title: How to capture user input in real time in NSTextField?
body: <p>I can capture string when user click the button.
  and I also use the following method</p>
  
  <pre><code>- (BOOL)control:(NSControl *)control textShouldBeginEditing:(NSText *)fieldEditor
  </code></pre>
  
  <p>which declared in NSControlTextEditingDelegate Protocol.
  And when user begin editing, the button will be available.</p>
  
  <p><strong>My Question is:</strong>
  How to make the button disable when user delete all text (make the textField empty without clicking button)?</p>
  
  <p>The above method seems can not do it...</p>