New experimental operator to switch the types on the completion of empty Observables.

[abersnaze]

For the issue #3037 a new operator that let consumer of an empty Observable tie additional work to follow after the completion.

[davidmoten]

Seems like an ok idea to me but switchEmpty and switchIfEmpty are confusing to distinguish though I like them being close together alphabetically for discoverability.

[NiteshKant]

aah @abersnaze I did not see that you have the changes here for #3037. I have created a PR #3060 for it too, although it contains an additional operator mergeError

[NiteshKant]

I implemented switchEmpty, as you call it, as a switchMap().concatWith(), is there a reason why that does not work?

[abersnaze]

I tried to reuse the switchIfEmpty and merge the switchEmpty functionality but couldn't get the generics to work. I decided to copy and edit because switchIfEmpty looked complicated enough with backpressure support.

***************
*** 65,79 ****

          @Override
          public void onCompleted() {
!             if (!empty) {
!                 child.onCompleted();
!             } else if (!child.isUnsubscribed()) {
                  subscribeToAlternate();
              }
          }
--- 64,76 ----

          @Override
          public void onCompleted() {
!             if (!child.isUnsubscribed()) {
                  subscribeToAlternate();
              }
          }
***************
*** 85,92 ****

          @Override
          public void onNext(T t) {
!             empty = false;
!             child.onNext(t);
              arbiter.produced(1);
          }
      }
--- 82,88 ----

          @Override
          public void onNext(T t) {
!             child.onError(new RuntimeException("switchEmpty used on a non empty observable. Possible fix is to add .ignoreElements() before .switchEmpty()."));
              arbiter.produced(1);
          }
      }

The overhead of the switchMap and concatWith seem pretty high for such a constrained use case.

[NiteshKant]

@abersnaze I updated my PR with the things we discussed offline and it now has custom operator implementations, optimized for this usecase where the source Observable is never expected to emit any item.

[benjchristensen]

Closing out so we can merge these efforts with #3060 after agreeing upon naming: #3037 (comment)