title: Xcode storyboard cancel push if a condition is not met
body: <p>I have a storyboard with a button in a Navigation View Control, this control has textfields in as well,  which uses the storyboard segue <code>push</code>. 
  However, I have some input checking, and I alert the user if any of the text fields are empty.</p>
  
  <p>I can do this, BUT it alerts the user, and then switches to the other view.</p>
  
  <p>Is there a way to cancel this <code>push</code> if an if condition is not met in the button Pressed method?</p>
  
  <p>N.B This push was created by the storyboard interface, not by code.</p>