Question

Hello i am using NSOperationQueue to download images in the background. I have created a custom NSOperation to download the images. I put the images in table cells. The problem is if I do [operationQueue setMaxConcurrentOperationCount: 10] and i scroll down several cells the program crashes with EXC_BAD_ACCESS. Every time it crashes at the same place in the table. There are 3 cells one after the other which are for the same company and have the same logo so basically it should download the images 3 times. Every other time it works fine.

- (void) main
{
    NSAutoreleasePool * pool = [[NSAutoreleasePool alloc] init];

    NSURL *url = [[NSURL alloc] initWithString:self.imageURL];
    debugLog(@"downloading image: %@", self.imageURL);
    //NSError *error = nil;
    NSData *data = [[NSData alloc] initWithContentsOfURL:url];
    [url release];

    UIImage *image = [[UIImage alloc] initWithData:data];
    [data release];
    if (image)
    {
        if (image.size.width != ICONWIDTH && image.size.height != ICONHEIGHT)
        {
            UIImage *resizedImage;
            CGSize itemSize = CGSizeMake(ICONWIDTH, ICONHEIGHT);

                        //!!! UIGraphicsBeginImageContext NOT THREAD SAFE
            UIGraphicsBeginImageContext(itemSize);
            CGRect imageRect = CGRectMake(0.0, 0.0, itemSize.width, itemSize.height);
            [image drawInRect:imageRect];
            resizedImage = UIGraphicsGetImageFromCurrentImageContext();
            UIGraphicsEndImageContext();

            self.theImage = resizedImage;
        }
        else
        {
            self.theImage = image;
        }
        [image release];
    }
    [delegate didFinishDownloadingImage: self];
    [pool release];
}

This is how i handle downloading the images. If i comment out

[delegate didFinishDownloadingImage: self];

in the function above it doesn’t crash but of course it is useless.

-(void) didFinishDownloadingImage:(ImageDownloadOperation *) imageDownloader
{
    [self performSelectorOnMainThread: @selector(handleDidFinishDownloadingImage:) withObject: imageDownloader waitUntilDone: FALSE];
}

-(void) handleDidFinishDownloadingImage:(ImageDownloadOperation *)imageDownloadOperation
{
    NSArray *visiblePaths = [self.myTableView indexPathsForVisibleRows];
    CompanyImgDownloaderState *stateObject = (CompanyImgDownloaderState *)[imageDownloadOperation stateObject];

    if ([visiblePaths containsObject: stateObject.indexPath])
    {
        //debugLog(@"didFinishDownloadingImage %@ %@", imageDownloader.theImage);

        UITableViewCell *cell = [self.myTableView cellForRowAtIndexPath: stateObject.indexPath];
        UIImageView *imageView = (UIImageView *)[cell viewWithTag: 1];
        if (imageDownloadOperation.theImage)
        {
            imageView.image = imageDownloadOperation.theImage;
            stateObject.company.icon = imageDownloadOperation.theImage;
        }
        else
        {
            imageView.image = [(TestWebServiceAppDelegate *)[[UIApplication sharedApplication] delegate] getCylexIcon];
            stateObject.company.icon = [(TestWebServiceAppDelegate *)[[UIApplication sharedApplication] delegate] getCylexIcon];
        }

    }
}

Answer 1

According to this mailing list post, UIGraphicsBeginImageContext isn’t thread safe. The post hints that CGBitmapContextCreate and related functions are the safe way to do it.

http://osdir.com/ml/cocoa-dev/2009-10/msg00035.html