combine
fun <T, C : Collection<T>, R, O> Observable<C>.combineLatestCollection(transformer: (T) -> Observable<R>, aggregator: (List<R>) -> O): Observable<O>
Maps every element of the current collection to an observable through transformer, and aggregates their current values through aggregator. The result is recomputed whenever the collection changes or any mapped observable emits; observables of elements leaving the collection are unsubscribed.