1.iOS13获取keywindow,不推荐使用 [UIApplication sharedApplication].keyWindow，过期了，
@property(nullable, nonatomic,readonly) UIWindow *keyWindow API_DEPRECATED("Should not be used for applications that support multiple scenes as it returns a key window across all connected scenes", ios(2.0, 13.0));
需要使用
[UIApplication sharedApplication].windows[0];
2.present一个控制器后的keywindow，和页面前的，不一定是一个window。需要页面展示完成获取当前页的主窗口，进行操作。
//iOS13下面不推荐的方法
[self presentViewController:navContoller animated:YES completion:NULL];
UIWindow *keyWindow = [UIApplication sharedApplication].keyWindow;
//不一定是页面展示完成的window，故view不一定展示
[keyWindow addSubview:view];
//推荐的做法
[self presentViewController:navContoller animated:YES completion:^{
UIWindow *keyWindow = [UIApplication sharedApplication].keyWindow;
[keyWindow addSubview:view];
}];
。。。待续
