Protip: UIViewControllerAnimatedTransitioning + Snapshots

When creating custom animations for view controller transitions, it is recommended to use snapshots of the views being animated. However, UIKit doesn’t always make things crystal clear.

If you find yourself with broken autolayout or views incorrectly configured after enabling your transition, try creating the snapshots after adding your view to containerView.

func animateTransition(using transitionContext: UIViewControllerContextTransitioning) {
    guard let fromVC = transitionContext.viewController(forKey: .from),
        let toVC = transitionContext.viewController(forKey: .to),
        let fromAnimatedView = fromVC.animatedView.snapshotView(afterScreenUpdates: true),
        let toAnimatedView = toVC.animatedView.snapshotView(afterScreenUpdates: true) else {
        return transitionContext.completeTransition(false)
    }

    let containerView = transitionContext.containerView

    [fromAnimatedView, toAnimatedView].forEach(containerView.addSubview(_:))

    // Now you got yourself broken views. Hooray!
    ...
}

The code above might not work. Your toVC’s view might end up completely broken and your snapshots may have the wrong frame and contents. To fix it, simply move the snapshots creation to after adding your destination view controller’s view to transitionContext.containerView.

If you just try it after your toVC has ended initializing its views (i.e. after viewDidLoad has been called) it won’t work, even if you force the view to be loaded. You truly need to create your snapshots after the containerView.

   func animateTransition(using transitionContext: UIViewControllerContextTransitioning) {
      guard let fromVC = transitionContext.viewController(forKey: .from),
          let toVC = transitionContext.viewController(forKey: .to) else {
              return transitionContext.completeTransition(false)
      }

      let containerView = transitionContext.containerView
      containerView.addSubview(toVC.view)

      guard let fromAnimatedView = fromVC.animatedView.snapshotView(afterScreenUpdates: true),
          let toAnimatedView = toVC.animatedView.snapshotView(afterScreenUpdates: true) else {
              return transitionContext.completeTransition(true) // toVC.view was already added to the container
      }

      ...
  }

Boom! Done. Now it works 🎉