ViewController.swift

func imagePickerController(_ picker: UIImagePickerController, didFinishPickingMediaWithInfo info: [UIImagePickerController.InfoKey : Any]) {
    picker.dismiss(animated: true)
    guard let image = info[.originalImage] as? UIImage else {
        return
    }

    imageView.image = image

    // layout logic, fit image view size to image ratio...
    ...
  
    generateFaceBoundingBoxes(for: image)
}

func generateFaceBoundingBoxes(for image: UIImage) {
    debugRectContainerView.subviews.forEach {
        $0.removeFromSuperview()
    }

    for faceRect in self.imageProcessingService.getFaceRects(in: image, normalizedTo: self.imageView.frame) {
        let rectView = UIView(frame: faceRect)
        rectView.backgroundColor = nil
        rectView.layer.borderColor = UIColor.green.cgColor
        rectView.layer.borderWidth = 1.5
        debugRectContainerView.addSubview(rectView)
    }
}