Custom UITableViewCell with large UIImageView causing scroll lag

Question

I have a custom UITableViewCell that consists of a UIImageView and a UILabel. The cell is 320x104px and the imageView takes up the whole area with the label in front. There are only 8 cells.

in ViewDidLoad I am creating all needed images up front and caching them in a dictionary at the correct dimensions.

When I scroll the UITableView there is a noticable lag every time a new cell is encountered. This makes no sense to me as the image it is using is already created and cached. All that I'm asking of the cell is for its UIImageView to render the image.

I am using a custom cell with its view in a xib and configuring my UITableView to use it with:

[self.tableView registerNib:[UINib nibWithNibName:@"ActsCell" bundle:nil] forCellReuseIdentifier:myIdentifier];

Cell creation and configuration:

- (UITableViewCell *)tableView:(UITableView *)tableView 
         cellForRowAtIndexPath:(NSIndexPath *)indexPath 
{  
    NSString* reuseIdentifier = @"ActsCell";
    ActsCell* cell = [tableView dequeueReusableCellWithIdentifier:reuseIdentifier];
    // Configure the cell...
    [self configureCell:cell atIndexPath:indexPath];
    return cell;
}

- (void)configureCell:(ActsCell *)cell atIndexPath:(NSIndexPath *)indexPath 
{
    Act* act = [self.acts objectAtIndex:indexPath.row];
    cell.selectionStyle = UITableViewCellSelectionStyleNone;
    cell.title.text = act.name;
    cell.imageView.image = [self.imageCache objectForKey:act.uid];
}

What could be causing the lag? There would seem to be no benefit in trying to do anything Async as all the time-intensive work is done.

Answer 1

Do you load images from local files by any chance?

By using Instruments I've found that there is some lazy loading mechanism in UIImage - the real image data was decompressed from PNG only at the stage of rendering it on the main thread which caused the lag during scrolling.

So, after the loading of UIImage with -initWithContentsOfFile: method, I've added code to render the contents of this image to offscreen context, saved that context as new UIImage and used it for UIImageView in UITableViewCell, this made the scroll to be smooth and pleasant to the eye again.

In case of reference there is the simple code I'm using to force reading of image contents in separate thread (using ARC):

UIImage *productImage = [[UIImage alloc] initWithContentsOfFile:path];

CGSize imageSize = productImage.size;
UIGraphicsBeginImageContext(imageSize);
[productImage drawInRect:CGRectMake(0, 0, imageSize.width, imageSize.height)];
productImage = UIGraphicsGetImageFromCurrentImageContext();
UIGraphicsEndImageContext();

And I think the surface of UIImage created in such a way will be in the supported format suited for rendering, which will also offload work needed to render it on main thread.

EDIT: The docs for UIGraphicsGetImageFromCurrentImageContext() say that it should be only used from main thread, but searching on the net or SO shows that starting from iOS 4 the UIGraphics.. methods became thread safe.