flat
fun <T> Observable<T>.flatMapCompletable(maxConcurrency: Int = Int.MAX_VALUE, mapper: (T) -> Completable): Completable
Calls the mapper for each element emitted by the Observable and subscribes to the returned inner Completable. The maximum number of concurrently subscribed inner Completables is determined by the maxConcurrency argument.
By default, all inner Completables are subscribed concurrently without any limits.
Please refer to the corresponding RxJava document.