Scaling image in background thread crashes - UIGraphicsImageRenderer

Question

private func scale(image ciImage: CIImage?, for scale: CGFloat) -> UIImage {
    guard let ciImage = ciImage else {
        return UIImage()
    }

    let image = UIImage(ciImage: ciImage)
    let size = CGSize(width: image.size.width * scale, height: image.size.height * scale)

    let renderer = UIGraphicsImageRenderer(size: size)

    return renderer.image(actions: { (context) in
        image.draw(in: CGRect(origin: .zero, size: size))
    })
}

I am using this method to scale (and generate before that) images. This is done in background thread and the result is then dispatched back to main thread. However, on image.draw - sometimes 1 out of 10 times, the app crashes with EXC_BAD_ACCESS.

What am I doing wrong? Can I do that in background?

THANKS

Answer 1

Don't do any UI actions in a background thread. All UI changes must be done in the main thread

Use a dispatch Queue to work in main thread.Try it like this

DispatchQueue.main.async { 
   image.draw(in: CGRect(origin: .zero, size: size))
}

Check this answer to know why we use main thread to perform UI actions

Answer 2

It looks like bug in CoreUI working with vector images. I'm having a similar issue right now with a following code

let scaledImage = renderer.image { _ in
        self.draw(in: CGRect(origin: .zero, size: scaledImageSize))
}

leads to the following error:

Looking into [_CUIThemeSVGRendition rawData] you can see the following release statement: I was not able to find related retain so I guess because of that memory is released twice which eventually leads to a crash. It looks like if(r8 & 0x3c) == 0x10) is checking bitmapEncoding (see https://developer.limneos.net/index.php?ios=15.2.1&framework=CoreUI.framework&header=CUIThemeRendition.h ) but I have no idea what should be inside. Maybe we should file a bug report to Apple.