How to synchronise UIViewControllerInteractiveTransitioning and CABasicAnimation

I have a button in a view controller that has a shadow. This shadow is applied with an animation, in viewWillAppear.

The button is inside an empty view called 'buttonContainer'.

override func viewWillAppear(_ animated: Bool) {
    super.viewWillAppear(animated)
    
    CATransaction.begin()
    CATransaction.setCompletionBlock {
        self.buttonContainer.layer.shadowOpacity = 1
    }
    
    let animation = CABasicAnimation(keyPath: "shadowOpacity")
    animation.fromValue = buttonContainer.layer.shadowOpacity
    animation.toValue = 1
    animation.duration = 0.6
    buttonContainer.layer.add(animation, forKey: animation.keyPath)
    
    CATransaction.commit()
}

I want the shadow to animate with the push animation of the view controller. I started implementing 'UIViewControllerAnimatedTransitioning' and I couldn't find a way to tie in the animation duration with CABasicAnimation.

func animateTransition(using transitionContext: UIViewControllerContextTransitioning) {

    let containerView = transitionContext.containerView
    let button = containerView.viewWithTag(68) as! UIButton

    let animation = CABasicAnimation(keyPath: "shadowOpacity")
    animation.fromValue = ?
    animation.toValue = ?
}

And I suppose I would need to implement UIViewControllerInteractiveTransitioning but I can't think of a way to tie in CABasicAnimation. Any help appreciated