仅显示一次警报消息，但在应用程序启动时再次出现

我正在尝试找出一种方法，当我的一个视图控制器被选中时，让一条消息出现在我的一个视图控制器上(这个视图不是应用程序上的第一个视图，所以我不能在启动).我只希望它出现一次，并且在重新选择该视图时不再出现；但是，如果应用程序重新启动，我希望它再次出现.

I'm trying to figure out a way to get a message to appear on one of my view controllers when that screen controller is selected (this view is NOT the first view on the app, so I can't do it at start-up). I only want it to appear ONCE and not appear again when that view is re-selected; however, I want it to appear again if the app is restarted.

到目前为止，我能够让它毫无问题地出现一次，并且在使用该应用程序时它不会再次弹出，这是我想要的，因为我经常返回其他视图并返回.我遇到的问题是，当我再次启动应用程序时，我希望在选择适当的视图时出现警报消息，但由于存储在 NSUserDefaults 中的数据，它不会出现.

So far, I am able to get it to appear once without any problem AND it won't pop up again while using the app, which is what I want as I am often going back into other views and returning. The problem I am having, is when I start the app up again, I want the alert message to appear when I select the appropriate view, but because of the data stored in NSUserDefaults it won't appear.

有什么想法吗?这出现在 ViewWillAppear 中:

Any ideas? This appears in ViewWillAppear:

 NSUserDefaults *defaults = [NSUserDefaults standardUserDefaults];
    NSString *alertshown = [defaults stringForKey:@"alertshown"]; {
        if (alertshown == nil) {

UIAlertView *alertView = [[UIAlertView alloc] initWithTitle:@"Press the Code button" message:@"Take a photo of this screen and show it to get your code" delegate:nil cancelButtonTitle:@"OK" otherButtonTitles:nil];

            [alertView show];
            [alertView release];

            [defaults setObject:@"alertwasdisplayed" forKey:@"alertshown"];
        }}