code: public static <R> Observable<R> ifThen(Func0<Boolean> condition, Observable<? extends R> then, Scheduler scheduler) {
          return ifThen(condition, then, Observable.<R> empty().subscribeOn(scheduler));
      }
docs: Return an Observable that emits the emissions from a specified Observable
  if a condition evaluates to true, otherwise return an empty Observable
  that runs on a specified Scheduler.
  <p>
  <img width="640" src="https://raw.github.com/wiki/ReactiveX/RxJava/images/rx-operators/ifThen.s.png" alt="">
  
  @param <R>
  the result value type
  @param condition
  the condition that decides whether to emit the emissions
  from the <code>then</code> Observable
  @param then
  the Observable sequence to emit to if {@code condition} is {@code true}
  @param scheduler
  the Scheduler on which the empty Observable runs if the
  in case the condition returns false
  @return an Observable that mimics the {@code then} Observable if the {@code condition} function evaluates to true, or an empty
  Observable running on the specified Scheduler otherwise