UINavigationControllerDelegate方法，在iOS 8中使用导航控制器进行自定义转换

我尝试在弹出或推送导航控制器时创建自定义过渡。我创建了一个符合UINavigationControllerDelegate的transitionManager类。我已经创建了这个transitionManager的对象，并将其作为transitioningDelegate添加到我的navigationController中。

推送动画运行得很好，但是当我尝试跳回到上一个viewController时，我只会看到黑屏。

我已经浏览了许多其他文章来使其工作并尝试了手头的一切，但是当我弹出时它仍然没有显示以前的ViewController。

这里的transitionManager的代码是：

import UIKit

class BRTransitionManager: NSObject, UINavigationControllerDelegate, UIViewControllerAnimatedTransitioning {

    private var presenting = true

    func navigationController(navigationController: UINavigationController, animationControllerForOperation operation: UINavigationControllerOperation, fromViewController fromVC: UIViewController, toViewController toVC: UIViewController) -> UIViewControllerAnimatedTransitioning? {

        if  operation == UINavigationControllerOperation.Push{
            self.presenting = true
        } else if operation == UINavigationControllerOperation.Pop{
            self.presenting = false
        }

        return self
    }

    func animateTransition(transitionContext: UIViewControllerContextTransitioning) {
        let container = transitionContext.containerView()
        let fromView = transitionContext.viewForKey(UITransitionContextFromViewKey)!
        let toView = transitionContext.viewForKey(UITransitionContextToViewKey)!

        // set up from 2D transforms that we'll use in the animation
        let offScreenRight = CGAffineTransformMakeTranslation(container.frame.width, 0)
        let offScreenLeft = CGAffineTransformMakeTranslation(-container.frame.width, 0)

        // prepare the toView for the animation
        if (self.presenting == true) {
            // add the both views to our view controller
            container.addSubview(toView)
            // container.addSubview(fromView)

            toView.transform = offScreenRight
        } else {
            toView.transform = offScreenLeft
        }

        let duration = self.transitionDuration(transitionContext)
        UIView.animateWithDuration(duration, delay: 0.0, usingSpringWithDamping: 0.5, initialSpringVelocity: 0.8, options: nil, animations: {

            if (self.presenting == true) {
                fromView.transform = offScreenLeft
            } else {
                fromView.transform = offScreenRight
            }

            toView.transform = CGAffineTransformIdentity

        }, completion: { finished in

            transitionContext.completeTransition(true)
        })
    }

    func transitionDuration(transitionContext: UIViewControllerContextTransitioning) -> NSTimeInterval {
        return 0.8
    }   
}