与在Swift的类扩展函数中使用’self’类似，您可以定义一个通用的辅助方法，该方法可以从调用上下文中推断出self的类型：
extension UIViewController{ class func instantiateFromStoryboard(storyboardName: String, storyboardId: String) -> Self { return instantiateFromStoryboardHelper(storyboardName, storyboardId: storyboardId) } private class func instantiateFromStoryboardHelper<T>(storyboardName: String, storyboardId: String) -> T { let storyboard = UIStoryboard(name: storyboardName, bundle: nil) let controller = storyboard.instantiateViewControllerWithIdentifier(storyboardId) as! T return controller }}然后
let vc = MyViewController.instantiateFromStoryboard("name", storyboardId: "id")进行编译，并将类型推断为
MyViewController。
Swift 3 更新 ：
extension UIViewController{ class func instantiateFromStoryboard(storyboardName: String, storyboardId: String) -> Self { return instantiateFromStoryboardHelper(storyboardName: storyboardName, storyboardId: storyboardId) } private class func instantiateFromStoryboardHelper<T>(storyboardName: String, storyboardId: String) -> T { let storyboard = UIStoryboard(name: storyboardName, bundle: nil) let controller = storyboard.instantiateViewController(withIdentifier: storyboardId) as! T return controller }}另一种可能的解决方案，使用
unsafeDowncast：
extension UIViewController{ class func instantiateFromStoryboard(storyboardName: String, storyboardId: String) -> Self { let storyboard = UIStoryboard(name: storyboardName, bundle: nil) let controller = storyboard.instantiateViewController(withIdentifier: storyboardId) return unsafeDowncast(controller, to: self) }}
