Get total step count for every date in HealthKit

Modified @sebastianr's answer using core Swift classes, for just for testing I am returning only steps for just one day, once you have more days you can create a dictionary of Dates and step count and return it

func getStepCountPerDay(completion:@escaping (_ count: Double)-> Void){

    guard let sampleType = HKObjectType.quantityType(forIdentifier: .stepCount)
        else {
            return
    }
    let calendar = Calendar.current
    var dateComponents = DateComponents()
    dateComponents.day = 1

    var anchorComponents = calendar.dateComponents([.day, .month, .year], from: Date())
    anchorComponents.hour = 0
    let anchorDate = calendar.date(from: anchorComponents)

    let stepsCumulativeQuery = HKStatisticsCollectionQuery(quantityType: sampleType, quantitySamplePredicate: nil, options: .cumulativeSum, anchorDate: anchorDate!, intervalComponents: dateComponents
    )

    // Set the results handler
    stepsCumulativeQuery.initialResultsHandler = {query, results, error in
        let endDate = Date()
        let startDate = calendar.date(byAdding: .day, value: 0, to: endDate, wrappingComponents: false)
        if let myResults = results{
            myResults.enumerateStatistics(from: startDate!, to: endDate as Date) { statistics, stop in
                if let quantity = statistics.sumQuantity(){
                    let date = statistics.startDate
                    let steps = quantity.doubleValue(for: HKUnit.count())
                    print("\(date): steps = \(steps)")
                    completion(steps)
                    //NOTE: If you are going to update the UI do it in the main thread
                    DispatchQueue.main.async {
                        //update UI components
                    }
                }
            } //end block
        } //end if let
    }
    HKHealthStore().execute(stepsCumulativeQuery)
}

Here is a translation that currently works for Swift 2.0, using the SwiftDate library.

    let type = HKSampleType.quantityTypeForIdentifier(HKQuantityTypeIdentifierStepCount)
    let startDate = NSDate().beginningOfDay().oneWeekAgo()
    let interval = NSDateComponents()
    interval.day = 1

    let predicate = HKQuery.predicateForSamplesWithStartDate(startDate, endDate: NSDate(), options: .StrictStartDate)
    let query = HKStatisticsCollectionQuery(quantityType: type!, quantitySamplePredicate: predicate, options: [.CumulativeSum], anchorDate: NSDate().begginingOfDay(), intervalComponents:interval)

    query.initialResultsHandler = { query, results, error in


        let endDate = NSDate()
        let startDate = NSDate().beginningOfDay().oneWeekAgo()
        if let myResults = results{
            myResults.enumerateStatisticsFromDate(startDate, toDate: endDate) {
                statistics, stop in

                if let quantity = statistics.sumQuantity() {

                    let date = statistics.startDate
                    let steps = quantity.doubleValueForUnit(HKUnit.countUnit())
                    print("\(date): steps = \(steps)")
                }
            }
        }
    }

    healthKitStore.executeQuery(query)

You should use HKStatisticsCollectionQuery:

NSCalendar *calendar = [NSCalendar currentCalendar];
NSDateComponents *interval = [[NSDateComponents alloc] init];
interval.day = 1;

NSDateComponents *anchorComponents = [calendar components:NSCalendarUnitDay | NSCalendarUnitMonth | NSCalendarUnitYear
                                                 fromDate:[NSDate date]];
anchorComponents.hour = 0;
NSDate *anchorDate = [calendar dateFromComponents:anchorComponents];
HKQuantityType *quantityType = [HKObjectType quantityTypeForIdentifier:HKQuantityTypeIdentifierStepCount];

// Create the query
HKStatisticsCollectionQuery *query = [[HKStatisticsCollectionQuery alloc] initWithQuantityType:quantityType
                                                                       quantitySamplePredicate:nil
                                                                                       options:HKStatisticsOptionCumulativeSum
                                                                                    anchorDate:anchorDate
                                                                            intervalComponents:interval];

// Set the results handler
query.initialResultsHandler = ^(HKStatisticsCollectionQuery *query, HKStatisticsCollection *results, NSError *error) {
    if (error) {
        // Perform proper error handling here
        NSLog(@"*** An error occurred while calculating the statistics: %@ ***",error.localizedDescription);
    }

    NSDate *endDate = [NSDate date];
    NSDate *startDate = [calendar dateByAddingUnit:NSCalendarUnitDay
                                             value:-7
                                            toDate:endDate
                                           options:0];

    // Plot the daily step counts over the past 7 days
    [results enumerateStatisticsFromDate:startDate
                                  toDate:endDate
                               withBlock:^(HKStatistics *result, BOOL *stop) {

                                   HKQuantity *quantity = result.sumQuantity;
                                   if (quantity) {
                                       NSDate *date = result.startDate;
                                       double value = [quantity doubleValueForUnit:[HKUnit countUnit]];
                                       NSLog(@"%@: %f", date, value);
                                   }

                               }];
};

[self.healthStore executeQuery:query];

Port to Swift with no dependency to SwiftDate library

    let calendar = NSCalendar.current
    let interval = NSDateComponents()
    interval.day = 1

    var anchorComponents = calendar.dateComponents([.day, .month, .year], from: NSDate() as Date)
    anchorComponents.hour = 0
    let anchorDate = calendar.date(from: anchorComponents)

    // Define 1-day intervals starting from 0:00
    let stepsQuery = HKStatisticsCollectionQuery(quantityType: stepsCount!, quantitySamplePredicate: nil, options: .cumulativeSum, anchorDate: anchorDate!, intervalComponents: interval as DateComponents)

    // Set the results handler
    stepsQuery.initialResultsHandler = {query, results, error in
        let endDate = NSDate()
        let startDate = calendar.date(byAdding: .day, value: -7, to: endDate as Date, wrappingComponents: false)
        if let myResults = results{
            myResults.enumerateStatistics(from: startDate!, to: endDate as Date) { statistics, stop in
            if let quantity = statistics.sumQuantity(){
                let date = statistics.startDate
                let steps = quantity.doubleValue(for: HKUnit.count())
                print("\(date): steps = \(steps)")
                //NOTE: If you are going to update the UI do it in the main thread
                DispatchQueue.main.async {
                    //update UI components
                }

            }
            } //end block
        } //end if let
    }
    healthStore?.execute(stepsQuery)