When using ExtJS I am able to use the prepareData method to modify variables before they are applied to a template. The Touch dataview documentation indicates there is a prepareData method but when I create a 'Basic Function' as I do when using ExtJS the designer reports

A valid function must have fn set. In addition, a function of a non top level component must match a configuration.
Is it possible to use prepareData? Should I create an override?