Iphone 为什么我的翻转动画没有更改为其他视图？

这是我的翻转动画代码，我使用两个视图，即viewMain和viewStart

- (IBAction)readyBtn:(id)sender {
    [UIView beginAnimations:nil context:nil];
    [UIView setAnimationDuration:1.0];
    [UIView setAnimationTransition:UIViewAnimationTransitionFlipFromRight forView:[self view] cache:YES];
    [[self view] addSubview:viewStart];
    [UIView commitAnimations];
}

- (IBAction)startBtn:(id)sender {
    [UIView beginAnimations:nil context:nil];
    [UIView setAnimationDuration:1.0];
    [UIView setAnimationTransition:UIViewAnimationTransitionFlipFromLeft forView:[self view] cache:YES];
    [viewStart removeFromSuperview];
    [UIView commitAnimations];
}

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运行后，我的viewMain将翻转到同一个viewMain，它无法将翻转更改为viewStart。如何修复它？

但是为什么我的viewStart会在我的viewMain完成翻转后显示？很奇怪

(IBAction)readyBtn:(id)sender {
    [UIView beginAnimations:nil context:NULL];
    [UIView setAnimationDuration:1];
    [UIView setAnimationTransition:UIViewAnimationTransitionFlipFromRight forView:viewMain cache:YES];
    viewStart.frame = viewMain.bounds;
    [viewMain addSubview:viewStart];
    [UIView commitAnimations];