How do I get NSPopUpButtonCell to re-render itself?

I am creating a custom subclass of NSPopUpButton and NSPopUpButtonCell to display a single icon on top of a button instead of plain text.

To do this, I override

- (void)drawInteriorWithFrame:(NSRect)cellFrame inView:(NSView *)controlView

      

        
        
        
      

    

in my subclass of the button cell to draw the icon at the appropriate size and location. When the user clicks on the I button, I want to change the image. My only problem is I can't figure out how to re-render the buttons of the buttons (by calling drawInterior ... again). By default, it only appears when the button is pressed again or when the focus is moved to another window. I've tried setNeedsDisplay: YES, but it doesn't look like this.