code: def on_canvas_change(self, event):
          
          # Connect key events from canvas to camera. 
          # TODO: canvas should keep track of a single node with keyboard focus.
          if event.old is not None:
              event.old.events.key_press.disconnect(self.viewbox_key_event)
              event.old.events.key_release.disconnect(self.viewbox_key_event)
          if event.new is not None:
              event.new.events.key_press.connect(self.viewbox_key_event)
              event.new.events.key_release.connect(self.viewbox_key_event)
docs: Canvas change event handler
  
          Parameters
          ----------
          event : instance of Event
              The event.