翻转视图时出现问题

- (void)loadFlipsideViewController {
    ProblemViewFlipController *viewController = [[ProblemViewFlipController alloc] initWithNibName:@"ProblemViewFlip" bundle:nil];
    self.problemViewFlipController = viewController;
    [viewController release];
}


- (void) flipView {

    if (problemViewFlipController == nil) {
        [self loadFlipsideViewController];
    }

    UIView *mainView = self.view;
    UIView *flipView = problemViewFlipController.view;

    [UIView beginAnimations:nil context:NULL];
    [UIView setAnimationDuration:1];

    [UIView setAnimationTransition:([mainView superview] ? UIViewAnimationTransitionFlipFromLeft : UIViewAnimationTransitionFlipFromRight) forView:self.view cache:YES];

    if ([flipView superview])
    {
        [flipView removeFromSuperview];
        [self.view addSubview:mainView];
    }
    else
    {
        [mainView removeFromSuperview];
        [self.view addSubview:flipView];
    }

    [UIView commitAnimations];
}

问题是,当我调用flipView时,该视图将替换为空白视图(即,我要翻转的视图中不会显示任何内容)。

这里有什么明显的我遗漏的吗?(我怀疑有!)