Detect when back is tapped on navigation controller without subclassing

gistfile1.m

//http://stackoverflow.com/questions/12748911/do-things-when-back-button-is-pressed

-(void)viewWillDisappear:(BOOL)animated {
  NSUInteger ind = [[self.navigationController viewControllers] indexOfObject:self];
  if (ind == NSNotFound) {
      // do something, we're coming off the stack.
  }
}