遇到了iOS11 切换rootVC后之前的子控制器没有删除的问题。为了解决这个问题，采用了循环删除旧rootVC的子控制器的解决办法。但是这个办法会闪一下，为了解决这个问题，在切换之前做了截屏处理。

+ (void)replaceRootViewControllerTo:(UIViewController*) vc :(void(^)(void))callback {
    AppDelegate* del = (AppDelegate*)[UIApplication sharedApplication].delegate;
    UIViewController* root = del.window.rootViewController;

    UIGraphicsBeginImageContextWithOptions(del.window.bounds.size, FALSE, UIScreen.mainScreen.scale);
    [del.window drawViewHierarchyInRect:del.window.bounds afterScreenUpdates:YES];
    UIImage* result = UIGraphicsGetImageFromCurrentImageContext();
    UIGraphicsEndImageContext();
    UIImageView* view = [[UIImageView alloc] initWithImage:result];
    [del.window addSubview:view];

    [self dismissChildVCFrom:root :^{
        del.window.rootViewController = vc;
        //[del.window bringSubviewToFront:view];
        [view removeFromSuperview];
        if (callback) {
            dispatch_async(dispatch_get_main_queue(), ^{
                callback();
            });
        }
    }];
}

+ (void)dismissChildVCFrom:(UIViewController *)vc :(void(^)(void))callback {
    if ([vc presentedViewController]) {
        UIViewController *nextRootVC = [vc presentedViewController];
        [self dismissChildVCFrom:nextRootVC :^{
            [nextRootVC dismissViewControllerAnimated:NO completion:callback];
        }];
        
    } else if ([vc isKindOfClass:[UITabBarController class]]) {
        UIViewController *nextRootVC = [(UITabBarController *)vc selectedViewController];
        [self dismissChildVCFrom:nextRootVC :callback];
        
    } else if ([vc isKindOfClass:[UINavigationController class]]){
        UIViewController *nextRootVC = [(UINavigationController *)vc visibleViewController];
        [self dismissChildVCFrom:nextRootVC :callback];

    } else {
        callback();
    }
}