对UINavigationController的开始/结束外观过渡的不平衡调用

Question

我正在使用TabBarControllerDelegate方法shouldSelectViewController在TabBarController中进行shouldSelectViewController 。 我收到以下警告

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

我相信这是因为我在目标ViewController上调用viewWillAppear(true) （在下面用***标记）的行（这将平滑过渡到新控制器）。

我看到这个问题和另一个问题相似但不完全相同。 我想我应该告诉屏幕上当前的View Controller，他将被删除（因为添加标记行时此警告开始出现），但是我不确定如何删除。

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;
}