FlatMapInsteadOfStartWith.java

@Override
public Observable<T> call(Observable<T> upstream) {
    final LastSeen<T> lastSeen = new LastSeen<>();
    return upstream
          .doOnNext(lastSeen.consumer)
          .share()
          .flatMap(new Func1<T, Observable<T>>() {
              @Override
              public Observable<T> call(T t) {
                  return lastSeen.producer;
              }
          });
}