title: UITabBarController detect tab clicks
body: <p>I'm creating an app using tab bar controller. It has 2 tabs. In first tab, it will have a text field and a submit button. user will enter some value in text field and clicks submit. 
  Now my problem:
  on click of submit button, some result "X" is computed depending on value entered in text field and it will open second tab. here result "X" has to be shown in label. without entering some value in text field, if user clicks on second tab, it must show an alert that "enter some value in text field"
  How can i achieve this. please help me.</p>