PostRepository.kt

@ExperimentalCoroutinesApi
class PostRepository {
    fun getPostData(): Flow<State<PostModel>> = callbackFlow {
        offer(State.Loading())
        val postDocument = Firebase.firestore
                .collection(AppConfig.POSTS_COLLECTION)
                .document("post1")

        val subscription = postDocument.addSnapshotListener { snapshot, exception ->
            exception?.let {
                offer(State.Failed(it.message.toString()))
                cancel(it.message.toString())
            }
            if (snapshot!!.exists()) {
                offer(State.Success(snapshot.toObject(PostModel::class.java)!!))
            }
        }
        awaitClose { subscription.remove() }
    }
}