Device motion updates in background - ios

Question

I am trying to get device motion updates on an iPhone or iPad in the background using CMMotionManager. I have reviewed all previous posts on this topic and thought that I had code that would work. My app also uses background audio, and this works properly in the foreground and background. In Info.plist, I have background audio and background location updates enabled.
For testing purposes, I have declared "var motionManager = CMMotionManager()" in AppDelegate, and included the following code in didFinishLaunchingWithOptions:
motionManager.deviceMotionUpdateInterval = 0.10
let queue = NSOperationQueue()
motionManager.startDeviceMotionUpdatesToQueue(queue, withHandler: {
data, error in
let accelerationVector = sqrt(pow(data!.userAcceleration.x, 2) + pow(data!.userAcceleration.y, 2) + pow(data!.userAcceleration.z, 2))
print("\(accelerationVector)")
})
When I run the app on my device, the code executes in the foreground as expected, but when I press the home button, I get about 10 more readings before it stops. When I tap on the app icon, the readings start again. I have also placed breakpoints on the code in the handler, and get similar results.
What am I missing?

Answer 1

Do NOT trust the NSLog or other similar log output
I used to meet this problem. I want collect the motion data in background and I made a demo about this. I found I can get all the data and log when my app in active, but the xCode log console output nothing when the application in background.
I used to think the problem is the CoreMotion data can only be collected in foreground, but I am wrong. All the callbacks still working when application enter background, Just the NSLog stop telling me the data.
If you don't believe, just collect all the data into a NSMutableArray or other collections, then check the data collected when app in background.
e.g.
#property (nonatomic, strong) NSMutableArray *arrAltimeters;
...
[self.altimeter startRelativeAltitudeUpdatesToQueue:self.operationQueue withHandler:^(CMAltitudeData * _Nullable altitudeData, NSError * _Nullable error) {
[self.arrAltimeters addObject:altitudeData];
NSLog(#"Altimate data count = %ld", self.arrAltimeters.count);
}];

Answer 2

I have declared the motion manager in ViewDidlLoad
CMMotionManager *motionManager;
/*--Initialising Motion Manager--*/
motionManager = [[CMMotionManager alloc] init];
motionManager.deviceMotionUpdateInterval = 1.0;
As whenever the app goes in background the motionManager stop providing callbacks in this case , You need to restart the motionmanager when the app goes in background or in foreground.To do this we need to follow the process below:
1) we first need to Register for the app transition notification :
[[NSNotificationCenter defaultCenter] addObserver:self selector:#selector(appDidEnterBackground) name:UIApplicationDidEnterBackgroundNotification object:nil];
[[NSNotificationCenter defaultCenter] addObserver:self selector:#selector(appDidBecomeActive) name:UIApplicationDidBecomeActiveNotification object:nil];
2)Notification callback functions, in which you will restart the motion manager
-(void)appDidEnterBackground{
[self restartMotionUpdates];
}
-(void)appDidBecomeActive{
[self restartMotionUpdates];
}
3)This function which will restart the motion manager
-(void)restartMotionUpdates{
[motionManager stopDeviceMotionUpdates];
[motionManager startDeviceMotionUpdatesToQueue:[NSOperationQueue mainQueue] withHandler:^(CMDeviceMotion *motion, NSError *error) {
NSLog(#"x=%f y=%f z=%f",fabs(motion.userAcceleration.x),fabs(motion.userAcceleration.y),fabs(motion.userAcceleration.z));
}];
}

Answer 3

After doing more research, I figured out the problem. I was using mpmusicplayercontroller to play background audio. This will play the audio in the background, but this does not keep my app running in the background. When I switched to using avplayer to play audio, the app runs in the background including the device motion updates.