remove subview from NSTextFieldCell

I added an NSProgressIndicator to a subclass of a NSTextFieldCell but when the NSTextFieldCell is removed from the table, my progress indicator is still being drawn and isn't actually removed until another cell is drawn in its place (one without a progress indicator). I guess that the NSTextFieldCell has not actually been removed from display but its still there and its contents are not being drawn, except for my progress indica开发者_运维百科tor, which the superclass knows nothing about. How can I know when the NSTextFieldCell is no longer being displayed so I can remove the progress indicator?