infectmac.com blog post code sample about UITableView and GCD race condition

UITableViewRace.m

- (UITableViewCell *)tableView:(UITableView *)tableView cellForRowAtIndexPath:(NSIndexPath *)indexPath {
    static NSString *CellIdentifier = @"Cell";
    
    UITableViewCell *cell = [tableView dequeueReusableCellWithIdentifier:CellIdentifier];
    if (cell == nil) {
        cell = [[[UITableViewCell alloc] initWithStyle:UITableViewCellStyleSubtitle reuseIdentifier:CellIdentifier] autorelease];
    }

    NSDictionary* pictObj = [self.pictures objectAtIndex:indexPath.row];
    NSString* title = [pictObj objectForKey:@"title"];
    title = [title length] == 0 ? @"(no title)" : title;
    cell.textLabel.text = title;
    cell.detailTextLabel.text = [[pictObj objectForKey:@"description"] objectForKey:@"_content"];
    cell.accessoryType = UITableViewCellAccessoryDisclosureIndicator;

    //identify which image the cell should be displaying right now
    cell.tag = indexPath.row;

    Photo* photo = [Photo photoWithFlickrData:pictObj inManagedObjectContext:context];
    if (photo.thumbnail) {
        cell.imageView.image = [UIImage imageWithData:photo.thumbnail];
    }
    else {
        cell.imageView.image = nil;
        [photo thumbnailWithBlock:^(UIImage* image) {
            //check to make sure the image should be updated
            if (cell.tag == indexPath.row) {
                cell.imageView.image = image;
                [self.tableView reloadRowsAtIndexPaths:[NSArray arrayWithObject:indexPath] withRowAnimation:UITableViewRowAnimationNone];
            }
        }];
    }

    return cell;
}