FirebaseProfileService.kt

fun getPosts(userId: String): Flow<List<Post>> {
    val db = FirebaseFirestore.getInstance()
    return callbackFlow {
        val listenerRegistration = db.collection("users")
                .document(userId)
                .collection("posts")
                .addSnapshotListener { querySnapshot: QuerySnapshot?, firebaseFirestoreException: FirebaseFirestoreException? ->
                    if (firebaseFirestoreException != null) {
                        cancel(message = "Error fetching posts",
                                cause = firebaseFirestoreException)
                        return@addSnapshotListener
                    }
                    val map = querySnapshot.documents.
                            .mapNotNull { it.toPost() }
                    offer(map)
        }
        awaitClose {
            Log.d(TAG, "Cancelling posts listener")
            listenerRegistration.remove()
        }
    }
}