something.m

@implementation MainViewController {
  NSURLSession *_session;
}

- (void) doSomethingElse
{
  [self doSomethingWithCompletionHandler: ^(){
    // create session.. add download tasks..
    // _session = blah blah
    // _session.createDownloadTask etc
    
    [self performSelector:@selector(cancel) withObject: nil afterDelay: 1];
  }];
}

- (void) cancel
{
  if(_session) {
    [_session invalidateAndCancel];
  }
}

@end