1. VC跳转，收不到通知

在SecondVC中：

- (void)viewDidLoad {
    [super viewDidLoad];
    // Do any additional setup after loading the view.
    self.view.backgroundColor = [UIColor whiteColor];
    [[NSNotificationCenter defaultCenter] addObserver:self selector:@selector(lookupTheInfo:) name:@"kFirstToSecondNotification" object:nil];
}

在FirstVC中一个点击事件：

- (void)buttonTouchUpInside:(id)sender {
    RXNotificationSecondViewController *vc = [RXNotificationSecondViewController new];
    [self.navigationController pushViewController:vc animated:YES];
    NSDictionary *userInfo = @{
        @"name":@"Notification",
        @"age":@"18",
        @"height":@"188cm"
    };
    [[NSNotificationCenter defaultCenter] postNotificationName:@"kFirstToSecondNotification" object:nil userInfo:userInfo];
}

因为SecondVC还没有注册，是无法收到通知。
所以可以改成：

- (void)buttonTouchUpInside:(id)sender {
    RXNotificationSecondViewController *vc = [RXNotificationSecondViewController new];
    [self.navigationController pushViewController:vc animated:YES];
    [[NSOperationQueue mainQueue] addOperationWithBlock:^{
        NSDictionary *userInfo = @{
            @"name":@"Notification",
            @"age":@"18",
            @"height":@"188cm"
        };
        [[NSNotificationCenter defaultCenter] postNotificationName:@"kFirstToSecondNotification" object:nil userInfo:userInfo];
    }];
}

2. VC跳转，SecondVC 需要单独处理一些事情

- (void)buttonTouchUpInside:(id)sender {
    RXNotificationSecondViewController *secondVC = [RXNotificationSecondViewController new];
    [self.navigationController pushViewController:secondVC animated:YES];
    [[NSOperationQueue mainQueue] addOperationWithBlock:^{
         [secondVC doSomething];
    }];
}

实际上，以上也可用如下的方法：

- (void)method3 {
    RXNotificationSecondViewController *vc = [RXNotificationSecondViewController new];
    [self.navigationController pushViewController:vc animated:YES];
    
    dispatch_after(dispatch_time(DISPATCH_TIME_NOW, (int64_t)(0.1 * NSEC_PER_SEC)), dispatch_get_main_queue(), ^{
        NSDictionary *userInfo = @{
            @"name":@"Notification",
            @"age":@"18",
            @"height":@"188cm"
        };
        [[NSNotificationCenter defaultCenter] postNotificationName:@"kFirstToSecondNotification" object:nil userInfo:userInfo];
    });
}

但是这个是需要估计一个时间，所以不是一个好方法。

以下的方法是不可以的：

- (void)method4 {
    RXNotificationSecondViewController *vc = [RXNotificationSecondViewController new];
    [self.navigationController pushViewController:vc animated:YES];
    dispatch_async(dispatch_get_main_queue(), ^{
        NSDictionary *userInfo = @{
            @"name":@"Notification",
            @"age":@"18",
            @"height":@"188cm"
        };
        [[NSNotificationCenter defaultCenter] postNotificationName:@"kFirstToSecondNotification" object:nil userInfo:userInfo];
    });
}