在 UIGraphicsImageRenderer 工作后释放 memory

Question

I have many images saved in a Assets.xcassets . I'm resizing them, making them smaller. Then later I don't need these images anymore. Images were not shown, just were prepared. So the class that got images, resized them and kept is now successfully deinited .
But the memory after UIGraphicsImageRenderer resized the images not released. Memory usage stayed on the same level. Even if I didn't use the images at all like in the example code below. I think something is wrong. Actually, I resized the images to use less memory but it contrary - resized images use more memory and it do not releasing after the class owner has been deinited.

How to release the memory?

Apples documentation says: " ...An image renderer keeps a cache of Core Graphics contexts, so reusing the same renderer can be more efficient than creating new renderers ." - but I don't need it? How to switch it off. With 28 images it seems not big deal, But I have about 100-300 images that should be resized. cropped and other actions with UIGraphicsImageRenderer that at the end of the day uses about 800-900 Mb of the memory that just cache of some render's job that already done.

You can take the code below and try.

class ExampleClass {
    
    func start() {
        Worker().doWork()
    }
}

class Worker {
    
    deinit {
        print("deinit \(Self.self)")
    }
    
    func doWork() {
        
        var urls: [String] = []
        _ = (1...28).map({ urls.append("pathToTheImages/\($0)") })
        // images with resolution 1024.0 x 1366.0 pixels
        
        for url in urls {
            let img = UIImage(named: url)!  // Memory usage: 11.7 MB
            //let cropped = resizedImage(original: img, to: UIScreen.main.bounds.size)
            //With this line above - Memory usage: 17.5 MB even after this class has been deinited
        }
    }
    
    // from 2048 × 3285 pixels >>> to >>> 768.0 x 1024.0 pixels  --- for iPad Pro (9.7-inch)
    func resizedImage(original: UIImage, to size: CGSize) -> UIImage {
        let result = autoreleasepool { () -> UIImage in
            let renderer = UIGraphicsImageRenderer(size: size)
            let result = renderer.image { (context) in
                original.draw(in: CGRect(origin: .zero, size: size))
            }
            return result
        }
        return UIImage(cgImage: result.cgImage!, scale: original.scale, orientation: original.imageOrientation)
    }
}

Answer 1

Asset catalogs are not intended for the use to which you are putting them. The purpose of an image in the asset catalog is to display it, directly. If you have a lot of images that you want to load and resize and save elsewhere without displaying, you need to keep them in your app bundle at the top level, so that you can call init(contentsOfFile:) which does not cache the image.