[flexcoders] cairngorm store example - newbie binding question

[normc_actionscript]

Greetings Flex pros:

In the modified cairngorm store (if you're familiar), file
ProductsAndCheckoutControlBar.mxml, there's:

<mx:Image
source="{ getProductThumbnailsButton( model.workflowState ) }"

That function (getProductThumbnailsButton) is not marked as 'bindable'
so how does the binding happen?

The var model is the only thing marked as bindable in the file.

This is the function:
    public function getProductThumbnailsButton( enforceBining : Number )
: Class
    {
     var model : ShopModelLocator = ShopModelLocator.getInstance();
     var result : Class = model.assets.thumbOff;

     if ( model.workflowState ==
ShopModelLocator.VIEWING_PRODUCTS_IN_THUMBNAILS )
     {
      result = model.assets.thumbOn;
     }

     return result;
    }