Is shareIn sharing Firestore queries correctly in Kotlin?

Below is a detailed answer addressing your two main concerns: using shareIn and handling potential errors such as not being signed in.

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1. Using shareIn

Your current use of shareIn looks like it shares the upstream Firestore snapshot stream within the lifetime of repoScope. In other words, if you have multiple subscribers during the five‐second “keep around” window (from SharingStarted.WhileSubscribed(5000)) you'll share the same query connection. However, note that if you call the function like this…

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val user = getProfile().first()

…each call to getProfile() creates a new shared flow instance. That is, you’re not reusing one shared instance because getProfile() builds a new flow every time you invoke it. If you want to keep a single connection during the app’s lifetime or during a ViewModel’s lifetime, consider storing the shared Flow as a property instead of reconstructing it in your repository function. For example:

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class UserRepository {
    private val repoScope = MainScope()

    // Create the flow only once.
    val profileFlow: CommonFlow<User> = firestore.collection("User")
        .where { "userID" equalTo auth.currentUser?.uid ?: throw IllegalStateException("Not signed in") }
        .snapshots
        .mapLatest { users ->
            users.documents.first()
                .data<User>() ?: throw IllegalStateException("User data missing")
        }
        .shareIn(repoScope, SharingStarted.WhileSubscribed(5000), replay = 1)
        .asCommonFlow()
}

Then subscribers can use profileFlow without re-launching the underlying source every time.

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2. Handling potential errors

You mentioned that there’s a crash when the user isn’t signed in (because of using auth.currentUser!!). To improve error handling, consider these steps:

a. Avoid force unwrapping

Instead of forcing the non-null with !!, check whether the user is signed in. For example, you could throw an exception or return an empty flow if no user exists:

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val userId = auth.currentUser?.uid 
    ?: throw IllegalStateException("User not signed in")

b. Use flow operators for error handling

If you want to emit errors gracefully or allow downstream handling rather than crashing immediately, you can use the catch operator. For instance:

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fun getProfile(): CommonFlow<User> =
    flow {
        val currentUserId = auth.currentUser?.uid
            ?: throw IllegalStateException("User not signed in")
        val snapshot = firestore.collection("User")
            .where { "userID" equalTo currentUserId }
            .snapshots
            .first() // or collect the flow here appropriately
        val userData = snapshot.documents.firstOrNull()?.data<User>()
            ?: throw IllegalStateException("User data missing")
        emit(userData)
    }
    .catch { e ->
        // You can handle or log the error here.
        // For example, rethrow if you want to let the caller know:
        throw e
        // Or emit a fallback empty user object (if applicable):
        // emit(User.empty())
    }
    .shareIn(repoScope, SharingStarted.WhileSubscribed(5000), replay = 1)
    .asCommonFlow()

In the above code:

• We first check if auth.currentUser is null—if it is, we immediately throw a clear error.
• We then safely extract the user document.
• The catch operator lets you decide whether to forward the error, log it, or emit a fallback value.

c. Consider using a sealed class/result type

For more robust error management, consider encapsulating success and failures in a sealed class or using Kotlin’s Result type. For example:

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sealed class ProfileResult {
    data class Success(val user: User) : ProfileResult()
    data class Error(val exception: Throwable) : ProfileResult()
}

fun getProfileResult(): CommonFlow<ProfileResult> =
    flow {
        val currentUserId = auth.currentUser?.uid
            ?: throw IllegalStateException("User not signed in")
        val snapshot = firestore.collection("User")
            .where { "userID" equalTo currentUserId }
            .snapshots
            .first()
        val userData = snapshot.documents.firstOrNull()?.data<User>()
            ?: throw IllegalStateException("User data missing")
        emit(ProfileResult.Success(userData))
    }
    .catch { e ->
        emit(ProfileResult.Error(e))
    }
    .shareIn(repoScope, SharingStarted.WhileSubscribed(5000), replay = 1)
    .asCommonFlow()

Then your caller can handle both the success and error cases appropriately.

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Summary

• shareIn Usage:
  Your shareIn operator is set up as intended to share the Firestore connection among multiple subscribers. However, creating a new flow instance on each call (via calling getProfile() repeatedly) means you’re not reusing a single shared instance. Consider caching the flow if persistent sharing is desired.

• Error Handling:
  Avoid using !! for auth.currentUser. Instead, check for null and handle errors gracefully using flow operators like catch or by employing a sealed class or Result type to model errors.

These improvements should help you manage Firestore connections efficiently and handle error cases like unauthenticated access without crashing your application.