解決類似網易新聞客戶端收到新聞推送後，彈出一個UIAlert，然後跳轉到新聞詳情頁面這種需求

1.提供一個UIView的分類方法，這個方法通過響應者鏈條獲取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.通過控制器的布局視圖可以獲取到控制器實例對象

modal的展現方式需要取到控制器的根視圖

+ (UIViewController *)currentViewController
{
    UIWindow *keyWindow = [UIApplication sharedApplication].keyWindow;
    // modal展現方式的底層視圖不同
    // 取到第一層時，取到的是UITransitionView，通過這個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;
}