@NonNull @CheckResult public static Completable runTransaction(@NonNull DatabaseReference ref,
      boolean fireLocalEvents, @NonNull Function<MutableData, Transaction.Result> task) {
    return Completable.create(new RunTransactionOnSubscribe(ref, fireLocalEvents, task));
  }
@ NonNull @ CheckResult public static Completable runTransaction ( @ NonNull DatabaseReference ref , boolean fireLocalEvents , @ NonNull Function < MutableData , Transaction . Result > task ) { return Completable . create ( new RunTransactionOnSubscribe ( ref , fireLocalEvents , task ) ) ; }
@see DatabaseReference#runTransaction(Transaction.Handler, boolean)