILocalNotification *localNotification = [[UILocalNotification alloc] init];
                    localNotification.alertBody = @"All files downloaded";
                    [[UIApplication sharedApplication] presentLocalNotificationNow:localNotification];

                }];
            }
        }
    }];
}

Answer 1

So, as I mentioned in my comments, the issue is that each File requires a unique NSURLSession, and each NSURLSession requires a NSURLSessionConfiguration with a unique identifier.

I think that you were close - and probably more proper than me in certain aspects... You just need to create a structure to pass unique IDs into unique Configurations, to populate unique Sessions (say that 10x fast).

Here's what I did:

/* * Retrieves the List of Files to Download * Also uses the size of that list to instantiate items * In my case, I load a character returned text file with the names of the files that I want to download */

- (void) getMediaList {

    NSString *list = @"http://myserver/media_list.txt";
    NSURLSession *session = [NSURLSession sharedSession]; // <-- BASIC session
    [[session dataTaskWithURL:[NSURL URLWithString:list]
            completionHandler:^(NSData *data, NSURLResponse *response, NSError *error) {

                NSString *stringFromData = [[NSString alloc] initWithData: data encoding: NSUTF8StringEncoding];

                // Populate Arrays
                REMOTE_MEDIA_FILE_PATHS = [stringFromData componentsSeparatedByString:@"\n"];
                [self instantiateURLSessions:[REMOTE_MEDIA_FILE_PATHS count]]; 


                // Start First File
                [self getFile:[REMOTE_MEDIA_FILE_PATHS objectAtIndex:downloadCounter]:downloadCounter]; // this variable is 0 at the start
            }]
     resume];
}

/* * This sets Arrays of Configurations and Sessions to the proper size * It also gives a unique ID to each one */

- (void) instantiateURLSessions : (int) size {

    NSMutableArray *configurations = [NSMutableArray array];
    NSMutableArray *sessions = [NSMutableArray array];

    for (int i = 0; i < size; i++) {
        NSString *index = [NSString stringWithFormat:@"%i", i];
        NSString *UniqueIdentifier = @"MyAppBackgroundSessionIdentifier_";
        UniqueIdentifier = [UniqueIdentifier stringByAppendingString:index];

        [configurations addObject: [NSURLSessionConfiguration backgroundSessionConfigurationWithIdentifier:UniqueIdentifier]];
        [sessions addObject:[NSURLSession sessionWithConfiguration: [configurations objectAtIndex:i]  delegate: self delegateQueue: [NSOperationQueue mainQueue]]];
    }

    NSURL_BACKGROUND_CONFIGURATIONS = [NSArray arrayWithArray:configurations];
    NSURL_BACKGROUND_SESSIONS = [NSArray arrayWithArray:sessions];
}

/* * This sets up the Download task for each file, based off of the index of the array * It also concatenates the path to the actual file */

- (void) getFile : (NSString*) file :(int) index {
    NSString *fullPathToFile = REMOTE_MEDIA_PATH; // Path To Server With Files
    fullPathToFile = [fullPathToFile stringByAppendingString:file];

    NSURL *url = [NSURL URLWithString:fullPathToFile];
    NSURLSessionDownloadTask *downloadTask = [[NSURL_BACKGROUND_SESSIONS objectAtIndex:index ] downloadTaskWithURL: url];
    [downloadTask resume];

}

/* * Finally, in my delegate method, upon the completion of the download (after the file is moved from the temp data), I check if I am done and if not call the getFiles method again with the updated counter for the index */

-(void)URLSession:(NSURLSession *)session downloadTask:(NSURLSessionDownloadTask *)downloadTask didFinishDownloadingToURL:(NSURL *)location
{

    // Get the documents directory URL
    NSArray *paths = NSSearchPathForDirectoriesInDomains(NSDocumentDirectory, NSUserDomainMask, YES);
    NSString *documentsDirectory = [paths objectAtIndex:0];
    NSString *dataPath = [documentsDirectory stringByAppendingPathComponent:LOCAL_MEDIA_PATH];
    NSURL *customDirectory = [NSURL fileURLWithPath:dataPath];

    // Get the file name and create a destination URL
    NSString *sendingFileName = [downloadTask.originalRequest.URL lastPathComponent];
    NSURL *destinationUrl = [customDirectory URLByAppendingPathComponent:sendingFileName];

    // Move the file
    NSError *error = nil;
    NSFileManager *fileManager = [NSFileManager defaultManager];
    if ([fileManager moveItemAtURL:location toURL:destinationUrl error: &error]) {

        // List
        [self listCustomDirectory];

        if(downloadCounter < [REMOTE_MEDIA_FILE_PATHS count] -1) {
            // Increment Counter
            downloadCounter++;

            // Start Next File
            [self getFile:[REMOTE_MEDIA_FILE_PATHS objectAtIndex:downloadCounter]:downloadCounter];
        }
        else {
            // FINISH YOUR OPERATION / NOTIFY USER / ETC
        }
    }
    else {
        NSLog(@"Damn. Error %@", error);
        // Do Something Intelligent Here
    }  
}