类型UIViewController的值永远不能为零,不允许比较

该函数的声明说,fromVC是可选的(这是什么?后缀的意思),那toVC是不可选的(因为它没有?后缀).只有一个Optional可以nil.

另外,常见的Swift风格是使用an if-let来打开可选项.试试这个:

private func switchViewController(from fromVC: UIViewController?, to toVC: UIViewController) {
    if let fromVC = fromVC {
        // In this scope, fromVC is a plain UIViewController, not an optional.
        fromVC.willMoveToParentViewController(nil)
        fromVC.view.removeFromSuperview()
        fromVC.removeFromParentViewController()
    }

    self.addChildViewController(toVC)
    self.view.insertSubview(toVC.view, atIndex: 0)
    toVC.didMoveToParentViewController(self)
}