为什么我的应用程序永远不会呈现UI？

Question

在我的项目中我有主视图控制器，我更换其内容取决于App Logic

所以这个主视图控制器将有不同的内容，我通过这段代码做到了：

    var contentViewController: UIViewController! {
    didSet {
        // remove any previous views
        for view in self.contentView.subviews {
            view.removeFromSuperview()
        }

        // add the view
        // the ViewController must be aded to the children list to avoid being released (as it become a Zombie)
        self.addChildViewController(self.contentViewController)

        if registerDoneFlag {

            //do some animation
            let animation =  CATransition()
            animation.duration = 0.3
            animation.type = kCATransitionPush
            animation.subtype = kCATransitionFromTop
            animation.timingFunction = CAMediaTimingFunction(name: kCAMediaTimingFunctionEaseInEaseOut)

            self.contentView.layer.addAnimation(animation, forKey: "SwitchToView1")

            registerDoneFlag = false
        }
        if backFlag {

            //do some animation
            let animation =  CATransition()
            animation.duration = 0.3
            animation.type = kCATransitionPush
            animation.subtype = kCATransitionFromBottom
            animation.timingFunction = CAMediaTimingFunction(name: kCAMediaTimingFunctionEaseInEaseOut)

            self.contentView.layer.addAnimation(animation, forKey: "SwitchToView1")

            backFlag = false
        }


        self.contentView.addSubview(self.contentViewController.view)

        self.contentViewController.didMoveToParentViewController(self)

    }
}

在某些情况下，内容从未改变过，虽然我调试项目时执行了上一个代码，但布局从未改变，即应用程序似乎永远渲染 UI 。

所以当这个案例发生时，我的应用程序冻结在最后一个VC中，我无法导航到应用程序，直到我杀死该应用程序。

有没有人遇到这样的问题？

请帮助，

感谢。