如果您要在登录后导航到其他页面，则类似于：

final FirebaseAuth firebaseAuth = FirebaseAuth.instance;
firebaseAuth
    .signInWithEmailAndPassword(
        email: emailController.text, password: passwordController.text)
    .then((result) {
  firestoreInstance
      .collection('Users')
      .document(usernameController.text)
      .get()
      .then((value) {
      var userType = (value.data)['userType'];
      if (userType == "firstType") {
        Navigator.pushReplacementNamed(context, '/homepage');
      }
      else if (userType == "secondType") {
        Navigator.pushReplacementNamed(context, '/anotherpage');
      }
    });
  }