title: Show and hide p:blockUI in managed bean
body: <p>I have a page with 2 dropdowns (We'll call then drop 1 and drop 2). When the user selects something from drop 1 I have a method that is called and runs a query that will populate drop 2 based on what drop 1 is. What I need to do is block the UI when drop 1 is changed and then unblock it when the query has returned and populated drop 2. I have a stub of how I think it should work but not sure how to access block UI from code.</p>
  
  <pre><code>public void onPrimaryQueryDataSourceChanged() {
      // ** Block UI **
      try {
          // Run query, get results, and populate dropdown
      } catch (SQLException e) {
          e.printStackTrace();
  
      } finally {
          // ** Unblock UI **
      }
  }
  </code></pre>
  
  <p>How can I achieve this?</p>