您不必等待构建，您应该构建一些东西，向用户显示应用程序正在加载东西（或黑屏），然后在功能结束时重新构建。

您可以在创建窗口小部件时将Widget变量设置为默认值，例如使用CircularProgressIndicator，然后使用setState对其进行更改，如下所示：

class YourWidgetState extends State<YourWidget> {
  Widget _body = CircularProgressIndicator();  // Default Body

  @override
  void initState(){
    _gotoHomeScreen();
  }

  @override
  Widget build(BuildContext context){
    return _body;
  }

  Widget _gotoHomeScreen() {
    AuthService.getuserPrefEmail().then((email){
      AuthService.email = email;
      if (email == null) {
        setState(() => _body = LoginScreen());
      } else {
        AuthService.uid = email;
        setState(() => _body = HomeMenuScreen());
      }
    });
  }
}

另一种方法是使用变量来通知您有关加载情况，例如bool finishedLoading，并使用完成后调用setState来更改值。 email变量，您设置为知道用户何时登录并进行条件构建，例如：

bool loading = true;

@override
Widget build(BuildContext context){
  if(loading) return CircularProgressIndicator();

  if(AuthService.email == null)
    return LoginScreen();
  else
    return HomeMenuScreen();
}

Widget _gotoHomeScreen() {
  AuthService.getuserPrefEmail().then((email){
    AuthService.email = email;
    if (email != null) {
     AuthService.uid = email;
    }
    setState((){ loading = false; });
  });
}

使用一个简单的FutureBuilder！

https://api.flutter.dev/flutter/widgets/FutureBuilder-class.html

FutureBuilder<Email>(
future: AuthService.getuserPrefEmail(),builder: (BuildContext context,AsyncSnapshot<String> snapshot) {
  switch (snapshot.connectionState) {
    case ConnectionState.active:
    case ConnectionState.waiting:
     return CircularProgressIndicator();
    case ConnectionState.done:
     if (snapshot.hasError) {
       return Text('Error: ${snapshot.error}');
      }
     .... here route to your screen or set it how you want
  }
 },)