UIView snapshots on custom UIViewController transitions - ios

Question

I am creating a custom transition for my app but when I try to create a snapshot of the destination view it always appears as a blank white rectangle. It is important I note that this is a custom push transition and not a modal presentation of the view. When presenting modally the snapshot appears to work. Is this the normal behaviour for custom push/pop transitions?
The code I've written is as follows:
func animateTransition(using transitionContext: UIViewControllerContextTransitioning) {
guard let toViewController = transitionContext.viewController(forKey: .to) as? CultureViewController else {
return
}
let containerView = transitionContext.containerView
let finalFrame = transitionContext.finalFrame(for: toViewController)
let snapshot = toViewController.view.snapshotView(afterScreenUpdates: true)
snapshot?.frame = UIScreen.main.bounds
containerView.addSubview(snapshot!)
toViewController.view.transform = CGAffineTransform(translationX: 0, y: toViewController.view.bounds.height)
//containerView.addSubview(toViewController.view)
let duration = transitionDuration(using: transitionContext)
UIView.animateKeyframes(withDuration: duration, delay: 0, options: .calculationModeCubic, animations: {
UIView.addKeyframe(withRelativeStartTime: 0, relativeDuration: 1, animations: {
toViewController.view.frame = finalFrame
})
}, completion: { _ in
transitionContext.completeTransition(!transitionContext.transitionWasCancelled)
})
}

Answer 1

Taking a snapshot of a view before it is rendered, such as is the case with a transition before you call containerView.addSubview(toViewController.view), returns a blank view, per UIView's documentation.
If the current view is not yet rendered, perhaps because it is not yet onscreen, the snapshot view has no visible content.