如何将小部件叠加在另一个小部件上

Question

我有一个看起来像这样的应用

class MainScreen extends StatefulWidget {

  @override
  State<StatefulWidget> createState() => MainScreenState();
}

class MainScreenState extends State<MainScreen> {

  @override
  Widget build(BuildContext context) {
    return new Container(
      child: FlatButton(
        onPressed:() {
          // Want pop up to launch
        },
      ),
    )
  }
}

class PopUp extends StatefulWidget {
  PopUp({
    this.postId
  })

  @override
  State<StatefulWidget> createState() => PopUpState();
}

class PopUpState extends State<PopUp> {
  // api logic

  @override
  Widget build(BuildContext context) {
    return new Dialog()
  }
}

是否可以在不使用showDialog的情况下将PopUp小部件覆盖在MainScreen小部件之上？因为我试图使PopUp成为有状态的，因为它会根据处理后的数据进行处理和重新呈现？