title: How to catch the deselection event in Add-on SDK in Firefox
body: <p>I use the Add-on Builder &amp; SDK to develop Firefox add-on. I catch the event when users select a piece of text by the snippet:</p>
  
  <pre><code>var selection = require("sdk/selection");
  selection.on('select', function () {
  //Doing something
  });
  </code></pre>
  
  <p>However, what I want also is to do other things when users does not select that text anymore, but I can not figure it out how to do it. Anyone can help me with this? Thank you very much.</p>