Question

The code below shows how I’m cutting my sprites, but the memory usage grows constantly. How can I fix?

CGImageRef imgRef = [imgSprite CGImage];
[imgView setImage:[UIImage imageWithCGImage:CGImageCreateWithImageInRect(imgRef, CGRectMake(column*width, line, width, height))]];
CGImageRelease(imgRef);

This code is called by the NSTimer in an interval of 0.1.

Answer 1

Since you haven’t posted the declaration of imgSprite, I’ll assume that its class follows Cocoa naming conventions.

In:

CGImageRef imgRef = [imgSprite CGImage];

that method (a non-NARC¹ method) returns an object that you do not own, hence you should not release it.

In:

[imgView setImage:[UIImage imageWithCGImage:CGImageCreateWithImageInRect(imgRef, CGRectMake(column*width, line, width, height))]];

the argument is the expression:

CGImageCreateWithImageInRect(imgRef, CGRectMake(column*width, line, width, height))

CGImageCreateWithImageInRect() (a function whose name follows the Create Rule²) returns an image that you do own, hence you should release it, which you don’t.

In:

CGImageRelease(imgRef);

you’re releasing an image that you do not own, so you should not release it.

You have two problems: you’re (potentially over)releasing imgRef and you’re leaking the image returned by CGImageCreateWithImageInRect().

You should do the following instead:

// you do not own imgRef, hence you shouldn’t release it
CGImageRef imgRef = [imgSprite CGImage];

// use a variable for the return value of CGImageCreateWithImageInRect()
// because you own the return value, hence you should release it later
CGImageRef imgInRect = CGImageCreateWithImageInRect(imgRef, CGRectMake(column*width, line, width, height));

[imgView setImage:[UIImage imageWithCGImage:imgInRect]];

CGImageRelease(imgInRect);

You might want to read the Memory Management Programming Guide and the Memory Management Programming Guide for Core Foundation.

¹NARC = new, alloc, retain, copy

²The Create Rule states that if you call a function whose name contains Create or Copy then you own the return value, hence you should release it when you don’t need it any longer.