Detect VoiceOver Status Changes

addObserver.m

- (void)viewDidLoad {
    ...
    [[NSNotificationCenter defaultCenter]
     addObserver:self
     selector:@selector(didChangeVoiceOverStatus:)
     name:UIAccessibilityVoiceOverStatusChanged
     object:nil];
    ...
}

- (void)didChangeVoiceOverStatus:(NSNotification *)notification {
    if (UIAccessibilityIsVoiceOverRunning()) {
        NSLog(@"VoiceOver is ON.");
    } else {
        NSLog(@"VoiceOver is OFF.");
    }
}

removeObserver.m

- (void)didReceiveMemoryWarning {
    [super didReceiveMemoryWarning];
    [[NSNotificationCenter defaultCenter] removeObserver:self];
    ...
}