UIImage decompression causing scrolling lag

Question

I have this app with a full screen tableView that displays a bunch of tiny images. Those images are pulled from the web, processed on a background thread, and then saved to disk

Answer 1

Jason's tip about pre-drawing the image to decompress it is the key, but you'll get even better performance by copying the whole image and discarding the original.

Images created at runtime on iOS seem to be better optimised for drawing than ones that have been loaded from a file, even after you've forced them to decompress. For that reason, you should load like this (it's also a good idea to put the decompressed image into an NSCache so you don't have to keep reloading it):

- (void)loadImageWithPath:(NSString *)path block:(void(^)(UIImage *image))block
{
    static NSCache *cache = nil;
    if (!cache)
    {
        cache = [[NSCache alloc] init];
    }

    //check cache first
    UIImage *image = [cache objectForKey:path];
    if (image)
    {
        block(image);
        return;
    }

    //switch to background thread
    dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_LOW, 0), ^{

        //load image
        UIImage *image = [UIImage imageWithContentsOfFile:path];

        //redraw image using device context
        UIGraphicsBeginImageContextWithOptions(image.size, YES, 0);
        [image drawAtPoint:CGPointZero];
        image = UIGraphicsGetImageFromCurrentImageContext();
        UIGraphicsEndImageContext();

        //back to main thread
        dispatch_async(dispatch_get_main_queue(), ^{

            //cache the image
            [cache setObject:image forKey:path];

            //return the image
            block(image);
        });
    });
}