HealthKit background delivery running many times in a minute

Question

I've registered for background delivery of four types of HealthKit data in my delegate's didFinishLaunching method. The data types are steps, sleep, workouts, and energyConsumed.

I've noticed my observer queries for these data types are being called by HealthKit/iOS many, many times a second in some cases. Here's an example from some device logs of the sleep observer query being called:

2017/03/27 07:21:58:821 Delegate - steps observer query received new data

2017/03/27 07:21:58:894 Delegate - steps observer query received new data

2017/03/27 07:21:58:936 Delegate - steps observer query received new data

2017/03/27 07:21:58:973 Delegate - steps observer query received new data

2017/03/27 07:21:58:993 Delegate - steps observer query received new data

2017/03/27 07:21:59:000 Delegate - steps observer query received new data

2017/03/27 07:21:59:024 Delegate - steps observer query received new data

2017/03/27 07:21:59:130 Delegate - steps observer query received new data

2017/03/27 07:21:59:145 Delegate - steps observer query received new data

2017/03/27 07:21:59:156 Delegate - steps observer query received new data

2017/03/27 07:21:59:169 Delegate - steps observer query received new data

2017/03/27 07:21:59:309 Delegate - steps observer query received new data

2017/03/27 07:21:59:328 Delegate - steps observer query received new data

2017/03/27 07:21:59:346 Delegate - steps observer query received new data

2017/03/27 07:21:59:404 Delegate - steps observer query received new data

2017/03/27 07:21:59:480 Delegate - steps observer query received new data

2017/03/27 07:21:59:499 Delegate - steps observer query received new data

2017/03/27 07:21:59:520 Delegate - steps observer query received new data

2017/03/27 07:21:59:547 Delegate - steps observer query received new data

2017/03/27 07:21:59:561 Delegate - steps observer query received new data

2017/03/27 07:21:59:571 Delegate - steps observer query received new data

2017/03/27 07:21:59:583 Delegate - steps observer query received new data

Here's the code in my app delegate where this log is created:

- (void)setUpStepsObserverQuery {

__weak typeof(self) weakSelf = self;

HKSampleType *sampleType = [HKObjectType quantityTypeForIdentifier:HKQuantityTypeIdentifierStepCount];
HKObserverQuery *query = [[HKObserverQuery alloc] initWithSampleType:sampleType predicate:nil updateHandler:^(HKObserverQuery *query, HKObserverQueryCompletionHandler completionHandler, NSError *error) {

    if (completionHandler) {
        completionHandler();
    }
           
    if (error) {
        DDLogDebug(@"App Delegate - An error occured while setting up the stepCount observer: %@", error.localizedDescription);
    
    } else {
        DDLogDebug(@"Delegate - steps observer query received new data");
        
        weakSelf.activityTask = [[UIApplication sharedApplication] beginBackgroundTaskWithExpirationHandler:^{
            [weakSelf endBackgroundTask:weakSelf.activityTask];
        }];

// At this point I run an anchor query to check if there really is new data, and if there is I go ahead and send the new HealthKit data to our server.

    }];

    [_healthStore executeQuery:query]; // _healthStore is a shared instance of HKHealthStore I created earlier
}

This is how I enable background delivery for steps data. I call this method in didFinishLaunching and it calls the method I've shared a snippet of above, setUpStepsObserverQuery

if ([defaults boolForKey:HK_ACTIVITY_SYNC]) {
    [self setUpStepsObserverQuery];
    [_healthStore enableBackgroundDeliveryForType:[HKObjectType quantityTypeForIdentifier:HKQuantityTypeIdentifierStepCount]
                                        frequency:HKUpdateFrequencyImmediate
                                   withCompletion:^(BOOL success, NSError * _Nullable error) {
                                       DDLogDebug(@"Delegate - enabled step count background updates");
                                   }];

Any ideas on why HealthKit would be calling my observer queries so many times in the same minute?

Answer 1

This is because every time your observe query gets called, your didFinishLaunching function is also called, and enableBackgroundDeliveryForType gets also called and each time it's enabled, the query is being performed.

you have to check if background delivery is enabled, don't enable it again!

Answer 2

This might be a bug rather than expected behavior, so you should file a radar with Apple.

You could also try specifying less frequent HKUpdateFrequency such as HKUpdate​Frequency​Hourly. As mentioned in the documentation for HKHealthStore, the system is actually intended to have a minimum frequency of hourly for high volume sample types such as steps and active energy.