UITabbarController，点击一个tab 怎样以模态弹出一个viewcontroller？

- (BOOL)tabBarController:(RDVTabBarController *)tabBarController shouldSelectViewController:(UIViewController *)viewController
{
    UINavigationController *temp = (UINavigationController *)viewController;
    //这是我的点击第五个的时候，
    if ([[temp.viewControllers objectAtIndex:0] isKindOfClass:[FiveTabVC class]]) {
    //你可以在这里判断是否登录，如果登录了 就返回YES 没登陆走下边的
        UIViewController *ff = [[UIViewController alloc] init];
        [self.window.rootViewController presentViewController:ff animated:YES completion:^{

        }];
        return NO;
    }
    return YES;
}