更改pushViewController和popViewController的动画效果

iPhone SDK的NavigationController提供的动画效果默认只有一种，如何实现各种不同的呢？
下面是来自three20的实现，大家只要把这两个方法作为UINavigationController的Category方法调用就可以了

 (void)pushAnimationDidStop {
}

- (void)pushViewController: (UIViewController*)controller
    animatedWithTransition: (UIViewAnimationTransition)transition {
  [self pushViewController:controller animated:NO];

  [UIView beginAnimations:nil context:nil];
  [UIView setAnimationDuration:TT_FLIP_TRANSITION_DURATION];
  [UIView setAnimationDelegate:self];
  [UIView setAnimationDidStopSelector:@selector(pushAnimationDidStop)];
  [UIView setAnimationTransition:transition forView:self.view cache:YES];
  [UIView commitAnimations];
}

- (UIViewController*)popViewControllerAnimatedWithTransition:(UIViewAnimationTransition)transition {
  UIViewController* poppedController = [self popViewControllerAnimated:NO];

  [UIView beginAnimations:nil context:NULL];
  [UIView setAnimationDuration:TT_FLIP_TRANSITION_DURATION];
  [UIView setAnimationDelegate:self];
  [UIView setAnimationDidStopSelector:@selector(pushAnimationDidStop)];
  [UIView setAnimationTransition:transition forView:self.view cache:NO];
  [UIView commitAnimations];

  return poppedController;
}