UIViewController that frosts a portion of the screen. The `toggleBlur` method is hooked up to a button on the story board.

BlurViewController.swift

class BlurViewController: UIViewController {
    
    var blurView : UIVisualEffectView!
    
    override func viewDidLoad() {
        let frame = CGRect(x: view.bounds.maxX * 0.1, y: view.bounds.maxY * 0.4, width: 350, height: 200)
        blurView = UIVisualEffectView(frame: frame)
        blurView.effect = UIBlurEffect(style: .systemUltraThinMaterial)
        view.addSubview(blurView)
    }

    @IBAction func toggleBlur() {
        blurView.isHidden = !blurView.isHidden
    }

}