Flutter + SharedPreferences: a build function returned null - flutter

Question

I'm trying to get access to the SharedPreferences and create a permanent file named "first_run".
At the first start of the application it should return "true" and then change it to false.
I declared a future function that return true or false based on that.
Now i got a Wrapper() widget that shows either Loading... , the HomeScreen() or the LoginScreen()
based on the result of the future function.
Why is it that the build function returns null ?
How can I avoid the "first_run" to get deleted when I update the app ?
Here's the code:
import 'package:flutter/material.dart';
import 'package:firebase_auth/firebase_auth.dart';
import 'package:shared_preferences/shared_preferences.dart';
import '../screens/home_screen.dart';
import '../screens/login_screen.dart';
import '../providers/auth_provider.dart';
class Wrapper extends StatefulWidget {
#override
_WrapperState createState() => _WrapperState();
}
class _WrapperState extends State<Wrapper> {
FirebaseAuth auth = FirebaseAuth.instance;
#override
void initState() {
AuthProvider().isUserLoggedIn();
super.initState();
}
#override
Widget build(BuildContext context) {
FutureBuilder(
future: hasAlreadyStarted(),
builder: (context, snapshot) {
if (snapshot.hasData == true) {
return LoginScreen();
} else {
return CircularProgressIndicator(
backgroundColor: Colors.deepOrange,
);
}
},
);
}
Future <bool> hasAlreadyStarted() async {
SharedPreferences prefs = await SharedPreferences.getInstance();
if (prefs.getBool("first_run") == true) {
prefs.setBool("first_run", false);
return true;
} else {
return false;
}
}
}

Answer 1

You need return keyword
#override
Widget build(BuildContext context) {
return FutureBuilder(
future: hasAlreadyStarted(),
builder: (context, snapshot) {
if (snapshot.hasData == true) {
return LoginScreen();
} else {
return CircularProgressIndicator(
backgroundColor: Colors.deepOrange,
);
}
},
);
}

Answer 2

Apart from the fact that you need a return, (which the other answer already pointed out), you should also not produce a new Future every time the build method is called.
Future<bool> _yourFuture;
#override
void initState() {
AuthProvider().isUserLoggedIn();
super.initState();
_yourFuture = hasAlreadyStarted(); // <= start it once
}
#override
Widget build(BuildContext context) {
return FutureBuilder(
future: _yourFuture, // <= reference it for every build