Dynamic Legend in Flex

I am using compiled version of ArcGIS Viewer for Flex (v 3.6) to create web map. I am using Dynamic Legend widget. I want to set the legend to only show classes that are visible in the current map extent. It is possible? I am using ArcGIS Server 10.1 and my layers tipe are "tiled", "feature" or "dynamic". Now the dynamic legend widget can be set so that the legend in web map to only show layers that are available at the current map scale. But I read that the dynamic legend can also said function. Does anyone know how to do it? Thank you