使用Swift设置游戏中心

我在设置游戏中心排行榜时遇到了问题,因为我遇到了错误, Cannot assign a value of type 'ViewController' to a value of type 'GKGameCenterControllerDelegate!' 。我认为这与我放在顶部的代码有关:

class ViewController: UIViewController, GKGameCenterControllerDelegate, ADBannerViewDelegate {
//stuff
}

以及我在其中一个动作按钮中输入的代码:

//shows leaderboard screen
            func showLeader() {
                var vc = self.view?.window?.rootViewController
                var gc = GKGameCenterViewController()
                gc.gameCenterDelegate = self            //<------This is the problem child.
                vc?.presentViewController(gc, animated: true, completion: nil)
            }

发生了什么事?谢谢