Force redisplay of NSTextField after NSFormatter subclass change

FROM : Jonathan Fewtrell
DATE : Tue Nov 27 17:27:23 2007

I have an NSTextField bound to a certain key in a Core Data model 
entity. A switch in the interface allows the user to display the 
value in either of two formats. The formats are quite different, so I 
have two different NSFormatter subclasses and I set the appropriate 
one when the user toggles the switch. I find that the displayed value 
immediately after a switch is spurious, So I want to force the field 
to redisplay. Nothing I try seems to get it to redisplay. I have 
tried the following:

sending -setNeedsDisplay: to the field
sending -display to the field
setting the field hidden and then unhidden
sending a -willChangeValueForKey: / -didChangeValueForKey; pair to 
the model object

It appears that because the underlying value of the key (which is an 
NSNumber) does not change, the field refuses to update. The only way 
I can get it to work is to change the value of the key and change it 
back again. But this seems a clumsy hack. Is there a better way?