I'm new in Xcode and Objective-C, so I need your help.  

I wan't to have an app which detect an Screenshot if this App is in the Background, for example you pressed the home button, so you be on the Home Screen, if you take a screenshot now I want that the app show an alert on the Home Screen.
I try this:

in my AppDelegate.m

```
- (void)applicationDidEnterBackground:(UIApplication *)application {
[[NSNotificationCenter defaultCenter] addObserver:self selector:@selector(userDidTakeScreenshot:) name:UIApplicationUserDidTakeScreenshotNotification object:nil];
}

- (void)userDidTakeScreenshot:(NSNotification *)notification{
    NSLog(@"screenshot detected");

    UIAlertView *alert = [[UIAlertView alloc] initWithTitle:@"Message" message:@"screenshot detected" delegate:self cancelButtonTitle:@"OK" otherButtonTitles:@"nil];
    [alert show];
}

```

But if the App enter the background and then i take a screenshot the alert don't show.
I hope you will understand my problem. 

And I excuse for my english.