title: How to return value in rxJava
body: <p>I'm new into rxJava and it's making my head spin. Basically I'm pulling data from youtube api with retrofit which gives back Observable and with youtubeDataMapper I'm mappng it into Youtube Pojo object which contains String videoID. So my question is, how to make this method return that string instead of Completable? </p>
  
  <p>This is my method:</p>
  
  <pre><code>@Override
    public Completable downloadVideoUrl(String query) {
  
      addSubscription(youtubeApi.getYoutubeId(query, Constants.youtubeApi)
                                .map(youtubeDataMapper::map)
                                .subscribeOn(subscribeScheduler)
                                .observeOn(observeScheduler)
                                .subscribe());
      return Completable.complete();
  }
  </code></pre>