query-registration-example.kt

// Please don't actually use this method, it's just a nice example of how painful the listener registration API is.
// I wrote it with a case of YAGNI before I really understood the Firestore APIs—you should
// be using `Query#get()` if you need the most up-to-date copy of your documents.
fun Query.getFromServer(): Task<QuerySnapshot> = TaskCompletionSource<QuerySnapshot>().apply {
    val listener = object : EventListener<QuerySnapshot> {
        lateinit var registration: ListenerRegistration

        override fun onEvent(snapshot: QuerySnapshot?, e: FirebaseFirestoreException?) {
            if (e == null) {
                if (!snapshot!!.metadata.isFromCache) {
                    setResult(snapshot)
                    registration.remove()
                }
            } else {
                FirebaseCrash.report(e)
                setException(e)
                registration.remove()
            }
        }
    }

    listener.registration = addSnapshotListener(
            QueryListenOptions().includeQueryMetadataChanges(),
            listener)
}.task