How to release memory quickly inside receiving method? - ios

Question

In my iPhone app, I have a large image that I've cached to disk and I retrieve it just before I hand the image to a class that does a lot processing on that image. The receiving class only needs the image briefly for some initialization and I want to release the memory that the image is taking up as soon as possible because the image processing code is very memory intensive, but I don't know how.
It looks something like this:
// inside viewController
- (void) pressedRender
{
UIImage *imageToProcess = [[EGOCache globalCache] imageForKey:#"reallyBigImage"];
UIImage *finalImage = [frameBuffer renderImage:imageToProcess];
// save the image
}
// inside frameBuffer class
- (UIImage *)renderImage:(UIImage *)startingImage
{
CGContextRef context = CGBitmapCreateContext(....)
CGContextDrawImage(context, rect, startingImage.CGImage);
// at this point, I no longer need the image
// and would like to release the memory it's taking up
// lots of image processing/memory usage here...
// return the processed image
CGImageRef tmpImage = CGBitmapContextCreateImage(context);
CGContextRelease(context);
UIImage *renderedImage = [UIImage imageWithCGImage:tmpImage];
CGImageRelease(tmpImage);
return renderedImage;
}
This may be obvious, but I'm missing something. Thank you.

Answer 1

#Jonah.at.GoDaddy is on the right track, but I would make all of this more explicit rather than relying on ARC optimizations. ARC is much less aggressive in debug mode, and so your memory usage may become too high when you're debugging unless you take steps.
UIImage *imageToProcess = [[EGOCache globalCache] imageForKey:#"reallyBigImage"];
First, I'm going to assume that imageForKey: does not cache anything itself, and does not call imageNamed: (which caches things).
The key is that you need to nil your pointer when you want the memory to go away. That's going to be very hard if you pass the image from one place to another (which Jonah's solution also fixes). Personally, I'd probably do something like this to get from image->context as fast as I can:
CGContextRef CreateContextForImage(UIImage *image) {
CGContextRef context = CGBitmapCreateContext(....)
CGContextDrawImage(context, rect, image.CGImage);
return context;
}
- (void) pressedRender {
CGContextRef context = NULL;
// I'm adding an #autoreleasepool here just in case there are some extra
// autoreleases attached by imageForKey: (which it's free to do). It also nicely
// bounds the references to imageToProcess.
#autoreleasepool {
UIImage *imageToProcess = [[EGOCache globalCache] imageForKey:#"reallyBigImage"];
context = CreateContextForImage(imageToProcess);
}
// The image should be gone now; there is no reference to it in scope.
UIImage *finalImage = [frameBuffer renderImageForContext:context];
CGContextRelease(context);
// save the image
}
// inside frameBuffer class
- (UIImage *)renderImageForContext:(CGContextRef)context
{
// lots of memory usage here...
return renderedImage;
}
For debugging, you can make sure that the UIImage is really going away by adding an associated watcher to it. See the accepted answer to How to enforce using `-retainCount` method and `-dealloc` selector under ARC? (The answer has little to do with the question; it just happens to address the same thing you might find useful).

Answer 2

you can autorelease objects right away in the same method. I think you need to try to handle the "big-image" process within one methods to use #autorelease:
-(void)myMethod{
//do something
#autoreleasepool{
// do your heavy image processing and free the memory right away
}
//do something
}