It is difficult to anticipate all of the app-specific needs when setting property values in FXML. Allowing scripting in FXML is one approach to allowing the ability to extend the FXML environment to be more flexible.

Another approach which allows an entire area of extensions to FXML to occur is through using a simple markup "escape" capability. When a property's value needs to be set, and the escape is activated, say with !#{markup escape content}, the escape is processed in away similar to a MarkupExtension in XAML.

This would allow you to add bi-directional binding easily or let clients develop their own binding approach from within fxml.

Related to RT-17646