iphone - Last In-First Out Stack with GCD?

Question

I have a UITableView that displays images associated with contacts in each row. In some cases these images are read on first display from the address book contact image, and where there isn't one they are an avatar rendered based on stored data. I presently have these images being updated on a background thread using GCD. However, this loads the images in the order they were requested, which means during rapid scrolling the queue becomes lengthy and when the user stops scrolling the current cells are the last to get updated. On the iPhone 4, the problem isn't really noticeable, but I am keen to support older hardware and am testing on an iPhone 3G. The delay is tolerable but quite noticeable.

It strikes me that a Last In-First Out stack would seem likely to largely resolve this issue, as whenever the user stopped scrolling those cells would be the next to be updated and then the others that are currently off-screen would be updated. Is such a thing possible with Grand Central Dispatch? Or not too onerous to implement some other way?

Note, by the way, that I am using Core Data with a SQLite store and I am not using an NSFetchedResultsController because of a many-to-many relationship that has to be traversed in order to load the data for this view. (As far as I am aware, that precludes using an NSFetchedResultsController.) [I've discovered an NSFetchedResultsController can be used with many-to-many relationships, despite what the official documentation appears to say. But I'm not using one in this context, yet.]

Addition: Just to note that while the topic is "How do I create a Last In-First Out Stack with GCD", in reality I just want to solve the issue outlined above and there may be a better way to do it. I am more than open to suggestions like timthetoolman's one that solves the problem outlined in another way; if such a suggestion is finally what I use I'll recognize both the best answer to the original question as well as the best solution I ended up implementing... :)

Answer 1

Because of the memory constraints of the device, you should load the images on demand and on a background GCD queue. In the cellForRowAtIndexPath: method check to see if your contact's image is nil or has been cached. If the image is nil or not in cache, use a nested dispatch_async to load the image from the database and update the tableView cell.

- (UITableViewCell *)tableView:(UITableView *)tableView cellForRowAtIndexPath:(NSIndexPath*)indexPath
   {
       static NSString *CellIdentifier = @"Cell";
       UITableViewCell *cell = [tableView dequeueReusableCellWithIdentifier:CellIdentifier];
       if (cell == nil) {
            cell = [[UITableViewCell alloc] initWithStyle:UITableViewCellStyleDefault reuseIdentifier:CellIdentifier];
       }
       // If the contact object's image has not been loaded, 
       // Use a place holder image, then use dispatch_async on a background queue to retrieve it.

       if (contact.image!=nil){
           [[cell imageView] setImage: contact.image];
       }else{
           // Set a temporary placeholder
           [[cell imageView] setImage:  placeHolderImage];

           // Retrieve the image from the database on a background queue
           dispatch_queue_t queue = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_HIGH, 0);
           dispatch_async(queue, ^{
               UIImage *image = // render image;
               contact.image=image;

               // use an index path to get at the cell we want to use because
               // the original may be reused by the OS.
               UITableViewCell *theCell=[tableView cellForRowAtIndexPath:indexPath];

               // check to see if the cell is visible
               if ([tableView visibleCells] containsObject: theCell]){
                  // put the image into the cell's imageView on the main queue
                  dispatch_async(dispatch_get_main_queue(), ^{
                     [[theCell imageView] setImage:contact.image];
                     [theCell setNeedsLayout];
                  });
               }
           }); 
       }
       return cell;
}

The WWDC2010 conference video "Introducing Blocks and Grand Central Dispatch" shows an example using the nested dispatch_async as well.

another potential optimization could be to start downloading the images on a low priority background queue when the app launches. i.e.

 // in the ApplicationDidFinishLaunchingWithOptions method
 // dispatch in on the main queue to get it working as soon
 // as the main queue comes "online".  A trick mentioned by
 // Apple at WWDC

 dispatch_async(dispatch_get_main_queue(), ^{
        // dispatch to background priority queue as soon as we
        // get onto the main queue so as not to block the main
        // queue and therefore the UI
        dispatch_queue_t lowPriorityQueue = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_BACKGROUND, 0)
        dispatch_apply(contactsCount,lowPriorityQueue ,^(size_t idx){
               // skip the first 25 because they will be called
               // almost immediately by the tableView
               if (idx>24){
                  UIImage *renderedImage =/// render image
                  [[contactsArray objectAtIndex: idx] setImage: renderedImage];
               }

        });
 });

With this nested dispatch, we are rendering the images on an extremely low priority queue. Putting the image rendering on the background priority queue will allow the images being rendered from the cellForRowAtIndexPath method above to be rendered at a higher priority. So, because of the difference in priorities of the queues, you will have a "poor mans" LIFO.

Good luck.