IOS UIDeviceBatteryLevelDidChangeNotification not firing in background mode - ios

Question

I'm writing an IOS application which will record ( over time ) the current battery level of the iPhone device.
I have working code executing within the foreground using UIDeviceBatteryLevelDidChangeNotification, this fires a notification ( which is handled successfully ) when the app in in foreground mode.
Code is as follows:
- (BOOL)application:(UIApplication *)application didFinishLaunchingWithOptions:(NSDictionary *)launchOptions {
[[UIDevice currentDevice] setBatteryMonitoringEnabled:YES];
[[NSNotificationCenter defaultCenter] addObserver:self selector:#selector(batteryChanged:) name:UIDeviceBatteryLevelDidChangeNotification object:nil];
return YES;
}
- (void)batteryChanged:(NSNotification *)notification
{
NSLog(#"Battery Changed From Observer");
[self UpdateBatteryStatus];
}
-(void)UpdateBatteryStatus
{
UIDevice *myDevice = [UIDevice currentDevice];
double batLeft = (float)[myDevice batteryLevel] * 100;
NSLog(#"Battery level: %f", batLeft);
// Do some stuff related to battery status
}
This executes ( output via log proves it ) every 1% increment ( or decrement ) on the battery status on the phone, which works exactly as expected.
When the app is moved to the background however, the events stop firing. Upon moving the app back to the foreground, the events fire ( it's almost like they are queued up until it reaches the foreground ).
I have searched stack overflow, and most answers are all the same, basically this is a background-mode permissions problem. So I ticked all the boxes:
I added everything just to make sure it wasn't a background persmission problem. Unfortunately, even after a clean, and rebuild ( also a shutdown of Xcode, and re-plugin of iPhone ) the events still do not fire.
Please bear in mind, I have no intention of releasing this to the app store, I'm aware that faking background-mode use cases is a quick and easy rejection. But I would like to get it working for personal usage.
Edit: ios target 11.3

Answer 1

Your app won't get NSNotificationCenter notifications while being in background/suspended. What you could try is to use fetch background mode to mimic remote downloads. Note however downsides of this approach - system itself will decide when to call the callback.
- (BOOL)application:(UIApplication *)application didFinishLaunchingWithOptions:(NSDictionary *)launchOptions {
[application setMinimumBackgroundFetchInterval:UIApplicationBackgroundFetchIntervalMinimum];
return YES;
}
- (void)application:(UIApplication *)application performFetchWithCompletionHandler:(void (^)(UIBackgroundFetchResult))completionHandler
{
UIDevice *myDevice = [UIDevice currentDevice];
double batLeft = (float)[myDevice batteryLevel] * 100;
NSLog(#"Battery level: %f", batLeft);
completionHandler(UIBackgroundFetchResultNewData);
}
If you as stated do not plan to release your app in AppStore you can try another hacks with background modes like infinite playing a muted sound in background.