The function SliceNavigationController::ConnectGeometrySliceEvent( ) adds an observer. Later it's needed to remove the observer and the tag id is needed.
I put here the code:
template <typename T>
unsigned long ConnectGeometrySliceEvent(T* receiver, bool connectSendEvent=true)
{
typedef typename itk::ReceptorMemberCommand<T>::Pointer
ReceptorMemberCommandPointer;
ReceptorMemberCommandPointer eventReceptorCommand =
itk::ReceptorMemberCommand<T>::New();
eventReceptorCommand->SetCallbackFunction(receiver, &T::SetGeometrySlice);
unsigned long tag = AddObserver(GeometrySliceEvent(NULL,0), eventReceptorCommand);
if(connectSendEvent)
ConnectGeometrySendEvent(receiver);
return tag;
}