How to wait for a flow to complete emitting the values - kotlin

Question

I have a function "getUser" in my Repository which emits an object representing a user based on the provided id.
flow function
fun getUser(id: String) = callbackFlow {
val collectionReference: CollectionReference =
FirebaseFirestore.getInstance().collection(COLLECTION_USERS)
val query: Query = collectionReference.whereEqualTo(ID, id)
query.get().addOnSuccessListener {
val lst = it.toObjects(User::class.java)
if (lst.isEmpty())
offer(null)
else
offer(it.toObjects(User::class.java)[0])
}
awaitClose()
}
I need these values in another class. I loop over a list of ids and I add the collected user to a new list. How can I wait for the list to be completed when I collect the values, before calling return?
collector function
private fun computeAttendeesList(reminder: Reminder): ArrayList<User> {
val attendeesList = arrayListOf<User>()
for (friend in reminder.usersToShare) {
repoScope.launch {
Repository.getUser(friend).collect {
it?.let { user ->
if (!attendeesList.contains(user))
attendeesList.add(user)
}
}
}
}
return attendeesList
}
I do not want to use live data since this is not a UI-related class.

Answer 1

There are multiple problems to address in this code:
getUser() is meant to return a single User, but it currently returns a Flow<User>
which will never end, and never return more than one user.
the way the list of users is constructed from multiple concurrent query is not thread safe (because multiple launches are executed on the multi-threaded IO dispatcher, and they all update the same unsafe list directly)
the actual use case is to get a list of users from Firebase, but many queries for a single ID are used instead of a single query
Solution to #1
Let's tackle #1 first. Here is a version of getUser() that suspends for a single User instead of returning a Flow:
suspend fun getUser(id: String): User {
val collectionReference = FirebaseFirestore.getInstance().collection(COLLECTION_USERS)
val query = collectionReference.whereEqualTo(ID, id)
return query.get().await().let { it.toObjects(User::class.java) }.firstOrNull()
}
// use the kotlinx-coroutines-play-services library instead
private suspend fun <T> Task<T>.await(): T {
return suspendCancellableCoroutine { cont ->
addOnCompleteListener {
val e = exception
if (e == null) {
#Suppress("UNCHECKED_CAST")
if (isCanceled) cont.cancel() else cont.resume(result as T)
} else {
cont.resumeWithException(e)
}
}
}
}
It turns out that this await() function was already written (in a better way) and it's available in the kotlinx-coroutines-play-services library, so you don't need to actually write it yourself.
Solution to #2
If we could not rewrite the whole thing according to #3, we could deal with problem #2 this way:
private suspend fun computeAttendeesList(reminder: Reminder): List<User> {
return reminder.usersToShare
.map { friendId ->
repoScope.async { Repository.getUser(friendId) }
}
.map { it.await() }
.toList()
}
Solution to #3
Instead, we could directly query Firebase for the whole list:
suspend fun getUsers(ids: List<String>): List<User> {
val collectionReference = FirebaseFirestore.getInstance().collection(COLLECTION_USERS)
val query = collectionReference.whereIn(ID, ids)
return query.get().await().let { it.toObjects(User::class.java) }
}
And then consume it in a very basic way:
private suspend fun computeAttendeesList(reminder: Reminder): List<User> {
return Repository.getUsers(reminder.usersToShare)
}
Alternatively, you could make this function blocking (remove suspend) and wrap your call in runBlocking (if you really need to block the current thread).
Note that this solution didn't enforce any dispatcher, so if you want a particular scope or dispatcher, you can wrap one of the suspend function calls with withContext.