自定义 UIViewController 转换上的 UIView 快照

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.