The UI Framework, runs in the browser. Debugging it can be hard as the source code of the core widget is minified. (Some custom widgets from the app store are shipped with the code.
Try that in Eclipse. First in Studio Pro, press Ctrl-F5 “Deploy for Eclipse”. Then in Eclipse open the project, look for the widget, set the breakpoint and start the project in debug modus.
Why do you want to know how the class is applied to the field? What would like to achieve when you know how it is done?