更改rootViewController时添加动画

当需要更换Window的rootViewController时，怎么给这个过程添加动画呢？

[UIView transitionWithView:[[UIApplication sharedApplication].delegate window]
                  duration:0.3
                   options:UIViewAnimationOptionTransitionCrossDissolve
                animations:^{
                    [[[UIApplication sharedApplication].delegate window] setRootViewController:viewController];
                }
                completion:NULL];

这样写咋看咋对, 试试也是对的, 但当横屏时会发现动画很奇怪, 后来在stackoverflow看到一个解决办法

[UIView transitionWithView:[[UIApplication sharedApplication].delegate window]
                  duration:0.3
                   options:UIViewAnimationOptionTransitionCrossDissolve
                animations:^{
                    BOOL oldState = [UIView areAnimationsEnabled];
                    [UIView setAnimationsEnabled:NO];
                    [[[UIApplication sharedApplication].delegate window] setRootViewController:viewController];
                    [UIView setAnimationsEnabled:oldState];
                }
                completion:NULL];