title: Flatmap my observable to subject
body: <p>The question is a little tricky.</p>
  
  <p>I am trying to implement the observable interface, within it i need to start listen to another publicsubject once the observable meet some circustance, so i write some code like this:</p>
  
  <pre><code>public myAPI(){
     return  restAPI.call()
                  .flatmap{ ret -&gt;
                     if(ret == success) return myPublishSubject
                   }
  </code></pre>
  
  <p>can it guarantee the subscribe start subscribe to the publishsubject only after restAPI call is done successfully ? </p>