Remove the focus point when the camera sees ARImageAnchor and reveal the focus point when the camera does not see ARImageAnchor

I try to obtain focus point entity when camera not see ARImageAnchor, and remove after camera sees ARImageAnchor, and when camera not sees anchor obtain focus point again. I used arView.session.delegate, but delegate method maybe call one time, i don't know. how to make it? Thank u

  var focusEntity: FocusEntity! // (FocusEntity: Entity, HasAnchoring) 

  override func viewDidLoad() {
        super.viewDidLoad()
        // ...
        focusEntity = FocusEntity(on: arView, style: .classic(color: .systemGray4))
        arView.session.delegate = self
    }
}


extension CameraViewController: ARSessionDelegate {
    func session(_ session: ARSession, didAdd anchors: [ARAnchor]) {
        for anchor in anchors {
            if let imageAnchor = anchor as? ARImageAnchor {
                focusEntity.destroy()
                focusEntity = nil

                //... Obtain entity to image anchor
            }
        }
    }

    func session(_ session: ARSession, didUpdate frame: ARFrame) {
        //... ???
    }
}