// 淡入淡出更换rootViewController

- (void)restoreRootViewController:(UIViewController *)rootViewController
{
    typedef void (^Animation)(void);
    AppDelegate *appdelegate = (AppDelegate *)[UIApplication sharedApplication].delegate;
    UIWindow* window = appdelegate.window;
    
    rootViewController.modalTransitionStyle = UIModalTransitionStyleCrossDissolve;
    Animation animation = ^{
        BOOL oldState = [UIView areAnimationsEnabled];
        [UIView setAnimationsEnabled:NO];
        [UIApplication sharedApplication].keyWindow.rootViewController = rootViewController;
        [UIView setAnimationsEnabled:oldState];
    };
    
    [UIView transitionWithView:window
                      duration:0.5f
                       options:UIViewAnimationOptionTransitionCrossDissolve
                    animations:animation
                    completion:nil];
}

至于为什么不用 keyWindow 请参考http://www.jianshu.com/p/ae84cd31d8f0,
小心使得万年船,哈哈.