Question

In its simplest form, my app displays 10 UIImageViews, each containing an image. Even with all UIImageViews containing images, my app uses a small enough memory footprint. However, there is a button to clear all the UIImageViews by setting all their images to nil. The problem is, when checking Memory Monitor in Instruments, the memory held by the UIImageViews is NOT going away. This doesn’t appear in the Allocations instrument, confirming the remaining memory footprint is not an object, but instead graphics-based memory. If I resize the images to something smaller or larger, the memory remaining is also smaller or larger, respectively.

Why is the image data sticking around after the UIImageView’s image has been set to nil?

Answer 1

I believe UIKit keeps a cache of images for reuse. UIImageView might be releasing the object, but a copy is kept around for performance reasons.

These images, though, should be released on receiving a memory warning. If they’re not, there’s two places I’d check:

1. Make sure the UIImageView is being dealloc’d. Use Allocations Instrument to profile your app and do whatever you need to do in the program to load those images. Then unload the images and do a search for UIImageView. As long as you’re sure your program should have released all of them, if you find any in the search you know something is wrong.
2. I’d also check any places the image was created, for example: UIImage = [UIImage imageName:@"Foo.jpg"]; Make sure these are also being released. You can use allocations to find UIImage classes, but it’ll be harder to weed out the ones that should/should not be there.
3. Run the static analyzer: In Xcode 4 it’s under Products -> Analyze. This is a great tool for finding logic errors, over/under release (if you not using ARC) etc.