favRequest.java

public Observable<ResponseOrError<String>> addOrRemoveFromFavs(final int postId) {
    return getIsFavoriteObservable(postId)
        .take(1) // take first value from cache
        .doOnNext(isCurrentlyFavorite -> {
            updateFavorite(postId, !isCurrentlyFavorite); // toggle value before request for better User Experience
        })
        .switchMap(isCurrentlyFavorite -> favsApi.addOrRemoveFromFavs(isCurrentlyFavorite)
            .compose(ResponseOrError.toResponseOrErrorObservable()) // it's changing error event to onNext event to not finish a chain
            .doOnNext(responseOrError -> {
                if (responseOrError.isError()) {
                    updateFavorite(postId, isCurrentlyFavorite); // set back to previous value because of error
                }
            }));
}