[vtkusers] The vtkRenderWindowInteractor only can accept one observer, bug?

[kdsfinger at gmail.com]

I found this bug in the
Insight/InsightApplications/LiverTumorSegmentation/ImageSliceViewer.cpp
Althought it is used in itk, it's a vtk program.
In the function
void
  ImageSliceViewer
::SetInteractor( vtkRenderWindowInteractor * interactor )
{
  m_RenderWindow->SetInteractor( interactor );

  vtkInteractorStyleImage * interactorStyle = vtkInteractorStyleImage::New();
  interactor->SetInteractorStyle( interactorStyle );
  interactorStyle->Delete();
  interactor->AddObserver( ::vtkCommand::LeftButtonPressEvent,
m_InteractorObserver );
  interactor->AddObserver( ::vtkCommand::LeftButtonReleaseEvent,
m_InteractorObserver );
  interactor->AddObserver( ::vtkCommand::MouseMoveEvent, m_InteractorObserver );
}
The interactor only accept the first added observer and neglect the
rest. In the above case, only the LeftButtonPressEvent is observed. If
I switch the sequence such that the LeftButtonReleaseEvent is added
before LeftButtonPressEvent, the LeftButtonReleaseEvent will be
observed while the LeftButtonPressEvent is not.

How can I observe multiple mouse events? Thanks for comments.

zl2k