How to get audio volume level, and volume changed notifications on iOS?

Any chance you did your signature wrong for the volumeChanged: method? This worked for me, dumped in my appdelegate:

- (BOOL)application:(UIApplication *)application
didFinishLaunchingWithOptions:(NSDictionary *)launchOptions
{
    [[NSNotificationCenter defaultCenter]
     addObserver:self
     selector:@selector(volumeChanged:)
     name:@"AVSystemController_SystemVolumeDidChangeNotification"
     object:nil];
}

- (void)volumeChanged:(NSNotification *)notification
{
    float volume =
    [[[notification userInfo]
      objectForKey:@"AVSystemController_AudioVolumeNotificationParameter"]
     floatValue];

    // Do stuff with volume
}

My volumeChanged: method gets hit every time the button is pressed, even if the volume does not change as a result (because it’s already at max/min).