Program crashes when segueing back and dispatch queue has animation method - objective-c

Question

Ok so i have a uitableview and when an item is selected it segues to a new view controller to show an image (inside of a uiscrollview). The image begins downloading in a dispatch_queue from prepareforsegue. To be clear I am doing all UI updates in the main queue. The problem is that if i hit the back button quick enough then my program crashes with an exc_bad_address. I think the problem has to deal with zoomToRect animated:YES because when i set animated to NO i cant get it to crash. Plus the call stack below deals with animation. What is the best way to go about fixing this and is there a better way to get what i need done?
Also when debugging the problem print 'Block completed' and then crashes shortly after.
stack trace
Here is the method called. It is called in a setter of the destination controller in prepareForSegue.
-(void) updateDisplay {
dispatch_queue_t queue = dispatch_queue_create("Load Flickr Photo", NULL);
UIActivityIndicatorView *spinner = [[UIActivityIndicatorView alloc] initWithActivityIndicatorStyle:UIActivityIndicatorViewStyleGray];
self.navigationItem.rightBarButtonItem = [[UIBarButtonItem alloc] initWithCustomView:spinner];
[spinner startAnimating];
dispatch_async(queue, ^{
UIImage *image = [FlickrFetcher imageForPhoto:self.currentPhoto format:FlickrPhotoFormatLarge];
dispatch_async(dispatch_get_main_queue(), ^{
self.navigationItem.rightBarButtonItem = nil;
self.imageView.image = image;
self.title = [FlickrFetcher titleForPhoto:self.currentPhoto];
CGAffineTransform transform = CGAffineTransformMakeScale(1.0, 1.0);
self.imageView.transform = transform;
self.imageView.frame = CGRectMake(0, 0, self.imageView.image.size.width, self.imageView.image.size.height);
self.scrollView.maximumZoomScale = 4.0;
self.scrollView.minimumZoomScale = .2;
self.scrollView.zoomScale = 1;
self.scrollView.contentSize = self.imageView.bounds.size;
//i think problem is here
[self.scrollView zoomToRect:self.imageView.frame animated:YES];
NSLog(#"Block completed");
});
});
}

Answer 1

I think a potential problem could be that while your block retains your self the view controller while its active in queue...
If you send -zoomToRect:animated:NO, the block will still be active and blocking on the call which ensures that all the objects are still valid in memory.
If you send -zoomToRect:animated:YES, the block will exit potentially generating a race condition where your view controller would be released since storyboards will also release your view controller when you go back in a segue leaving it with an effective retain count of zero.