获取窗口当前显示的控制器

解决类似网易新闻客户端收到新闻推送后，弹出一个UIAlert，然后跳转到新闻详情页面这种需求

1.提供一个UIView的分类方法，这个方法通过响应者链条获取view所在的控制器

- (UIViewController *)parentController
{
    UIResponder *responder = [self nextResponder];
    while (responder) {
        if ([responder isKindOfClass:[UIViewController class]]) {
            return (UIViewController *)responder;
        }
        responder = [responder nextResponder];
    }
    return nil;
}

2.通过控制器的布局视图可以获取到控制器实例对象

   modal的展现方式需要取到控制器的根视图

+ (UIViewController *)currentViewController
{
    UIWindow *keyWindow = [UIApplication sharedApplication].keyWindow;
    // modal展现方式的底层视图不同
    // 取到第一层时，取到的是UITransitionView，通过这个view拿不到控制器
    UIView *firstView = [keyWindow.subviews firstObject];
    UIView *secondView = [firstView.subviews firstObject];
    UIViewController *vc = secondView.parentController;
    if ([vc isKindOfClass:[UITabBarController class]]) {
        UITabBarController *tab = (UITabBarController *)vc;
        if ([tab.selectedViewController isKindOfClass:[UINavigationController class]]) {
            UINavigationController *nav = (UINavigationController *)tab.selectedViewController;
            return [nav.viewControllers lastObject];
        } else {
            return tab.selectedViewController;
        }
    } else if ([vc isKindOfClass:[UINavigationController class]]) {
        UINavigationController *nav = (UINavigationController *)vc;
        return [nav.viewControllers lastObject];
    } else {
        return vc;
    }
    return nil;
}