问题：今天在项目中遇到这样一个需求，tabBar上有首页，商家，购物车，个人中心四个模块，当点击商家和购物车时要判断用户是否已经登录，如果没有就要跳转到登录界面，登录成功后回到指定的控制器，就拿购物车来说，登录成功后自动跳到购物车界面去

思路：在AppDelegate里面设置tabBarController的代理为self，在- (BOOL)application:(UIApplication *)application didFinishLaunchingWithOptions:(NSDictionary *)launchOptions方法中添加监听事件，登录成功发送通知，通知信息里面传递tabBar的selectIndex值，至于如何传递这个值有很多方法。然后实现- (BOOL)tabBarController:(UITabBarController *)tabBarController shouldSelectViewController:(UIViewController *)viewController方法，跳转到登录控制器，登录完成自动跳回

以下是AppDelegate核心代码：

- (BOOL)application:(UIApplication *)application didFinishLaunchingWithOptions:(NSDictionary *)launchOptions {
    [[NSNotificationCenter defaultCenter]addObserver:self selector:@selector(logSelect:) name:LoginSuccessNotification object:nil];//接收成功的通知
    }

- (BOOL)tabBarController:(UITabBarController *)tabBarController shouldSelectViewController:(UIViewController *)viewController
{
    //这里我判断的是当前点击的tabBarItem的标题
    if ([viewController.tabBarItem.title isEqualToString:@"购物车"])
    {
        //我的登录状态存放在本地
        if ([[[NSUserDefaults standardUserDefaults] objectForKey:LoginStatu] isEqualToString:successLogin]) {
            return YES;
        }
        else//未登录
        {
            //跳到登录页面，这里我是用单利来存储tabbar的selectIndex的，个人习惯
            LoginViewController *login = [[LoginViewController alloc] init];
            [MKJRequestHelper shareRequestHelper].selectIndex = @"2";
            login.hidesBottomBarWhenPushed = YES;
            [((UINavigationController *)tabBarController.selectedViewController) pushViewController:login animated:YES];
            return NO;
        }
    }

    //如果是商家
    else if([viewController.tabBarItem.title isEqualToString:@"商家"])
     {
            if ([[[NSUserDefaults standardUserDefaults] objectForKey:LoginStatu] isEqualToString:successLogin])
         {
            return YES;
         }
        else
        {
            //跳到登录页面
            LoginViewController *login = [[LoginViewController alloc] init];
             [MKJRequestHelper shareRequestHelper].selectIndex = @"1";
            login.hidesBottomBarWhenPushed = YES;
            [((UINavigationController *)tabBarController.selectedViewController) pushViewController:login animated:YES];
            return NO;
        }
    
    }
    return YES;
}


//登录成功接受到的方法，让tabBarConroller直接跳转到你想跳的界面中去
- (void)logSelect:(NSNotification *)text{
    
   _tabBarVC.selectedIndex = [text.userInfo[@"index"]integerValue];
    
}

登录控制器的代码就只有发送通知就可以了，