Images from URLs (RSS Reader) displaying in table view - problem - objective-c

Question

I'm having a problem with a table view and some custom cells. Here is my code:
- (UITableViewCell *)tableView:(UITableView *)tableView cellForRowAtIndexPath:(NSIndexPath *)indexPath {
NSLog(#"cellForRowAtIndexPath");
static NSString *MyIdentifier = #"customCell";
EventTableCell *cell = (EventTableCell *)[tableView dequeueReusableCellWithIdentifier:MyIdentifier];
if (cell == nil) {
//cell = [[[UITableViewCell alloc] initWithFrame:CGRectZero reuseIdentifier:MyIdentifier] autorelease];
[[NSBundle mainBundle] loadNibNamed:#"EventTableCell" owner:self options:nil];
cell = self.eventCell;
}
//Set up the cell
int storyIndex = [indexPath indexAtPosition: [indexPath length] - 1];
[[cell eventNameLabel] setText:[[stories objectAtIndex: storyIndex] objectForKey: #"title"]];
[[cell eventDateLabel] setText:[[stories objectAtIndex: storyIndex] objectForKey: #"date"]];
NSString * storyLink = [[stories objectAtIndex: storyIndex] objectForKey: #"link"];
// clean up the link - get rid of spaces, returns, and tabs...
storyLink = [storyLink stringByReplacingOccurrencesOfString:#" " withString:#""];
storyLink = [storyLink stringByReplacingOccurrencesOfString:#"\n" withString:#""];
storyLink = [storyLink stringByReplacingOccurrencesOfString:#" " withString:#""];
//NSString *string = [[NSString alloc] stringWithString:[[stories objectAtIndex: storyIndex] objectForKey: #"link"]];
NSURL *url = [NSURL URLWithString:storyLink];
NSData *data = [[NSData alloc] initWithContentsOfURL: url];
UIImage *image = [UIImage imageWithData: data];
[[cell eventImage] setImage:image];
return cell;
}
My table has a lot of these custom cells in it, more than the view can show at once. So therefore I need to scroll up/down to see all the cells. My problem is this, when I scroll it lags a lot and in the debugger console it calls cellForRowAtIndexPath as each new cell comes into the view. Each cell holds an image which is downloaded from a URL and hence has to be converted to a UIImage. I believe this is whats causing the lag.
Can anyone direct me as to what I need to do in order to download the images and have them display in the cells without causing major lag in the app?
IE: Where would I put the download/conversion code so that the image is already stored as an image before the cell needs to be dispayed?
Thanks,
Jack

Answer 1

If you want to pre-download the images, you could do it at any point in your application and cache them for later. For example, you could start the downloads just after you download and parse the RSS feed. You can save them in the directory returned by [NSSearchPathForDirectoriesInDomains(NSCachesDirectory, NSUserDomainMask, YES) objectAtIndex:0]. It would be best to use asynchronous method to download them, and have just a few downloads active at a time for best performance.
If you want to load them as needed, set a placeholder image when you allocate the cell and use an asynchronous NSURLConnection or the like to do the download. Once the download completes, replace the placeholder with the real image (and cache it for reuse later, of course).