title: RxJava- CombineLatest but only fire for one Observable's emission?
body: <p>Let's say I have two infinite Observables that can emit values at any moment. They combine to create a <code>Observable&lt;ProcessFileEvent&gt;</code>.</p>
  
  <pre><code>Observable&lt;Integer&gt;  selectedFileId= ...
  Observable&lt;MouseClick&gt; buttonClick = ...
  
  Observable&lt;ProcessFileEvent&gt; `processFileEvent` = Observable.combineLatest(selectedFileId, buttonClick, (s,b) -&gt; {
      //create ProcessFileEvent here
  });
  </code></pre>
  
  <p>The problem is I only want the processFileEvent to emit when <code>buttonClick</code> emits something, not <code>selectedFileId</code>. It's definitely not the behavior a user expects when a file ID is inputted and it kicks off a <code>ProcessFileEvent</code>. How do I combine but only emit when the <code>buttonClick</code> emits?</p>