How access frame of a toView object in custom animation - ios

Question

I want to make an animation like iOS home screen folders, and I have to know the frame of the final position of the "folder":
I'm using a custom transition and here is the code of the animation file:
class FolderAnimationController: NSObject, UIViewControllerAnimatedTransitioning {
let duration = 5.0
var presenting = true
func transitionDuration(_ transitionContext: UIViewControllerContextTransitioning?) -> TimeInterval {
return duration
}
func animateTransition(_ transitionContext: UIViewControllerContextTransitioning) {
let containerView = transitionContext.containerView()
let toViewC = transitionContext.viewController(forKey: UITransitionContextToViewControllerKey)! as! ProjectViewController
let fromViewC = transitionContext.viewController(forKey: UITransitionContextFromViewControllerKey)!as! ViewController
let cell : FolderCollectionViewCell = fromViewC.folderCollectionView.cellForItem(at: (fromViewC.folderCollectionView.indexPathsForSelectedItems()?.first!)!) as! FolderCollectionViewCell
let cellSnapshot: UIView = cell.snapshotView(afterScreenUpdates: false)!
let cellFrame = containerView.convert(cell.frame, from: cell.superview)
cellSnapshot.frame = cellFrame
cell.isHidden = true
toViewC.view.frame = transitionContext.finalFrame(for: toViewC)
toViewC.view.alpha = 0
containerView.addSubview(toViewC.view)
containerView.addSubview(cellSnapshot)
UIView.animate(withDuration: duration, animations: {
toViewC.view.alpha = 1.0
let finalFrame = containerView.convert(toViewC.containerView.frame, from: toViewC.view)
cellSnapshot.frame = finalFrame
}) { (_) in
cellSnapshot.removeFromSuperview()
transitionContext.completeTransition(true)
}
}
}
All works correctly, except let finalFrame = containerView.convert(toViewC.containerView.frame, from: toViewC.view) that set finalFrame values (is a CGRect variable) to (origin = (x = 0, y = 0), size = (width = 0, height = 0)).
I've followed this tutorial, writing my code in Swift
So, how can I access the property correctly?

Answer 1

The frame is the zero rect because at the point you are accessing it, the view's layout pass has not occurred. After you set the toVC.view frame, add this line:
toVC.view.layoutIfNeeded()
I've written about the use of snapshots in view controller transitions here if you're interested.