how to make a customized TDF_Attribute persistent ?

I am using OCAF and have written a class that inherits from TDF_Attribute.

I want to make it persistent (save it to an XML file).

I have also written a class that inherits from XmlMDF_ADriver.

I am using XmlXCAFDrivers::DefineFormat(mApplication) to register the format, but it doesn't work.

How can I complete this implementation?