objective c - iOS lazy-loading of table images

Question

I have successfully implemented lazy-loading of images in my app using the example provided by apple here. The thing is that, I want the image to load as I am scrolling, but the image loads into the cell only when I finish dragging (Release my finger from the screen. Until then the cell remains empty). I have modified the code as follows:

- (UITableViewCell *)tableView:(UITableView *)tableView cellForRowAtIndexPath:(NSIndexPath *)indexPath
{

    //NSLog(@"cell for row at indexpath, size - %f, current - %d", flowTable.contentSize.height, self.current);

    NSString *CellIdentifier = @"FlowCell";

    MyFlowCell *cell = (MyFlowCell *)[self.flowTable dequeueReusableCellWithIdentifier:CellIdentifier];

    if (cell == nil) {

        NSArray *nib = [[NSBundle mainBundle] loadNibNamed:@"FlowCell" owner:nil options:nil];
       // cell = [nib objectAtIndex:0];

        for(id currentObject in nib)

        {

        if([currentObject isKindOfClass:[MyFlowCell class]])

        {
            cell = (MyFlowCell *)currentObject;

            break;
        }
        }
    }

    ImageDetails *rowItem = [self.imageData objectAtIndex:indexPath.row];

    if (!rowItem.mainImage)
    {  
       // if (self.flowTable.dragging == NO && self.flowTable.decelerating == NO)
       // {

            [self startIconDownload:rowItem forIndexPath:indexPath];

        //}

        cell.mainImage.backgroundColor = [UIColor grayColor];

    }
    else
    {

            cell.mainImage.image = rowItem.mainImage;

    }
    }


    return cell;
}


- (void)scrollViewDidEndDragging:(UIScrollView *)scrollView willDecelerate:(BOOL)decelerate
{
    if (!decelerate)
    {
      //  [self loadImagesForOnscreenRows];
    }
}

- (void)scrollViewDidEndDecelerating:(UIScrollView *)scrollView
{
    //[self loadImagesForOnscreenRows];
}

Moreover, I found out that in my ImageDownloader class, the NSURLConnection doesn't even receive the response until I release my finger from the screen. I have been trying to find out why this is happening, but I haven't been successful so far. Please tell me if I've been missing something.

Answer 1

Updated answer:

The standard Apple LazyTableImages suffers from a few flaws:

1. It won't try to retrieve the images until the scrolling finished. I understand why they chose to do that (probably don't want to retrieve images that might be scrolling away), but this causes the images to appear more slowly than necessary.

2. It does not constrain the number of concurrent requests to some reasonable number. Yes, NSURLConnection will automatically freeze requests until the maximum number of requests falls to some reasonable number, but in a worst case scenario, you can actually have requests time out, as opposed to simply queueing properly.

3. Once it starts to download the image, it won't cancel it even if the cell subsequently scrolls off of the screen.

The simple fix is to retire IconDownloader and all of the scrolling/decelerating logic and just use the UIImageView category from either SDWebImage or AFNetworking.

If you're going to fix this yourself, it takes a little work to get all of this right. But here is a rendition of LazyTableImages that uses NSOperationQueue to ensure that we (a) limit concurrent requests; and (b) allows us to cancel requests. I confess I prefer the above UIImageView category implementations, better, but I tried to stay with the structure of Apple's original LazyTableImages, while remedying the flaws I've enumerated above.