A simple Rx based pager

Pager.java

public class Pager<I, O> {

    private static final Observable FINISH_SEQUENCE = Observable.never();

    private PublishSubject<Observable<I>> pages;
    private Observable<I> nextPage = finish();
    private Subscription subscription = Subscriptions.empty();

    private final PagingFunction<I> pagingFunction;
    private final Func1<I, O> pageTransformer;

    public static <T> Pager<T, T> create(PagingFunction<T> pagingFunction) {
        return new Pager<>(pagingFunction, UtilityFunctions.<T>identity());
    }

    public static <I, O> Pager<I, O> create(PagingFunction<I> pagingFunction, Func1<I, O> pageTransformer) {
        return new Pager<>(pagingFunction, pageTransformer);
    }

    Pager(PagingFunction<I> pagingFunction, Func1<I, O> pageTransformer) {
        this.pagingFunction = pagingFunction;
        this.pageTransformer = pageTransformer;
    }

    /**
     * Used in the paging function to signal the caller that no more pages are available, i.e.
     * to finish paging by completing the paged sequence.
     *
     * @return the finish token
     */
    @SuppressWarnings("unchecked")
    public static <T> Observable<T> finish() {
        return FINISH_SEQUENCE;
    }

    /**
     * Transforms the given sequence to have its subsequent pages pushed into the observer subscribed
     * to the new sequence returned by this method. You can advance to the next page by calling {@link #next()}
     *
     * @param source the source sequence, which would be the first page of the sequence to be paged
     * @return a new sequence based on {@code source}, where subscribers keep receiving pages through subsequent calls
     * to {@link #next()}
     */
    public Observable<O> page(final Observable<I> source) {
        return Observable.create(new Observable.OnSubscribe<O>() {
            @Override
            public void call(final Subscriber<? super O> subscriber) {
                pages = PublishSubject.create();
                subscription = Observable.switchOnNext(pages).subscribe(new PageSubscriber(subscriber));
                subscriber.add(subscription);
                pages.onNext(source);
            }
        });
    }

    /**
     * Returns the last page received from the pager. You may use this to
     * retry that observable in case it failed the first time around.
     */
    public Observable<O> currentPage() {
        return page(nextPage);
    }

    /**
     * @return true, if there are more pages to be emitted.
     */
    public boolean hasNext() {
        return nextPage != FINISH_SEQUENCE;
    }

    /**
     * Advances the pager by pushing the next page of items into the current observer, is there is one. If the pager
     * has been unsubscribed from or there are no more pages, this method does nothing.
     */
    public void next() {
        if (!subscription.isUnsubscribed() && hasNext()) {
            pages.onNext(nextPage);
        }
    }

    public interface PagingFunction<T> extends Func1<T, Observable<T>> {
    }

    private final class PageSubscriber extends Subscriber<I> {
        private final Subscriber<? super O> inner;

        public PageSubscriber(Subscriber<? super O> inner) {
            this.inner = inner;
        }

        @Override
        public void onCompleted() {
            inner.onCompleted();
        }

        @Override
        public void onError(Throwable e) {
            inner.onError(e);
        }

        @Override
        public void onNext(I result) {
            nextPage = pagingFunction.call(result);
            inner.onNext(pageTransformer.call(result));
            if (nextPage == FINISH_SEQUENCE) {
                pages.onCompleted();
            }
        }
    }
}

@mttkay I did a Pager based on yours:

public class Pager<I, P> {
  private PublishSubject<P> pages;
  private P nextPageToken;

  private final Func2<P, I, P> pagingFunction;
  private final Func1<P, Observable<I>> obtainFunction;

  public static <T, O> Pager<T, O> create(O firstPage, Func2<O, T, O> pagingFunction,
      Func1<O, Observable<T>> obtainFunction) {
    return new Pager<>(firstPage, pagingFunction, obtainFunction);
  }

  Pager(P firstPage, Func2<P, I, P> pagingFunction, Func1<P, Observable<I>> obtainFunction) {
    this.nextPageToken = firstPage;
    this.pagingFunction = pagingFunction;
    this.obtainFunction = obtainFunction;

    pages = PublishSubject.create();

    pages.doOnNext(p -> {
      if (p == null) {
        pages.onCompleted();
      }
    });
  }

  public Observable<I> getObservable() {
    return Observable.defer(() -> page(nextPageToken));
  }

  public boolean hasNext() {
    return nextPageToken != null;
  }

  public void next() {
    if (pages.hasObservers() && hasNext()) {
      pages.onNext(nextPageToken);
    }
  }

  private Observable<I> page(final P source) {
    return pages.startWith(source)
        .flatMap(obtainFunction)
        .doOnNext(page -> nextPageToken = pagingFunction.call(nextPageToken, page));
  }
}

The usage is:
Pager.create(initialPageToken, (oldPageToken, pageResult) -> pageResult.getNextPageToken(), datasource::getPage )
or if you want to use offset instead of token:
Pager.create(0, (offset, pageResult) -> offset + pageResult.size(), datasource::getPageWithOffset )
basically the first 2 parameter are like rxjava scan

I don't know if I may be introducing any problem, I just tried to simplify the code.

@mttkay Are you missing setting nextPage = source inside of the public Observable<O> page(final Observable<I> source) method? If the first request fails, then nextPage Observable is never updated and is still equal to FINISH_SEQUENCE. This means you can't retry the first request using the currentPage() method as per your comment :(. Otherwise this solution is great, thank you!