How to add observer to handle viewport changing(rotate) event?

Hello,I want to recalculate the normal of all actors in the window when view port changing(rotating) by dragging or left key pressing.Below approaches have been tried:

class Callback:

    def __init__(self):

    def __call__(self, caller, ev):

interactor.AddObserver(vtk.vtkCommand.RotateEvent, Callback())
renwin.AddObserver(vtk.vtkCommand.RotateEvent, Callback())
renderer.GetActiveCamera().AddObserver(vtk.vtkCommand.RotateEvent, Callback())

But none of them will print out “rotating”,question is

Where to register which type of event to handle rotate event?