AnimationNavigationControllerDelegate.cs

public class AnimationNavigationControllerDelegate : UINavigationControllerDelegate
{
  private TransitionAnimator _animator = new TransitionAnimator();
  private int _id;

  public override IUIViewControllerAnimatedTransitioning GetAnimationControllerForOperation(UINavigationController navigationController, UINavigationControllerOperation operation, UIViewController fromViewController, UIViewController toViewController)
  {
    if (operation == UINavigationControllerOperation.Push)
    {
      var image = fromViewController.View.ViewWithTag(_id);
      var convertedFrame = image.ConvertRectToView(image.Bounds, fromViewController.View);
      _animator.ThumbnailFrame = convertedFrame;
    }
    _animator.Operation = operation;
    return _animator;
  }

  internal void SetTransitionId(int id)
  {
    _id = id;
  }
}

AnimationNavigationRenderer.cs

public class AnimationNavigationRenderer : Xamarin.Forms.Platform.iOS.NavigationRenderer
{
  private AnimationNavigationControllerDelegate _delegate;

  public override void ViewDidLoad()
  {
    base.ViewDidLoad();
    _delegate = new AnimationNavigationControllerDelegate();
    Delegate = _delegate;

    MessagingCenter.Subscribe<object, int>(this, "TransitionId", (sender, arg) =>
    {
      _delegate.SetTransitionId(arg);
    });
  }
}