title: RxJS 3 conditional filter
body: <p>I have 3 conditional numeric dropdown with a values <code>-1</code>; <code>0</code>; <code>1</code>; and <code>Observable&lt;Thing[]&gt;</code> datasource that I need to filter on based on dropdown.  I need to display or hide a particular rows based on dropdown:</p>
  
  <ul>
  <li><ol>
  <li>keep all the items (no filter) (<code>-1</code>) </li>
  </ol></li>
  <li><ol start="2">
  <li>filter only that have no specific field (or field is empty)  (<code>0</code>)</li>
  </ol></li>
  <li><ol start="3">
  <li>filter only that have specific field (<code>1</code>)</li>
  </ol></li>
  </ul>
  
  <p>I am trying to figure out how to connect the filter and my dropdown so based on a particular value I have related items to display.</p>
  
  <pre><code>dataSource$ = originalDataSource$.filter((item:any) =&gt; { 
   // check the conditions here
  });
  </code></pre>
  
  <p>Any thoughts?</p>