title: Extjs - Tree Panel expand event
body: <p>I want to create a tree panel and add an event to it when expand node, but code seems not work for me. I've already searched google for hours but no luck. Could anyone take a look and tell me why?</p>
  
  <p>Here is my <a href="http://jsfiddle.net/Bayoneda/RDC9W/" rel="nofollow">code</a>: </p>
  
  <pre><code>Ext.onReady(function () {
      var store = Ext.create('Ext.data.TreeStore', {
          root: {
              expanded: true,
              children: [{
                  text: "detention",
                  leaf: true
              }, {
                  text: "homework",
                  expanded: false,
                  children: [{
                      text: "book report",
                      leaf: true
                  }, {
                      text: "alegrbra",
                      leaf: true
                  }]
              }, {
                  text: "buy lottery tickets",
                  leaf: true
              }]
          }
      });
  
      Ext.create('Ext.tree.Panel', {
          title: 'Simple Tree',
          width: 200,
          height: 150,
          store: store,
          rootVisible: false,
          renderTo: Ext.getBody(),
          listeners: {
              afteritemexpand: function ( node, index, item, eOpts ){
                  alert('HAHA');
              }
          }
      });
  });
  </code></pre>
  
  <p>Thanks in advance!</p>