Question

Game.m

#import "Game.h"
#import "CoreMotion.h"

@implementation Game

- (id) init 
{
    self = [super init];

    self.stopButtonPressed = NO;

    return self;
}
-(void) play
{
     CMMotionManager *motionManager;
     motionManager = [[CMMotionManager alloc] init];
     motionManager.deviceMotionUpdateInterval = 1.f/10.f;
     [motionManager startDeviceMotionUpdatesToQueue:[NSOperationQueue currentQueue]
                                        withHandler:^(CMDeviceMotion *motion, NSError *error)
                                        {
                                          NSLog(@"--------------> %i %i", motionManager.deviceMotionActive , motionManager.deviceMotionAvailable);

                                          NSLog(@"Degrees : %F",atan(motion.magneticField.field.y/fabs(motion.magneticField.field.x)) * 180.0 / M_PI);
                                        }
     ];
}

MyViewController.m

#import "MyViewController.h"
#import "Game.h"

@interface MyViewController()
{
    Game *game;
}
@end

@implementation MyViewController

-(void) viewDidLoad
{
    game = [[Game alloc] init];
}

- (IBAction)playPressed:(UIButton *)sender 
{
    // using a separate thread
    //[game performSelectorInBackground:@selector(play) withObject:nil];

    // not using a separate thread
    [game play] ;
}

- (IBAction)stopPressed:(UIButton *)sender 
{
    game.stopButtonPressed = YES;
}

@end

Answer 1

the magnetic field value is not available immediately after you call the method startDeviceMotionUpdates. You need to try to retrive the value in a later point (i.e. using a NSTimer and checking for updates.

Although this should work it isn’t a good practice. If you only need the magnetic field value you should take a look at the documentation of CMMotionManager and use the method startMagnetometerUpdatesToQueue:withHandler: like the following:

[motionManager startMagnetometerUpdatesToQueue:[NSOperationQueue currentQueue] withHandler:^(CMMagnetometerData *magnetometerData, NSError *error) {
  CMMagneticField field = magnetometerData.magneticField;
  NSLog(@"x: %f  y:%f  z:%f", field.x, field.y, field.z);
}];

cheers,
anka