按下音量按钮时如何执行方法？

Question

我正在尝试在用户按下音量按钮时执行方法，但“volumeChanged”行没有出现在日志中。我看了几个例子

iPhone Detect Volume Keys press.

iOS Detect system volume level change. private API or not? AVSystemController_SystemVolumeDidChangeNotification

-(void)volumeChanged{
    //do something
    NSLog(@"volumeChanged");
}

- (void)viewDidLoad {
    [super viewDidLoad];
    masterViewController = self;
    [masterViewController swipeInit];
    [masterViewController intervalPickerInit];

    // init audio session
    AudioSessionInitialize(NULL, NULL, NULL, NULL);
    AudioSessionSetActive(true);

    [[NSNotificationCenter defaultCenter] addObserver:self
                                         selector:@selector(volumeChanged:)
                                             name:@"AVSystemController_SystemVolumeDidChangeNotification"
                                           object:nil];