It sounds more like you want to create a tool. See the UI::Tool class in the API. Then read through and understand the "linetool.rb" script in the "Plugins/Examples" directory. Keep in mind that scripting within a GUI application's process, is event-driven programming, not a linear sequential exercise. You write a class with callback methods, that the application calls, in response to UI input, mouse movements, key presses, etc., from the user.

Did you read the "linetool.rb" file in the "Examples" directory ??