很多时候我们需要获取到app的当前ViewController，比如说用户点击推送消息需要跳转到指定页面的时候等等，如果你要从当前的视图控制器跳转，那么你需要获取当前是哪一个VC

-(UIViewController *)getCurrentVC
{
 UIViewController *result = nil;
    UIWindow * window = [[UIApplication sharedApplication] 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;
            }
        }
    }
    id  nextResponder = nil;
    UIViewController *appRootVC=window.rootViewController;
    //    如果是present上来的appRootVC.presentedViewController 不为nil
    if (appRootVC.presentedViewController) {
        nextResponder = appRootVC.presentedViewController;
    }else{

        NSLog(@"===%@",[window subviews]);
        UIView *frontView = [[window subviews] objectAtIndex:0];
        nextResponder = [frontView nextResponder];
    }

    if ([nextResponder isKindOfClass:[UITabBarController class]]){
        UITabBarController * tabbar = (UITabBarController *)nextResponder;
        UINavigationController * nav = (UINavigationController *)tabbar.viewControllers[tabbar.selectedIndex];
        //        UINavigationController * nav = tabbar.selectedViewController ; 上下两种写法都行
        result=nav.childViewControllers.lastObject;

    }else if ([nextResponder isKindOfClass:[UINavigationController class]]){
        UIViewController * nav = (UIViewController *)nextResponder;
        result = nav.childViewControllers.lastObject;
    }else{
        result = nextResponder;
    }
    return result;   
}