TransitionAnimator.cs

public class TransitionAnimator : UIViewControllerAnimatedTransitioning
{
  private const double _duration = 0.5;

  public override void AnimateTransition(IUIViewControllerContextTransitioning transitionContext)
  {
    var containerView = transitionContext.ContainerView;
    var toView = transitionContext.GetViewFor(UITransitionContext.ToViewKey);

    containerView.AddSubview(toView);
    toView.Alpha = 0;
    UIView.Animate(_duration, () =>
    {
      toView.Alpha = 1;
    }, () =>
    {
      transitionContext.CompleteTransition(true);
    });
  }
  
  public override double TransitionDuration(IUIViewControllerContextTransitioning transitionContext)
  {
    return _duration;
  }
}