iOS 获取window根视图（rootViewController）和顶层UIViewController

#pragma mark - 获取window根视图
- (UIViewController *)rootViewController{
    if (!_rootViewController) {
        _rootViewController = [UIApplication sharedApplication].keyWindow.rootViewController;
    }
    return _rootViewController;
}

#pragma mark - 获取window顶层的UIViewController
- (UIViewController *)topViewController {
    
    _topViewController = [self _topViewController:[[UIApplication sharedApplication].keyWindow rootViewController]];
    while (_topViewController.presentedViewController) {
        _topViewController = [self _topViewController:_topViewController.presentedViewController];
    }
    
    return _topViewController;
}

- (UIViewController *)_topViewController:(UIViewController *)vc {
    if ([vc isKindOfClass:[UINavigationController class]]) {
        return [self _topViewController:[(UINavigationController *)vc topViewController]];
    } else if ([vc isKindOfClass:[UITabBarController class]]) {
        return [self _topViewController:[(UITabBarController *)vc selectedViewController]];
    } else {
        return vc;
    }
    return nil;
}