Warning: Attempt to present xxx on xxx whose view is not in the window hierarchy!

在项目中遇到一个情况在VC1中有一个当网络连接断开后会弹框(UIAlertView)提醒用户网络断开，在iOS8以上使用的时UIAlertController，如果当前屏幕窗口显示的控制器是VC1通过present出来的VC2就会出现Warning: Attempt to present xxx on xxx whose view is not in the window hierarchy!
原因就是使用UIAlertController也是present的出来的，当VC1同时present两个控制器就会在控制台打印这种错误，并且不会弹出来提醒框。
解决方法：
1、在网络断开执行的方法把当前present的控制器干掉，提醒框才能弹出来

        UIViewController *topController = [UIApplication sharedApplication].keyWindow.rootViewController;
        while (topController.presentedViewController) {
            topController = topController.presentedViewController;
                [topController dismissViewControllerAnimated:NO           completion:nil];
            }
        }

2、获取当前present的控制器再进行弹框

UIAlertController *alertController = [UIAlertController alertControllerWithTitle:@"..." message:@"+++" preferredStyle:UIAlertControllerStyleAlert];
    UIAlertAction *action = [UIAlertAction actionWithTitle:@"ssss" style:UIAlertActionStyleDefault handler:nil];
    [alertController addAction:action];
    
    UIViewController *topVC = [UIApplication sharedApplication].keyWindow.rootViewController;
    while (topVC.presentedViewController) {
        topVC = topVC.presentedViewController;
    }
    [topVC presentViewController:alertController animated:YES completion:nil];

详见这里