Question

I’m developing a demo RSS reader for iPhone. I obviously have a tableview to display the feeds, and then a detailed view. Some of this feeds have a thumbnail that I want to display on cell.imageview of the table, and some don’t.

The problem is that when scrolling the table, loaded thumbnails start repeating on other cells, and I end up with a thumbnail on every cell.

Here’s a piece of my code. I may upload screenshots later

   - (UITableViewCell *)tableView:(UITableView *)tableView cellForRowAtIndexPath:(NSIndexPath *)indexPath{
        UITableViewCell * cell = [tableView dequeueReusableCellWithIdentifier: @"rssItemCell"];
        if(nil == cell){
            cell = [[[UITableViewCell alloc] initWithStyle:UITableViewCellStyleSubtitle reuseIdentifier:@"rssItemCell"]autorelease];
        }
        BlogRss *item = [[[self rssParser]rssItems]objectAtIndex:indexPath.row];
        cell.textLabel.text = [item title];
        cell.detailTextLabel.text = [item description];
        cell.accessoryType = UITableViewCellAccessoryDisclosureIndicator;

        // Thumbnail if exists
        if(noticia.imagePath != nil){
            NSData* imageData;
            @try {
                imageData = [[NSData alloc]initWithContentsOfURL:[NSURL URLWithString:item.imagePath]];
            }
            @catch (NSException * e) {
                //Some error while downloading data
            }
            @finally {
                item.image = [[UIImage alloc] initWithData:imageData];
                [imageData release];
            }
        }

        if(item.image != nil){
            [[cell imageView] setImage:item.image];
        }
        return cell;
    }

Any help will be very appreciated.

Answer 1

Easy one.

Cells are reused. Just ensure you clear the cell.imageView.image each time you fill the cell. You may just need to remove the if(item.image!=nil) line.

For a production application you probably also want to fetch the images in the background and implement a simple cache. There are plenty of examples of how to do that knocking around.

EDIT

RickiG makes a lot of good points : cellForRowAtIndexPath should be displaying the data from the model and as little as possible else!

The concept of just supplying a view-ready model is kind of good (I do it with ASP.NET MVC), but needs to be balanced against the JustInTime memory minimization techniques of iPhone and lets face it, you are not committing the real sin of trying to read back data from the controls on your tableview – that really doesn’t work!

The worst thing you are doing is reading web data on cellForRowAtIndexPath as that will block the UK. Instead you should display a blank or placeholder image and trigger a background fetch that will update the model with the data, and then trigger a reload – preferably of the specific cell.