there are two possibilities, independently whether you play with the dashboard or with some other diagramming app:
- if you're using MVVM the textbox would alter the data in the MVVM model and through this the data in the shapes would be updated (or triggering a recalculation or fetching new data or whatever the business rule is).
- if you are not using MVVM you can loop over the shapes contained in the diagram and alter either the Content or the DataContext or trigger a particular method of the individual shape.
Using the OnApplyTemplate, as you suggested, is not very practical since it's only executed when a new shape is added and the control template is applied. This means that the logic contained therein is oblivious to triggers after the template has been applied.
Hope this helps, Fr.