title: How to disable rightBarButtonItem
body: <p>How can I enable/disable <code>navigationItem.rightBarButton</code> item? I am calling the following code at <code>-viewDidLoad</code> but it does nothing.</p>
  
  <pre><code>self.navigationItem.rightBarButtonItem = 
        [[[UIBarButtonItem alloc]
              initWithTitle:@"Flow"
              style:UIBarButtonItemStyleDone
              target:self  action:@selector(flowWithMe:)] autorelease];
  
  self.navigationItem.rightBarButtonItem.enabled = NO;
  </code></pre>
  
  <p>I want to enable right Bar Button only on a particular action.</p>