I have an MVVM application with a TextBox control for which I want to validate that it's not empty. As the evaluation should be performed for every key stroke, I changed the binding so it contains

`Text={Binding ..., UpdateSourceTrigger=PropertyChanged}`

Now I'd also like to perform an action when the TextBox control loses its focus, so the default behaviour of the TextBox control would be suitable. My problem is that I don't know how to combine the two settings (if at all possible).

Any idea? How to react to key strokes **and** to the LostFocus event?