您需要首先创建一个statefull小部件，state小部件有一个名为initstate的方法，该方法在调用该小部件的build方法之前被调用。focusnode只能与输入小部件一起使用

class SettingsScreen extends StatefulWidget {
  @override
  _SettingsScreenState createState() => _SettingsScreenState();
}

class _SettingsScreenState extends State<SettingsScreen> {

  bool _showDialog = false;
  @override
  void initState() {
    // TODO: implement initState

    // do your thing, perhaps call showDialog ?
     _showDialog=  true;
    super.initState();
  }

  @override
  Widget build(BuildContext context) {

    if(_showDialog) {
      showDilaog()
   }

    return Container(

    );
  }
}