exportVisibleImages.swift

func exportVisibleImages() {
    for (windowIndex, window) in UIApplication.shared.windows.enumerated() {
        let prefix = "iOS\(ProcessInfo.processInfo.operatingSystemVersion.majorVersion)_window\(windowIndex)_images"
        exportImages(fromRootView: window, toDirectory: "/path/to/target/directory", prefix: prefix)
    }
}

func exportImages(fromRootView rootView: UIView, toDirectory directory: String, prefix: String) {
    let directoryURL = URL(fileURLWithPath: directory, isDirectory: true)
    try! FileManager.default.createDirectory(at: directoryURL, withIntermediateDirectories: true, attributes: nil)

    let imageViews = gatherImageViews(below: rootView)
    for (index, imageView) in imageViews.enumerated() {
        guard let image = imageView.image else { continue }
        let url = directoryURL.appendingPathComponent("\(prefix)_\(index)@\(Int(image.scale))x.png", isDirectory: false)
        try! UIImagePNGRepresentation(image)!.write(to: url)
    }
}

func gatherImageViews(below view: UIView) -> [UIImageView] {
    var imageViews: [UIImageView] = []
    for subview in view.subviews {
        if let imageView = subview as? UIImageView {
            imageViews.append(imageView)
        }
        imageViews += gatherImageViews(below: subview)
    }

    return imageViews
}