CMMotionManager not delivering any values

Question

I am currently trying to develop an iOS app that pulls the magnetometer data from CMMotionManager. I created a very simple test app, but even that app is not working on my iPhone 5. The code is as follows:

- (void)viewDidLoad
{
    [super viewDidLoad];

    CMMotionManager *motionManager = [[CMMotionManager alloc] init];

    [motionManager setMagnetometerUpdateInterval:1.0/30.0f];
    [motionManager  startMagnetometerUpdatesToQueue:[[NSOperationQueue alloc] init]
                    withHandler:^(CMMagnetometerData *magnetometerData, NSError *error) {
        NSLog(@"eek");
    }];
}

The problem is that the handler is simply never called. Likewise, calling [motionManager startMagnetometerUpdates] and then polling magnetometerData is not working (it returns nil). When executing [motionManager isMagnetometerActive] the result is always false.

I am stuck here, any ideas? I can't see why the code should not be working. Do I have to request access to the compass first in any way? The documentation of CMMotionManager did not mention anything besides the things I have done.

Answer 1

I think your motionManager gets autoreleased. Add a instance variable or property to keep the motion manager anyway.

Also check if magnetometerAvailable returns YES, otherwise it could be possible that your device does not support the magnetometer.