场景需求：
1：app在后台挂起状态下收到个远程推送，要进行页面跳转。
2：类似微信，来个视频电话，在收到来电监听后要在任意页面能吊起通话界面 进行跳转

上代码：

+(UIViewController*)getCurrentVC {
    
    UIWindow * window = [[UIApplication sharedApplication] keyWindow];

/*  
keyWindow和appdelegate.window可能不是同一个  所以还是查keyWindow 

    //app默认windowLevel是UIWindowLevelNormal，如果不是，找到UIWindowLevelNormal的
    if (window.windowLevel != UIWindowLevelNormal)
    {
        NSArray *windows = [[UIApplication sharedApplication] windows];
        for(UIWindow * tmpWin in windows)
        {
            if (tmpWin.windowLevel == UIWindowLevelNormal)
            {
                window = tmpWin;
                break;
            }
        }
    }
*/
    UIViewController* viewController = window.rootViewController;
    return [self findBestViewController:viewController];
}
//循环查找模态和视图容器  如果有自定义容器（自定义TabBarController这种）也要进行相关判断
+ (UIViewController *)findBestViewController:(UIViewController*)vc {
    if (vc.presentedViewController) {
        return [self findBestViewController:vc.presentedViewController];
    } else if ([vc isKindOfClass:[UISplitViewController class]]) {
        UISplitViewController* svc = (UISplitViewController*) vc;
        if (svc.viewControllers.count > 0)
            return [self findBestViewController:svc.viewControllers.lastObject];
        else
            return vc;
    } else if ([vc isKindOfClass:[UINavigationController class]]) {
        UINavigationController* navc = (UINavigationController*) vc;
        if (navc.viewControllers.count > 0)
            return [self findBestViewController:navc.visibleViewController];
        else
            return vc;
    } else if ([vc isKindOfClass:[UITabBarController class]]) {
        UITabBarController* tabVC = (UITabBarController*) vc;
        if (tabVC.viewControllers.count > 0)
            return [self findBestViewController:tabVC.selectedViewController];
        else
            return vc;
    } else {
        return vc;
    }
}