title: Conditionally Prevent NSButton Action
body: <p>I have a pair of check boxes, NSButton instances, where I want to prevent the user from deselecting both. Either button can be checked, or both, but never should both buttons be unchecked.</p>
  
  <p>How can I intercept the click event and test to see if it is the only button checked? If I set state back to checked in the IBAction, it toggles unchecked, then checked. I don't want this. I just want to ignore the mouse click in this case.</p>
  
  <p>I also do not want it disabled, or at least not to be greyed out. That would confuse the user.</p>
  
  <p>I would think there would be a delegate of sorts like NSTextFieldDelegate with a <code>shouldChangeState</code> type method but I haven't seen this anywhere.</p>
  
  <p>This seems like a very basic question, but I couldn't find an answer that satisfied me anywhere. </p>