top view controller

@implementation UIWindow (Additions)

- (UIViewController *)topViewController
{
    UIViewController *topViewController = self.rootViewController;

    while (topViewController.presentedViewController != nil) {
       topViewController = topViewController.presentedViewController;
    }

    while (topViewController.childViewController.count > 0) {
        if ([topViewController isKindOfClass:[UITabBarController class]]) {
            topViewController = ((UITabBarController *)topViewController).selectedViewController;
        } else if ([topViewController isKindOfClass:[UINavigationController class]]) {
            topViewController = ((UINavigationController *)topViewController).topViewController;
        } else { // 可以再加判斷自己的 container view controller
            break;
        }
    }

    return topViewController;
}