[flexcoders] help binding a property of a class inside a class to a Label

[Brian]

I'm trying to bind a Text control to a function to get the description
of an item, when a property of a class inside that item changes. Is
this at all possible? I've tried different variations of binding
functions, Binding tags, etc. and still can't get it to work. Here are
code snippets that will hopefully explain what I'm trying to do.

package src
{
  [Bindable]
  public class Toy
  {
    public getDescription():String {
      // will be overridden in subclasses
      return "";
    }
  }
}

package src
{
  [Bindable]
  public class ToyTruck extends Toy
  {
    public var color:String;

    public override function getDescription():String {
      return color + " truck";
    }
  }
}

package cart
{
  import src.*;
  [Bindable]
  public class CartItem
  {
    public var quantity:int
    public var toy:Toy;

    public function getDescription():String {
      return quantity + " " + toy.getDescription();
    }
  }
}

MXML component
<mx:VBox>
  <mx:Script>
    <![CDATA[
      import cart.CartItem;
      import src.*;

      [Bindable]
      public var _cartItem;

      public function getText(toy:Toy):String {
        return _packItem.getDescription();
      }
    ]]>
  </mx:Script>
  <mx:Text id="itemDesc" text="{getText(_cartItem.toy)}"/>
</mx:Vbox>

What I'd like to have happen is have the Text control update when the
ToyTruck.color is changed by other events. Is this possible? The above
code snippets is what I'm currently trying but the Tex is not
updating. Even though the toy is not used directly in the getText()
function, I thought having it as a parameter was how the function was
triggered. The docs say that when a bindable property as an argument
of the function changes, the function executes.

In reality, there are different properties for different subclasses of
Toy, which could be modified by several different events. This is why
I'm trying to get the binding to work, so I don't have to manually
update the Text control all over the place. I thought that was the
whole point of Binding, but maybe I'm missing something.

Or do I have to use a Binding tab for each of those different
properties of the subclasses? I haven't tried that yet but I don't
think that would work because the source has to be a string, right?

Thanks