readAverageValueFromHealthKitWithIdentifier.m

- (void)readAverageValueFromHealthKitWithIdentifier:(HKQuantityType *)identifier 
    unit:(HKUnit *)unit 
        options:(HKStatisticsOptions)options 
            startDate:(NSDate *)startDate 
            andEndDate:(NSDate *)endDate 
            completionBlock:(void (^)(NSError *error, NSDictionary *dict))completionBlock {
    
    // HKQueryOptionStrictStartDate requires any samples HealthKit 
    // returns to have start dates that fall within the period 
    // between the start and end dates we pass in here
    NSPredicate *predicate = [HKQuery predicateForSamplesWithStartDate:startDate 
                              endDate:endDate 
                              options:HKQueryOptionStrictStartDate];
    
    // HKStatisticsQuery is used for calculating minimums, maximums, and averages
    // We want an average of weight for each day
    // in case the user weighs in multiple times so the options parameter
    // will just be HKStatisticsOptionDiscreteAverage
    
    // The identifier in this case is HKQuantityTypeIdentifierBodyMass
    HKStatisticsQuery *statsQuery = [[HKStatisticsQuery alloc] initWithQuantityType:identifier 
                                     quantitySamplePredicate:predicate 
                                     options:options 
                                     completionHandler:^(HKStatisticsQuery * _Nonnull query, HKStatistics * _Nullable result, NSError * _Nullable error) {
        
        if (error) {
            // handle error and return
            
        }
        
        // try to get value from the result
        // if no value, return
        
        // If there is a value, format it:
        
        // Turn the startDate (An NSDate) into a string like "2018-01-15"
        // Create a dictionary that looks like this:
        { "2018-01-15": 90.5}
        // These dictionaries will be combined by the caller of this method
        // so a whole bunch of dates and values can be sent to the API at once
        // call completion block here and return the dictionary we just created
    }];
    
    // Execute the query
    [healthStore executeQuery:statsQuery];
}