为什么动画后UINavigationBar会缩小？

Question

我试图关注这个问题：Presenting a UINavigationController in a custom modal，但我的导航栏在动画后从64p缩小到44p。我使用的是自定义UIPresentationController和UIViewControllerAnimatedTransitioning。

我认为问题在于以下代码。当我注释掉动画时，导航栏开始并保持在44p

func animatePresentationWithTransitionContext(transitionContext: UIViewControllerContextTransitioning) {
    let presentedController = transitionContext.viewControllerForKey(UITransitionContextToViewControllerKey)! as DrawerNavigationController
    let presentedControllerView = transitionContext.viewForKey(UITransitionContextToViewKey)!
    let containerView = transitionContext.containerView()

    var finalFrame = transitionContext.finalFrameForViewController(presentedController)
    presentedControllerView.frame = CGRectOffset(finalFrame, 0, containerView.bounds.size.height)
    containerView.addSubview(presentedControllerView)

    UIView.animateWithDuration(self.duration, delay: 0.0, usingSpringWithDamping: 1.0, initialSpringVelocity: 0.0, options: .AllowUserInteraction, animations: {

        var height = presentedController.height()
        presentedControllerView.frame.origin.y = containerView.bounds.size.height - height

        }, completion: {(completed: Bool) -> Void in
            transitionContext.completeTransition(true)
    })
}