public class SetUnit extends ActionImpl implements Undoable {
	private Unit unit;
	private Unit oldUnit;

	@Override
	public boolean canDo() { // Checks whether the action can be executed
		return unit != null;
	}
	
	@Override
	protected void doActionBody() { // Execution of the action.
		oldUnit = ScaleRuler.getUnit();
		redo();
	}
	
	@Override
	public void undo() { // Undoes the action
		ScaleRuler.setUnit(oldUnit);
	}

	@Override
	public void redo() { // Redoes the action
		ScaleRuler.setUnit(unit);
	}

	@Override
	public String getUndoName() { // Provides a short textual description of this undoable action.
		return "Changing the unit";
	}
    //...
}

// An instrument is a controller/presenter/viewModel/component/etc: 
// it gather events produced by widgets as user interactions into actions that modify the system.
public class PreferencesSetter extends JfxInstrument implements Initializable {
    @FXML private ComboBox<String> unitChoice;
    //...

    @Override
    protected void configureBindings() throws IllegalAccessException, InstantiationException {
        //...
        // Defines an widget binding that binds a combobox to the action SetUnit
        bindComboBox(SetUnit.class,  // The type of the action to produce
            action -> action.setUnit(Unit.getUnit(unitChoice.getSelectionModel().getSelectedItem())), // The initialisation of the action
            unitChoice); // The source widget to listen
    }
}