如何解散/删除当前的rootViewController，然后弹出新设置的rootviewcontroller

Question

我有一个rootviewcontroller A，我想删除它，然后设置新的rootviewcontrollerB。但是，当我检查内存时，旧的rootviewcontroller内存尚未释放（deinit），请帮忙，非常感谢。 以下是我的代码：

    let appdelegate = UIApplication.shared.delegate as! AppDelegate
    let previousViewController = appdelegate.window!.rootViewController

    let storyBoard : UIStoryboard = UIStoryboard(name: "Main", bundle:nil)
    let newViewController = storyBoard.instantiateViewController(withIdentifier: "identifier")
    previousViewController?.dismiss(animated: true, completion: {
        previousViewController?.view.removeFromSuperview()
    })
    appdelegate.window!.rootViewController = newViewController