您不必在Interface Builder中使用Storyboard Reference。您可以通过编程方式创建引用，但无论如何，我发现这些引用都更容易理解。

界面生成器

请确保您设置FirstViewController并SecondViewController通过检查各自的故事板的初始控制器“是初始视图控制器”。

码

class MainViewController: UIViewController {
    @IBOutlet weak var contentView: UIView!

    // Initialize the first view controller of storyboard
    let firstViewController: FirstViewController = {
        let storyboard = UIStoryboard(name: "First", bundle: nil)
        return storyboard.instantiateInitialViewController() as! FirstViewController
    }()

    // Initialize the second view controller of storyboard
    let secondViewController: SecondViewController = {
        let storyboard = UIStoryboard(name: "Second", bundle: nil)
        return storyboard.instantiateInitialViewController() as! SecondViewController
    }()

    override func viewDidLoad() {
        super.viewDidLoad()
        show(childViewController: firstViewController)
    }

    @IBAction func showFirstVC(_ sender: Any) {
        // Don't show the first view controller again if it's already visible
        guard firstViewController.parent != self else { return }
        removeAllChildViewControllers()
        show(childViewController: firstViewController)
    }

    @IBAction func showSecondVC(_ sender: Any) {
        // Don't show the second view controller again if it's already visible
        guard secondViewController.parent != self else { return }
        removeAllChildViewControllers()
        show(childViewController: secondViewController)
    }

    // MARK: - Helper methods
    // Show a view controller in the `contentView`
    func show(childViewController vc: UIViewController) {
        self.addChildViewController(vc)
        self.contentView.addSubview(vc.view)
        vc.didMove(toParentViewController: self)
    }

    // Remove a view controller from the `contentView`
    func remove(childViewController vc: UIViewController) {
        vc.willMove(toParentViewController: nil)
        vc.removeFromParentViewController()
        vc.view.removeFromSuperview()
    }

    // Remove all child view controllers
    func removeAllChildViewControllers() {
        for childVC in self.childViewControllers {
            remove(childViewController: childVC)
        }
    }
}