title: Disable keyboard "Go" button when another text field is empty
body: <p>I need to disable the keyboard "Go" button when the following occur at the same time:</p>
  
  <ol>
  <li>Username textfield is empty AND</li>
  <li>Password textfield has focus (even if there is text already entered in the password field)</li>
  </ol>
  
  <p>I have two text fields "txtUsername" and "txtPassword" with the following properties set:</p>
  
  <pre><code>[txtUsername setReturnKeyType:UIReturnKeyNext];
  [txtPassword setReturnKeyType:UIReturnKeyGo];
  [txtPassword setEnablesReturnKeyAutomatically:YES];
  </code></pre>
  
  <p>The "Go" button gets disabled only when the password text field is empty (doesn't matter if there is text in the username text field or not)... I'm not sure how to go about solving it when the above two conditions are met..</p>