Question

I have an iOS app that has a UITableView with custom TableViewCells that contain a UIImageView. The image is loaded from a web service, so during the initial load, I display a “loading” image, and then use gcd to dispatch and get the image matching the data for that cell.

When I use a DISPATCH_QUEUE_PRIORITY_HIGH global queue to perform the image fetch, I sporadically get the wrong images loading in the tableview cells. If I use my own custom queue then the correct images get populated into the cells but the tableview performance is awful.

Here is the code…

    // See if the icon is in the cache
if([self.photoCache objectForKey:[sample valueForKey:@"api_id"]]){
    [[cell sampleIcon]setImage:[self.photoCache objectForKey:[sample valueForKey:@"api_id"]]];
}
else {
    NSLog(@"Cache miss");
        [cell.sampleIcon setImage:nil];
        dispatch_queue_t cacheMissQueue = dispatch_queue_create("cacheMissQueue", NULL);
        //dispatch_queue_t cacheMissQueue = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_HIGH, 0ul);
        dispatch_async(cacheMissQueue, ^{
            if(sample.thumbnailFilename && sample.api_id){
                NSData *thumbNailData = [[NSData alloc] initWithContentsOfFile:sample.thumbnailFilename];
                UIImage *thumbNailImage = [[UIImage alloc]initWithData:thumbNailData];
                if(thumbNailImage){
                    // Set the cell
                    dispatch_sync(dispatch_get_main_queue(), ^{
                        [[cell sampleIcon]setImage:thumbNailImage];
                        [cell setNeedsLayout];
                    });
                    // save it to cache for future references
                    NSLog(@"DEBUG: Saving to cache %@ for sample %@",sample.thumbnailFilename,[sample objectID]);
                    [self.photoCache setObject:thumbNailImage forKey:sample.api_id];
                }
            }
        });
        dispatch_release(cacheMissQueue);
}

Answer 1

Watching the WWDC 2012 session #211 helped a lot and I changed the code from using GCD to NSOperationQueue and it solved the problem.

New code…

[[self imgQueue]addOperationWithBlock:^{
            if(sample.thumbnailFilename && sample.api_id){
                NSData *thumbNailData = [[NSData alloc] initWithContentsOfFile:sample.thumbnailFilename];
                UIImage *thumbNailImage = [[UIImage alloc]initWithData:thumbNailData];
                if(thumbNailImage){
                    // Set the cell
                    [[NSOperationQueue mainQueue]addOperationWithBlock:^{
                        [[cell sampleIcon]setImage:thumbNailImage];
                        [cell setNeedsLayout];
                    }];
                    // save it to cache for future references
                    [self.photoCache setObject:thumbNailImage forKey:sample.api_id];
                }
            }

        }];