第三个参数必须是类型，而不是示例

extension UIViewController {
    func switchScreen<T : UIViewController>(storyboardName: String, viewControllerName: String, type: T.Type) {
        let mainStoryboard = UIStoryboard(name: storyboardName, bundle: Bundle.main)
        if let viewController = mainStoryboard.instantiateViewController(withIdentifier: viewControllerName) as? T {
            self.present(viewController, animated: true, completion: nil)
        }
    }
}

并称之为

switchScreen(storyboardName: "Main", 
             viewControllerName: "Foo", 
             type: LoginViewController.self)

但是由于instantiateViewController和present不关心静态类型，因此您不需要它，这就足够了

extension UIViewController {
    func switchScreen(storyboardName: String, viewControllerName: String) {
        let mainStoryboard = UIStoryboard(name: storyboardName, bundle: Bundle.main)
        let viewController = mainStoryboard.instantiateViewController(withIdentifier: viewControllerName)
        self.present(viewController, animated: true, completion: nil)
    }
}

为了避免崩溃，您可以添加一个具有可用视图控制器标识符的枚举，并在第二个参数中传递此类型，例如

enum ControllerIdentifier : String, RawRepresentable {
    case foo, bar
}

extension UIViewController {
    func switchScreen(storyboardName: String, viewControllerName: ControllerIdentifier) {
        let mainStoryboard = UIStoryboard(name: storyboardName, bundle: Bundle.main)
        let viewController = mainStoryboard.instantiateViewController(withIdentifier: viewControllerName.rawValue)
        self.present(viewController, animated: true, completion: nil)
    }
}

使用通用参数

extension UIViewController {
    func switchScreen<T: UIViewController>(storyboardName:String,viewControllerName:String,vcc:T) {
        let mainStoryboard = UIStoryboard(name: storyboardName, bundle: Bundle.main)
        if let viewController = mainStoryboard.instantiateViewController(withIdentifier: viewControllerName) as? T {
            self.present(viewController, animated: true, completion: nil)
        }
    }
}