title: Swift prepareForSegue cancel
body: <p>I'm trying to implement a login screen, when login is clicked it does the segue "login".</p>
  
  <p>I added a prepareForSegue() override to try to cancel it if the login fails but I don't see any method to cancel the segue if there is a failure.</p>
  
  <p>What is the best way to do this?</p>