iPhone: UITableView performance issue with imageWithContentsOfFile - iphone

Question

I have performance issue in UITableView scrolling when I use imageWithContentsOfFile instead of imageNamed. Following is my code:
- (UITableViewCell *)tableView:(UITableView *)tableView cellForRowAtIndexPath:(NSIndexPath *)indexPath {
static NSString *CellIdentifier = #"Cell";
UITableViewCell *cell = [tableView dequeueReusableCellWithIdentifier:CellIdentifier];
UniversalAppAppDelegate *delegate = (UniversalAppAppDelegate *) [[UIApplication sharedApplication] delegate];
NSDictionary *res = [[delegate comments] objectAtIndex:indexPath.section];
if (cell == nil)
cell = [[[UITableViewCell alloc] initWithStyle:UITableViewCellStyleValue1 reuseIdentifier:CellIdentifier] autorelease];
......
// This line has a problem...when I use imageNamed here instead of imageWithContentsOfFile, it works absolutely fine
imageView = [[UIImageView alloc] initWithImage:[UIImage imageWithContentsOfFile:imagepath]];
[imageView setFrame: CGRectMake(30, 10, 55, 55)];
[sectionHeaderView addSubview:imageView];
......
cell.backgroundView = sectionHeaderView;
return cell;
I have also tried caching image but still same issue. Only imageNamed works perfectly fine but I can't use it as I need to specify path for image stored in documents folder and changes dynamically. Could anyone please tell me how could I resolve this issue?
Caching code:
NSMutableDictionary *thumbnailCache = [[NSMutableDictionary alloc] init];
- (UIImage*)thumbnailImage:(NSString*)fileName
{
UIImage *thumbnail = [thumbnailCache objectForKey:fileName];
if (nil == thumbnail)
{
NSString *thumbnailFile = [self filePath:fileName];
thumbnail = [UIImage imageWithContentsOfFile:thumbnailFile];
[thumbnailCache setObject:thumbnail forKey:fileName];
}
return thumbnail;
}
Usage:
imageView = [[UIImageView alloc] initWithImage:[delegate thumbnailImage:[res objectForKey:#"ImageName"]]];

Answer 1

You could try building an array of UIImages in the viewDidLoad method of the controller, and use the images in that array in your cellForRowAtIndexPath.
This will get rid of the need to call imageWithContentsOfFile for each image when drawing the tableview.
You could for example build the array in a seperate thread, and show a loading indicator while this happens. It would mean a small loading time at first, but a smooth scrolling tableview.