Why is this React Component rendering first? - reactjs

Question

I could use your input on a quick question about Component loads.
The Goal
Return the <Login /> Component if the user isn't logged in, and the App if they are.
Expected Behavior
When a user is logged in, they see the App.
Observed Behavior
The <Login /> Component flickers (renders) for a moment, then the user sees the App.
My goal is to eliminate this flicker!
Code Samples
Index.js
export default function Index() {
let [isLoading, setIsLoading] = useState(true)
const router = useRouter()
// User object comes in from an Auth Context Provider
const { user } = useContext(AuthContext)
const { email } = user
useEffect(() => {
if (user) {
setIsLoading(false)
}
}, [])
// Returns the App if logged in, login screen if not
const getLoggedIn = () => {
if (user.loggedIn) {
return (
<>
// App goes here
</>
)
} else {
return <Login />
}
}
return (
<Box className="App">
{ isLoading
? <div className={classes.root}>
<LinearProgress />
</div>
: getLoggedIn()
}
</Box>
)
}
Auth Context
Note: I'm using Firebase for auth.
// Listens to auth state changes when App mounts
useEffect(() => {
// Calls setUser state update method on callback
const unsubscribe = onAuthStateChange(setUser)
return () => {
unsubscribe()
}
}, [])
// Brings data from auth to Auth Context user state via callback
const onAuthStateChange = callback => {
return auth.onAuthStateChanged(async user => {
if (user) {
const userFirestoreDoc = await firestore.collection('users').doc(user.uid).get()
const buildUser = await callback({
loggedIn: true,
email: user.email,
currentUid: user.uid,
userDoc: userFirestoreDoc.data()
})
} else {
callback({ loggedIn: false })
}
})
}
Stack
"next": "^8.1.0",
"react": "^16.8.6",
"react-dom": "^16.8.6"
Thanks so much for taking a look.

Answer 1

I had this exact problem and resolved it by storing the user in local storage
then on app start up do this:
const [user, setUser] = useState(JSON.parse(localStorage.getItem('authUser')))
and it'll use the details from localstorage and you wont see a flicker
(it's because onauthstate takes longer to kick in)

Answer 2

So I figured out a sort of 'hacky' way around this. One needs to set the value of the boolean on which the initial load of the App depends...
const getLoggedIn = () => {
// Right here
if (user.loggedIn) {
return (
<>
// App goes here
</>
)
} else {
return <Login />
}
...before making any asynchronous calls in the AuthContext. Like this:
const onAuthStateChange = callback => {
return auth.onAuthStateChanged(async user => {
if (user) {
// sets loggedIn to true to prevent flickering to login screen on load
callback({ loggedIn: true })
const userFirestoreDoc = await firestore.collection('users').doc(user.uid).get()
const buildUser = await callback({
loggedIn: true,
email: user.email,
currentUid: user.uid,
userDoc: userFirestoreDoc.data()
})
} else {
callback({ loggedIn: false })
}
})
}
I hope this helps someone.