Value of type UIViewController can never be nil, comparison isn't allowed

Question

I am following a tutorial on Multiview apps, and it said to write this function in one of the view controllers. I am getting an error on the second if statement that says UIViewController cant be nil, and comparison isn't allowed. Is this something I don't need to worry about anymore? The book is a little outdated so I'm assuming its occurring because of change in Swift.

private func switchViewController(from fromVC: UIViewController?, to toVC: UIViewController) {
    if fromVC != nil {
        fromVC!.willMoveToParentViewController(nil)
        fromVC!.view.removeFromSuperview()
        fromVC!.removeFromParentViewController()
    }

    if toVC != nil {
        self.addChildViewController(toVC)
        self.view.insertSubview(toVC.view, atIndex: 0)
        toVC.didMoveToParentViewController(self)
    }
}

Answer 1

The declaration of the function says that fromVC is optional (that's what the ? suffix means), and that toVC is not optional (because it has no ? suffix). Only an Optional<UIViewController> can be nil .

Also, the common Swift style is to use an if-let to unwrap the optional. Try this:

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)
}