title: Copy an Observable into an existing observable
body: <p>In my application, I have an Activity where an Observable is subscribed to listen to some new data.
  This Observable comes from a Controller of the Activity.</p>
  
  <p>The Controller is responsible of fetching some data and giving it to the Activity through this Observable.</p>
  
  <p>When the controller is asked to fetch some data, it first fetches data in a DB, gives the DB's data to the Activity and then perform a Retrofit request to fetch some more recent data and gives this new data to the Activity.</p>
  
  <p>The source of the data should be transparent for the Activity, a single Observable is used.</p>
  
  <p>So, whenever the Controller fetches some data from either the DB or with Retrofit, he should "gives" to the Activity Obervable all the new data.</p>
  
  <p>How can I copy an Observable values into another already existing Observable ?</p>
  
  <p>Thank you</p>