CODEWITHSUNDEEP
iphoneobjective-ciosmagnetometer
user1073400
user1073400

Reputation:

Why am I not reading any magnetic field values with this simple code?

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

Upvotes: 2

Views: 1411

Answers (3)

mattjgalloway
mattjgalloway

Reputation: 34912

You need to hold onto the CMMotionManager. So make an instance variable for it. For example:

Game.m

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

@interface Game ()
@property (nonatomic, strong) CMMotionManager *motionManager;
@end

@implementation Game

@synthesize motionManager = _motionManager;

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

    self.stopButtonPressed = NO;

    return self;
}
-(void) play
{
     self.motionManager = [[CMMotionManager alloc] init];
     self.motionManager.deviceMotionUpdateInterval = 1.f/10.f;
     [self.motionManager startDeviceMotionUpdatesToQueue:[NSOperationQueue currentQueue]
                                             withHandler:^(CMDeviceMotion *motion, NSError *error)
                                             {
         NSLog(@"--------------> %i %i", self.motionManager.deviceMotionActive , self.motionManager.deviceMotionAvailable);
         NSLog(@"Degrees : %F",atan(motion.magneticField.field.y/fabs(motion.magneticField.field.x)) * 180.0 / M_PI);
                                             }
     ];
}

The problem is that your CMMotionManager that you're creating is being deallocated at the end of the play method since nothing is holding onto it. So the handler never gets called back because your motion manager has gone away.

Upvotes: 1

Peter M
Peter M

Reputation: 7493

BTW One possibility is that the device you are running may not even have a magnetometer. See magnetometer-for-compass-on-ipod-touch-4g Where I point to apple-devices-with-magnetometer

Upvotes: 0

anka
anka

Reputation: 3857

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

Upvotes: 1

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