Detect face up position with sensors in iOS

Question

I want to detect face up position with sensor information. If position of the iPhone is face up, the button on the screen named as btnOpen should be seem, otherwise the button should be hidden. I'm checking z values and interval [-0.8, -1.0] Z values changing correctly but btnOpen.hidden = YES or NO working first time then doesn't working. I created a label to print z values on screen but it writes first z valu and the value didn't change. What is the problem ? How can I fix this problem ?

NSOperationQueue *theQueue = [[NSOperationQueue alloc] init];

    returnedData = [[CMAccelerometerData alloc] init];
    motionManager = [[CMMotionManager alloc] init];

    [motionManager startAccelerometerUpdatesToQueue:theQueue withHandler:^(CMAccelerometerData *accelerometerData, NSError *error) {

        returnedData = motionManager.accelerometerData;

        float z = returnedData.acceleration.z;

        NSLog(@"Z: %f", z);

        if(z > -1.0 && z < -0.8 ){
            btnOpen.hidden = NO;
        }
        else{
            btnOpen.hidden = YES;
        }

Answer 1

- (void)viewDidLoad {
   [[UIDevice currentDevice] beginGeneratingDeviceOrientationNotifications];
   [[NSNotificationCenter defaultCenter] addObserver:self selector:@selector(orientationChanged:) name:UIDeviceOrientationDidChangeNotification object:[UIDevice currentDevice]];
}


- (void)orientationChanged:(NSNotification *)note{
   UIDevice * device = note.object;
   //  NSLog(@"rotation main %d ",device.orientation;);

  switch(device.orientation){
    case 1:
        //Portrait
    break;

    case 4:
       //landscape right
    break;

    case 3:
        //landscape left
    break;

    default:
        //other orientations
    break;
   };
}

Answer 2

I solved the problem with using Scheduled Timer. Every 0.5 second calling method below and working.

- (void)viewDidLoad {
    [super viewDidLoad];

    theQueue = [[NSOperationQueue alloc] init];

    returnedData = [[CMAccelerometerData alloc] init];
    motionManager = [[CMMotionManager alloc] init];
    [NSTimer scheduledTimerWithTimeInterval:0.5f
                                     target:self selector:@selector(checkPhonePosition:) userInfo:nil repeats:YES];

}


-(void)checkPhonePosition:(NSTimer *)timer {


    [motionManager startAccelerometerUpdatesToQueue:theQueue withHandler:^(CMAccelerometerData *accelerometerData, NSError *error) {

        returnedData = motionManager.accelerometerData;

        z = returnedData.acceleration.z;

        NSLog(@"Z: %f", z);


    }];

    if(z > -1.0 && z < -0.8 ){
        btnOpen.hidden = NO;
    }
    else{
        btnOpen.hidden = YES;
    }
}