title: binding NSTextField to NSNumber
body: <p>I'm trying to use an NSTextField for integer user input. The text field is bound to an NSNumber property and in the setter method I cleanup the input value (make sure it's an int) and set the property if necessary. I send the willChangeValueForKey: and didChangeValueForKey:, but the UI doesn't update to the new values while that text field is still active. </p>
  
  <p>So for example I can type "12abc" in the text field that the setter method cleans up to "12", but the text field still shows "12abc".</p>
  
  <p>I have "Continuously Update Value" checked in the interface builder. </p>
  
  <p>(I've also noticed that the setter method receives an NSString, not an NSNumber. Is that normal?)</p>
  
  <p>What's the correct way of hooking up an NSTextField to an NSNumber? What does the setter method look like for the property? How to prevent non-numeric values from showing up in the text field?</p>