title: Prevent user from entering UITabBarController with login
body: <p>I have a login view which leads to a <code>UITabBarController</code> with four tabs.
  What I want is when the password is empty or wrong the user is prompted to stay in the same view (the login view) and not the tab bar controller. In other words I want to be able to view the tab bar if the password is OK (non-empty and correct). Is it possible to keep showing the login view until the provided password is correct? Any ideas?</p>