Question

I am trying to asynchronously download images for a UITableViewCell, but it is currently setting the same image to each cell.

Please can you tell me the problem with my code:

- (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];
    }

    SearchObject *so = (SearchObject *)[_tableData objectAtIndex:indexPath.row];
    cell.textLabel.text = [[[[so tweet] stringByReplacingOccurrencesOfString:@"&quot;" withString:@"\""] stringByReplacingOccurrencesOfString:@"&lt;" withString:@"<"] stringByReplacingOccurrencesOfString:@"&gt;" withString:@">"];
    cell.detailTextLabel.text = [so fromUser];
    if (cell.imageView.image == nil) {
        NSURLRequest *req = [NSURLRequest requestWithURL:[NSURL URLWithString:[so userProfileImageURL]]];
        NSURLConnection *conn = [NSURLConnection connectionWithRequest:req delegate:self];
        [conn start];
    }
    if ([_cellImages count] > indexPath.row) {
        cell.imageView.image = [UIImage imageWithData:[_cellImages objectAtIndex:indexPath.row]];
    }
    return cell;
}
- (void)connection:(NSURLConnection *)connection didReceiveData:(NSData *)data {
    [_cellData appendData:data];
    [_cellImages addObject:_cellData];
}
- (void)connectionDidFinishLoading:(NSURLConnection *)connection {
    [self.tableView reloadData];
}

Answer 1

You are appending the data from every image downloaded to the same data object. So in the best case the data object ends up with the data for image #1 immediately followed by the data for image #2 and so on; the image decoder is apparently taking the first image in the chunk of data and ignoring the garbage after. You also seem to be unaware that NSURLConnections’ connection:didReceiveData: will not necessarily be called in the order that the connections were started, that connection:didReceiveData: can be called zero or multiple times per connection (and probably will if your images are more than a few kibibytes), and that tableView:cellForRowAtIndexPath: is not guaranteed to be called for every cell in the table in order. All of which are going to totally screw up your _cellImages array.

To do this right, you need to have a separate NSMutableData instance for each connection, and you need to add it to your _cellImages array just once, and at the correct index for the row rather than at the arbitrary next available index. And then in connection:didReceiveData: you need to figure out the correct NSMutableData instance to append to; this could be done by using the connection object (wrapped in an NSValue using valueWithNonretainedObject:) as the key in an NSMutableDictionary, or using objc_setAssociatedObject to attach the data object to the connection object, or by making yourself a class that handles all the management of the NSURLConnection for you and hands you the data object when complete.