Which event is triggered when zooming?

Scripting
5

I wanted to GetWorldToScreenScale within the event handler but Rhino crashes.

This is what I do:

If you uncomment either of the commented lines rhino will crash.

import scriptcontext as sc
import Rhino
vp = None

class dc(Rhino.Display.DisplayConduit):
    def CalculateBoundingBox(self,e):
        global vp
        vp = e.Viewport#.GetWorldToScreenScale(pt)[1]
        print vp#.GetWorldToScreenScale(pt)[1]

def zoom_event_helper_func():
    #Event trigger helper function
    if sc.sticky.has_key('ZoomCallback'):
        callback = sc.sticky['ZoomCallback']
        if callback:
            callback.Enabled = False
            callback = None
            sc.sticky.Remove('ZoomCallback')
            print "zoom event listener stopped."
    else:
        callback = dc()
        callback.Enabled = True
        sc.sticky['ZoomCallback'] = callback
        print "zoom event listener started."

zoom_event_helper_func()

Can you think of any workaround, @dale?

Update:
Oh, now I see pt is not defined.