Releasing renderInContext result within a loop

Question

I have a method which is called in a loop, that looks something like this:

NSAutoreleasePool *pool = [[NSAutoreleasePool alloc] init];
UIImageView *background = [[UIImageView alloc] initWithFrame:CGRectMake(0, 0, PAGE_WIDTH, PAGE_HEIGHT)];
background.image = backgroundImg;

for (UIView *view in viewArray)
{
    [background addSubview:view];
}

UIGraphicsBeginImageContext(background.frame.size);
[background.layer renderInContext:UIGraphicsGetCurrentContext()];
UIImage *image = UIGraphicsGetImageFromCurrentImageContext();
UIGraphicsEndImageContext();

for (UIView *view in background.subviews)
    [view removeFromSuperview];

background.image = nil;
[background release];
[image retain];
[pool drain];
[image autorelease];
return image;

However, according to Instruments Memory Monitor, the memory usage goes up and up, and never comes down until the end of the loop. (It crashes.)

If I replace the UIGraphicsBeginImageContext to UIGraphicsEndImageContext with

UIImage *image = someotherimage;

then the memory does not spike, but is allocated and reduces on every iteration of the loop, as I would expect, due to the Autorelease pool. (It doesn't crash)

And if I just comment out the renderInContext line it works fine. (Doesn't crash)

So it looks as if renderInContext is holding onto the image somehow - how can I get it to release it? Or any alternative suggestions please :)?

Answer 1

Naturally after 3 days of experimenting, I find the answer (an answer, anyway, and I would be glad of comments on this) within the hour.

I add

background.layer.contents = nil;

after

UIGraphicsEndImageContext();

and the cached memory in the layer is uncached :).

Answer 2

I am not using a UIImageView, so setting layer.contents = nil did not work for me. However, I found a different solution that, while not ideal, does work. It appears that the memory allocated by renderInContext does not get freed until the main_queue is idle. So, I did the following:

dispatch_queue_t queue = dispatch_queue_create("com.example.imageprocessing", DISPATCH_QUEUE_SERIAL);

for (int k = 0; k < images.count; ++k) {
    dispatch_async(queue, ^{
        dispatch_sync(dispatch_get_main_queue(), ^{
            @autoreleasepool {
                ...
                UIGraphicsBeginImageContext(self.view.bounds.size);
                [view.layer renderInContext:UIGraphicsGetCurrentContext()];
                image = UIGraphicsGetImageFromCurrentImageContext();
                UIGraphicsEndImageContext();
                ...
            }
        }
    }
}

This solved my memory issue. I'm not processing a lot of images, so I don't know how the performance is affected.