Flutter: Question about async function to retrieve Firebase user ID - firebase

Question

App Flowchart
I have a question about async function in flutter. I write an that use Firebase authentication. I want to make it such that the app will read the Firebase User ID at the top level of the app(Root Page in this case) at the init state function and then pass the user object to its child widget. Since the function to retrieve the user ID is an async function, I run into problem that the child widget get a null value for user ID even though it should not be null. I have already use future builder in the children widget but it doesn't work. Does anyone know how to do it correctly.
The exact error I am getting is "A build function returned null. The offending widget is: FutureBuilder. Build functions must never return null."
RootPage (Parent)
class _RootPageState extends State {
AuthStatus authStatus = AuthStatus.notSignIn;
String cuerrentUserId;
#override
void initState() {
super.initState();
widget.auth.currentUser().then((userId) {
setState(() {
authStatus = userId == null ? AuthStatus.notSignIn : AuthStatus.signIn;
cuerrentUserId = userId;
});
});
}
#override
Widget build(BuildContext context) {
return new Scaffold(
body: new FutureBuilder<FirebaseUser>(
future: FirebaseAuth.instance.currentUser(),
builder: (BuildContext context, AsyncSnapshot<FirebaseUser> snapshot) {
switch(authStatus) {
case AuthStatus.notSignIn:
return new LoginPage(
auth: new Auth(),
onSignedIn: _signedIn,
);
case AuthStatus.signIn:
if (snapshot.connectionState == ConnectionState.done) {
return new HomePage(
auth: widget.auth,
onSignedOut: _signedOut,
userId: snapshot.data.uid,
);
}
else {
}
}
}
),
);
}
HomePage (child)
Future<String> setUserData() async {
currentUser = User(widget.userId);
await currentUser.loadUserData();
_userName = currentUser.name;
_userEmail = currentUser.email;
_userPicURL = currentUser.avatar;
print('current user');
print(currentUser.id);
print(currentUser.email);
return _userName;
}
#override
Widget build(BuildContext context) {
return UserProvider(
user: currentUser,
child: new Container(
child: new FutureBuilder(
future: setUserData(),
builder: (BuildContext context, AsyncSnapshot<String> snapshot) {
if (snapshot.data!=null) {
...

Answer 1

You could make your main function async in order to decide during app launch if you should show the login or home page as the first screen.
This could look like the following:
Future<void> main() async {
FirebaseUser currentUser = await FirebaseAuth.instance.currentUser();
bool showHomePage = currentUser != null;
runApp(MyApp(showHomePage));
}
You could use the showHomePage param inside MyApp now to determine which screen should be shown initially. That's it.
Bonus: With this approach you also don't need to show a screen for a friction of a second which may be replaced by another one (e.g. show the home screen --> user is not logged in --> replace with login screen). This could look like a glitch in your app.