[flexcomponents] How to let user create user defined function

[dorkie dork from dorktown]

In the Tree component you can assign a function to the labelFunction
property. I would like to do that in my component but I'm only receiving
errors. What I want is exactly the same as the Tree component. Here is the
closest I could get:

In my component class file:

   // user defined function. user should be able to define or redefine this
function
   private function _externalInterfaceFunction(value:Boolean):Boolean {
       return true;
   }

   // setter that lets user override default function with user defined
function
   public function set externalInterfaceFunction(value:Function) {

       if (value!=null) {
           _externalInterfaceFunction = value;
       }
   }