UIScreenBrightnessDidChangeNotification未正确触发

Question

我正在使用iPhone5和iOS7。我希望使用＆＃34; UIScreenBrightnessDidChangeNotification＆＃34;来跟踪设备的亮度。有时它被正确地射击，有时不被射击。这是我的设备或iOS7或我的方法的问题吗？

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

-(void)brightnessChanged
{
     NSLog(@"brightness is %f",[[UIScreen mainScreen] brightness]);
}