code: public static EventStream<Void> invalidationsOf(Observable observable) {
          return new EventStreamBase<Void>() {
              @Override
              protected Subscription observeInputs() {
                  InvalidationListener listener = obs -> emit(null);
                  observable.addListener(listener);
                  return () -> observable.removeListener(listener);
              }
          };
      }
docs: Creates an event stream that emits an impulse on every invalidation
  of the given observable.