更换UINavigationController根视图

AppDelegate* app =(AppDelegate *)[UIApplication sharedApplication].delegate;
            ViewController * vc = [[ViewController alloc]init];
            UINavigationController * navi = [[NavigationController alloc] initWithRootViewController:vc];
            VideoViewController *VideoVC = [[VideoViewController alloc]init];
            [navi pushViewController:VideoVC animated:NO];
            app.window.rootViewController = navi;
            //去显示

            [app.window makeKeyAndVisible];