[vtkusers] Disabling middle mouse button for vtkImageTracerWidget

[Dean Inglis]

> David,
> I think except for this line with overrides the pointer type for "self" 
> and
>  keeps the functionality in vtkImageTracerWidget...
>
>  vtkImageTracerWidget* self = reinterpret_cast<vtkImageTracerWidget
>
> It looks to me like ProcessEvents() is the callback function used for all 
> of
> the callback operations, and the way ProcessEvents() is coded it defeats 
> sub
> classing. Hope that helps,

>Hm, so I would have to implement the ProcessEvents() function and
>copy/paste everything except the middle mouse button stuff? That seems
>quite silly... Is there a reason that it is implemented like this
>instead of allowing subclassing?

David,

vtkImageTracerWidget is written in the old style of widgets (inherit from 
vtk3DWidget)
and so ProcessEvents has this method signature common to most/all(?) 
vtk3DWidget subclasses.
You could subclass the widget's AddObservers method or set the widget's 
interactor,
and after the SetEnabled call, you could remove the command from the 
interactor with
RemoveObservers.

Dean