title: Flex 4 COLLECTION_CHANGE event not firing
body: <p>I'm currently trying to achieve 2 way binding of an ArrayCollection object.  However, the COLLECTION_CHANGE event is not firing.</p>
  
  <p>App.mxml</p>
  
  <pre><code>&lt;?xml version="1.0" encoding="utf-8"?&gt;
  &lt;s:Application xmlns:fx="http://ns.adobe.com/mxml/2009" 
                 xmlns:s="library://ns.adobe.com/flex/spark" 
                 xmlns:mx="library://ns.adobe.com/flex/mx" 
                 xmlns:components="components.*"
                 creationComplete="handleCreationComplete(event)"&gt;
      &lt;fx:Script&gt;
          &lt;![CDATA[
              import mx.collections.ArrayCollection;
  
              [Bindable]
              public var booths:ArrayCollection;              
  
              public function handleCreationComplete(event:Event):void
              {
                  // ADD SOME BOOTHS
                  booths = new ArrayCollection();
                  booths.addItem( "item1" );
                  booths.addItem( "item2" );
              }
  
          ]]&gt;
      &lt;/fx:Script&gt;
  
      &lt;components:FloorplanGrid id="grid" width="400" height="300" booths="{booths}" /&gt;
  &lt;/s:Application&gt;
  </code></pre>
  
  <p>FloorplanGrid.as</p>
  
  <pre><code>package components
  {       
      import mx.collections.ArrayCollection;
      import mx.events.CollectionEvent;
  
      import spark.components.Group;
  
      [Event(name="collectionChanged", type="events.CollectionEvent")]
  
      public class FloorplanGrid extends Group
      {
          [Bindable]
          public var booths:ArrayCollection = new ArrayCollection();
  
          public function FloorplanGrid()
          {
              booths.addEventListener(CollectionEvent.COLLECTION_CHANGE, handleBoothsChange);
              super();
          }
  
          private function handleBoothsChange(event:CollectionEvent):void
          {
              trace("HANDLE BOOTHS CHANGE");
              /* draw booths */
          }
      }
  }
  </code></pre>
  
  <p>I'm trying to achieve 2-way binding with the <strong>Booths</strong> variable.  However, the COLLECTION_CHANGE event never fires even when I add 2 new items in the Booths variable in the App.mxml</p>