Asynchronous Image Loading in UICollectionViewCell

UICollectionView Lazy Loading

-(UICollectionViewCell *)collectionView:(UICollectionView *)collectionView cellForItemAtIndexPath:(NSIndexPath *)indexPath {
    static NSString *cellIdentifier = @"AfterpartyPhotoCell";
    AfterpartyPhotoCell *cell = (AfterpartyPhotoCell *)[collectionView dequeueReusableCellWithReuseIdentifier:cellIdentifier forIndexPath:indexPath];
    if (!cell)
        cell = [[AfterpartyPhotoCell alloc] init];
    
    [cell.downloadIndicator startAnimating];
    cell.downloadIndicator.center = cell.center;
    cell.imageView.contentMode = UIViewContentModeScaleAspectFill;
    dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_HIGH, 0), ^{
        BOOL shouldDownload = YES;
        UIImage *image = _thumbnailCacheDict[[NSString stringWithFormat:@"%lld", (long long)indexPath.item]];
        if (image != nil) {
            shouldDownload = NO;
            CGSize newSize = [self sizePhotoForColumn:image.size];
            dispatch_async(dispatch_get_main_queue(), ^{
                CGRect frame = cell.imageView.frame;
                frame.size = newSize;
                cell.imageView.frame = frame;
                cell.layer.borderColor = [[UIColor whiteColor] CGColor];
                cell.layer.borderWidth = 0.5f;
                [cell.downloadIndicator stopAnimating];
                cell.downloadIndicator.hidden = YES;
                [cell.imageView setImage:image];
            });
        }
        if (shouldDownload) {
            AfterpartyPhotoInfo *photoInfo = _photoMetadata[indexPath.item];
            CGSize newSize = [self sizePhotoForColumn:[photoInfo size]];
            [_cm downloadImageForRefID:[photoInfo thumbID] completion:^(NSData *imageData, NSError *error) {
                if (!error && imageData != nil) {
                    UIImage *image = [[UIImage alloc] initWithData:imageData];
                    [_thumbnailCacheDict setObject:image forKey:[NSString stringWithFormat:@"%lld", (long long)indexPath.item]];
                    dispatch_async(dispatch_get_main_queue(), ^{
                        CGRect frame = cell.imageView.frame;
                        frame.size = newSize;
                        cell.imageView.frame = frame;
                        cell.layer.borderColor = [[UIColor whiteColor] CGColor];
                        cell.layer.borderWidth = 0.5f;
                        [cell.downloadIndicator stopAnimating];
                        cell.downloadIndicator.hidden = YES;
                        [cell.imageView setImage:image];
                    });
                }
            }];
        }
    });

    return cell;
}