Memory Leak in Continuous Downloads with NSURLSession (iOS8)

#import "ViewController.h"

#define URL_MODEL_STRING@"http://sample.com/sample_%03d.pdf"
#define MAX_FILE_NUMBER60

@ interface ViewController()<NSURLSessionDownloadDelegate>{
    NSURLSessionConfiguration*_conf;
    NSURLSession*_session;
    NSURLSessionDownloadTask*_downloadTask;

    int_currentFileNumber;

   __weak IBOutlet UIButton*_startButton;
}

@end

@implementation ViewController

- (void) viewDidLoad{
    superviewDidLoad;

    _conf = NSURLSessionConfiguration defaultSessionConfiguration;
}


- (IBAction) startButtonPushed: (id) sender {

    _startButton.enabled = NO;

    _session = [NSURLSession sessionWithConfiguration:_conf]
                                             delete —self
                                        deleteQueue: [NSOperationQueue mainQueue];
    _currentFileNumber=1;

    [self downloadFileWithNumber:_currentFileNumber];
}


- (void) downloadFileWithNumber: (int) fileNumber {
    NSURL*url = [self urlWithFileNumber: fileNumber];
    _downloadTask=[_session downloadTaskWithURL:url];

    [_downloadTask resume];
}


- (NSURL*) urlWithFileNumber: (int) fileNumber {
    return [NSURL URLWithString: [NSString stringWithFormat: URL_MODEL_STRING, fileNumber]];
}


# pragma mark - NSURLSessionDownloadDelegate

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

    NSLog(@"fileNumber:%d location:%@",_currentFileNumber, location);

    // TODO: Move downloaded temporary file to another place.


    if(_currentFileNumber==MAX_FILE_NUMBER){
        NSLog(@"Download complete.");
        [_session invalidateAndCancel];
        _startButton.enabled = YES;
        return;
    }

    _currentFileNumber++;

    [self downloadFileWithNumber:_currentFileNumber];
}

- (void) DidReceiveMemoryWarning {
    super didReceiveMemoryWarning;

    NSLog (@"didReceiveMemoryWarning");
}

@end