code: function invokeCallback(expression, event, item) {
        return $parse(expression)(scope, {
          event   : event,
          index   : getPlaceholderIndex(),
          item    : item || undefined,
          external: !dndDragTypeWorkaround.isDragging,
          type    : dndDragTypeWorkaround.isDragging ? dndDragTypeWorkaround.dragType : undefined
        });
      }
docs: Invokes a callback with some interesting parameters and returns the callbacks return value.