Setting Combobox 'dataProvider' @runtime and using 'labelFunction'

Question

I have 2 comboboxes which are related i.e. when i change cb-1 i need to display values in cb-2 (cb-2 list varies with the value selected). I have simplified my case but this is what my problem is. After i set 'cb-2.dataProvider' when does 'labelFunction' for cb-2 get called?
If i am not clear enough please let me know. Here is the code.....
private function cb-1Change(event:ListEvent):void
{// This will be a dynamic one using the value selected in cb-1
cb-2.dataProvider = __model.someList.getItemAt(0).cb-2VOList;  // extracting from a Map kind object
// This Label function is never getting called
private function displayCB-2Label(item:Object):String{ 
     // something
<mx:ComboBox d="cb-1"           dataProvider="{__model.cb-1List}"
          change="cb-1Change(event)"
/>
<mx:ComboBox d="cb-2"
          labelFunction="displayCB-2Label(event)"
          change="cb-1Change(event)"
/>
          change="cb-1Change(event)"
/>  

Answer 1

Ryan,
        Thanks for the answer. I think i was not clear on my questions. Let me try it again:
Requirement:
       When ever user change cb-1 value it should populate a new list and assign to cb-2 dataProvider i.e. cb-2 list should refresh. cb-2 needs to display right label based on its 'labelFunction'.
cb1 -  List of Strings
cb2 -  List of VO's in the following format
ListVO:
       public var code:String;    // This Code is what user selects in cb-1
       public var  listValues:ArrayCollection;  // List of PlainVO's
PlainVO:
     public var name:String;
     public var value:String;
Also from your answer how can i call 'cb-2 label function in cb-1 change', i mean cb-2 labelFunction expects current 'ListVO' object...right?