我实现的最终解决方案是：

 FirebaseUser firebaseUser;
    try {
      firebaseUser = (await FirebaseAuth.instance.signInWithCredential(credential)).user;
    } catch(e) {
      print(e.toString());
    } finally {
      final userToSubmit = User(
          email: firebaseUser.email,id: firebaseUser.uid,name: firebaseUser.displayName,owner: false,restauranteAdded: false);
      DocumentReference dbRef =
      Firestore.instance.collection('users').document(firebaseUser.uid).collection('nameOfTheCollection').document('documentName');

      dbRef.get().then((data) {
        data.exists ? _goToHomeScreen(user) :  
          _createUser();
      });

通过这种方式，我检查用户文档是否存在于Firestore数据库中，如果不存在，它将创建该文档。

发生了很多事情。通常，您不会使用state方法更新任何setState()对象。 setState用于告诉窗口小部件某些state已更改-并且与更改一起被称为 ，但是在这里，您只是在检查文档是否存在，没有任何变化，这是不需要的。

而且看来您根本没有真正返回user对象，因此在我阅读本文时，此代码将始终返回false。

1. 您应该在此处完全删除setState（执行此操作不需要）
2. 运行登录方法，该方法返回您要执行操作的用户对象。
3. 一旦您知道已登录的用户（即带有已登录用户的uid），便可以访问数据库

这里是尝试的选项：

//Create a User class so that you don't have to call FireBase in your methods.
class User {
  User({
    @required this.uid,this.email,this.displayName,});
  final String uid;
  final String email;//these are whatever you want
  final String displayName;
}

//put sign in methods into an abstract class. Don't have to,but keeps code flexible if you don't stay with firebase
abstract class AuthBase {
 //add in other signin methods as needed
  Future<User> signInWithGoogle();

}

//Implement AuthBase in class Auth
class Auth implements AuthBase {
  final FirebaseAuth _firebaseAuth = FirebaseAuth.instance;

//Get a User from Firebase User
  User _userFromFirebase(FirebaseUser user) {
    if (user == null) {
      return null;
    }
    return User(
      uid: user.uid,displayName: user.displayName,email: user.email,);
  }

//your signinWithGoogle method
Future<User> signInWithGoogle() async {
  //where User is the class you've created
  GoogleSignIn googleSignIn = GoogleSignIn();
  GoogleSignInAccount googleUser = await googleSignIn.signIn();

  if (googleUser != null) {
    GoogleSignInAuthentication googleAuth = await googleUser.authentication;
    if (googleAuth.idToken != null && googleAuth.accessToken != null) {
      final authResult = await _firebaseAuth.signInWithCredential( //await your instance of FirebaseAuth
        GoogleAuthProvider.getCredential(
          idToken: googleAuth.idToken,accessToken: googleAuth.accessToken,),);

      return _userFromFirebase(authResult.user); //your signed in firebase user that you can do stuff with uid,email,displayname,etc.
    } else {
      throw PlatformException(
        code: 'ERROR_MISSING_GOOGLE_AUTH_TOKEN',message: '',);
    }
  } else {
    throw PlatformException(
      code: 'ERROR_ABORTED_BY_USER',);
  }
}