I have a view controller that displays the views of 2 sub view controllers in a given area of its view. The 2 sub view controllers are FlopVC and FipVC.

I want to animate the transition from one sub view to the other. The code I'm using is:

```
-(IBAction)flip:(id)sender{

    UIViewController *newVC = nil;

    if (self.isFlip) {
        newVC = [[FlopVC alloc] initWithNibName:nil bundle:nil];
    }else{
        newVC = [[FipVC alloc] initWithNibName:nil bundle:nil];
    }

    newVC.view.frame = CGRectMake(120, 20, 240, 260);
    [self.view addSubview:newVC.view];

    [UIView transitionFromView:self.currentVC.view
                        toView:newVC.view
                      duration:0.9
                       options:UIViewAnimationTransitionFlipFromLeft 
                    completion:^(BOOL finished) {
                        self.currentVC = newVC;
                        self.isFlip = ! self.isFlip;
                    }];

}

```

The sub views are swapped, but without any animation. What am I doing wrong?

PS the full project is [here](http://dl.dropbox.com/u/3978473/forums/tester.zip).