对UINavigationController的开始/结束外观转换的不平衡调用

Question

我使用TabBarControllerDelegate方法shouldSelectViewController在TabBarController中进行转换。我收到了以下警告

Unbalanced calls to begin/end appearance transitions for <UINavigationController: 0x1804dcf0>.

我认为这是因为我在目标ViewController上调用viewWillAppear(true)（在下面用***标记）的行（这样可以平滑过渡到新控制器）。

我看到this question和this other question相似但不完全相同。我想我应该告诉屏幕上的当前View Controller他将被删除（因为当添加标记的行时这个警告开始出现），但我不确定如何。

func tabBarController(tabBarController: UITabBarController, shouldSelectViewController viewController: UIViewController) -> Bool {

    let controllerIndex: Int = find( (self.viewControllers as! [UIViewController]), viewController)!

    if(controllerIndex == tabBarController.selectedIndex) {
        return false
    }

    // Get the views
    let fromView: UIView = tabBarController.selectedViewController!.view
    let toView: UIView = (tabBarController.viewControllers as! [UIViewController])[controllerIndex].view

***
    // Tell the destination view controller know it will appear on screen
    (tabBarController.viewControllers as! [UIViewController])[controllerIndex].viewWillAppear(true)
***

    let viewSize = fromView.frame
    let scrollRight = controllerIndex > tabBarController.selectedIndex;

    // Add the to view to the tab bar view.
    fromView.superview?.addSubview(toView)


    // Position it off screen.
    let screenWidth = UIScreen.mainScreen().bounds.size.width
    toView.frame = CGRectMake(
        scrollRight ? screenWidth : -screenWidth,
        viewSize.origin.y,
        screenWidth,
        viewSize.size.height
    )

    UIView.animateWithDuration(0.2, animations: { () -> Void in
        fromView.frame = CGRectMake(
            scrollRight ? -screenWidth : screenWidth,
            viewSize.origin.y,
            screenWidth,
            viewSize.size.height
        )

        toView.frame = CGRectMake(
            0,
            viewSize.origin.y,
            screenWidth,
            viewSize.size.height
        )
    }) { (finished) -> Void in
        if(finished) {
            fromView.removeFromSuperview()
            tabBarController.selectedIndex = controllerIndex
        }
    }

    return false;
}