Question How to collect Compass data when an application is in background?

Hi all,
I have few knowledge of Objective-C and Swift but after spending several hours to write a native plugin to collect compass data using a background thread in iOS i come up with a solution which compile and work for ~ 2s before being canceled by the SO. Below there is the plugin so far

Code (Boo):

1.  
2. // MyPlugin.m
3.  
4. extern "C" {
5. #import <CoreLocation/CoreLocation.h>
6. #import "MyPlugin.h"
7.  
8. @implementation MyPlugin
9.  
10. NSThread *_magneticHeadThread;
11. BOOL _shouldStop;
12. NSMutableArray *readings;
13.  
14. - (void)startBackgroundThread {
15.     dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_BACKGROUND, 0), ^{
16.         // Put your code here that should run in the background when the app enters background.
17.         NSLog(@"Background thread started!");
18.         readings = [NSMutableArray array];
19.         _magneticHeadThread = [[NSThread alloc] initWithTarget:self selector:@selector(retrieveMagneticHeadPosition) object:nil];
20.         [_magneticHeadThread start];
21.     });
22. }
23.  
24. - (void)applicationDidEnterBackground:(NSNotification *)notification {
25.     _shouldStop = NO;
26.     [self startBackgroundThread];
27. }
28.  
29. - (void)applicationWillEnterForeground:(NSNotification *)notification {
30.     _shouldStop = YES;
31.     [_magneticHeadThread cancel];
32.     NSLog(@"Magnetic head position: %lu", [readings count]);
33. }
34.  
35. + (instancetype)sharedInstance {
36.     static MyPlugin *sharedInstance = nil;
37.     static dispatch_once_t onceToken;
38.     dispatch_once(&onceToken, ^{
39.         sharedInstance = [[MyPlugin alloc] init];
40.     });
41.     return sharedInstance;
42. }
43.  
44. - (void)retrieveMagneticHeadPosition {
45.     CLLocationManager *locationManager = [[CLLocationManager alloc] init];
46.    
47.     locationManager.headingFilter = kCLHeadingFilterNone;
48.     [locationManager startUpdatingHeading];
49.     int c = 0;
50.     while (!_shouldStop) {
51.        
52.         NSLog(@"Magnetic head position: %i", c);
53.         c++;
54.         [readings addObject:@(locationManager.heading.magneticHeading)];
55.         [NSThread sleepForTimeInterval:0.1];
56.     }
57.    
58.     [locationManager stopUpdatingHeading];
59. }
60.  
61. @end
62.  
63.  
64. void UnityMyPlugin_StartBackgroundThread() {
65.     [[NSNotificationCenter defaultCenter] addObserver:[MyPlugin sharedInstance] selector:@selector(applicationDidEnterBackground:) name:UIApplicationDidEnterBackgroundNotification object:nil];
66.    
67.     [[NSNotificationCenter defaultCenter] addObserver:[MyPlugin sharedInstance] selector:@selector(applicationWillEnterForeground:) name:UIApplicationWillEnterForegroundNotification object:nil];
68. }
69. }
70.  

It is called in a MonoBehaviour in a Start function:

Code (CSharp):

1. void Start()
2.     {
3.         Application.runInBackground = true;
4. #if UNITY_IPHONE && !UNITY_EDITOR
5.         UnityMyPlugin_StartBackgroundThread();
6. #endif
7.     }

I've read that dispatch_async expects to finish in a fixed time, but i haven't find any clue for indefinite thread nor information about services to request informations such as MagneticField, can you point me out where to check or who to ask for a solution? Am I looking for anything possible? Did I make any bad choice in this code?
Thanx