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public func subscribeToHeartBeatChanges() {
// Creating the sample for the heart rate
guard let sampleType: HKSampleType =
HKObjectType.quantityType(forIdentifier: .heartRate) else {
/// Creating an observer, so updates are received whenever HealthKit’s
// heart rate data changes.
self.heartRateQuery = HKObserverQuery.init(
sampleType: sampleType,
predicate: nil) { [weak self] _, _, error in
guard error == nil else {
/// When the completion is called, an other query is executed
/// to fetch the latest heart rate
self.fetchLatestHeartRateSample(completion: { sample in
guard let sample = sample else {
/// The completion in called on a background thread, but we
/// need to update the UI on the main.
DispatchQueue.main.async {
/// Converting the heart rate to bpm
let heartRateUnit = HKUnit(from: "count/min")
let heartRate = sample
.doubleValue(for: heartRateUnit)
/// Updating the UI with the retrieved value
public func fetchLatestHeartRateSample(
completion: @escaping (_ sample: HKQuantitySample?) -> Void) {
/// Create sample type for the heart rate
guard let sampleType = HKObjectType
.quantityType(forIdentifier: .heartRate) else {
/// Predicate for specifiying start and end dates for the query
let predicate = HKQuery
withStart: Date.distantPast,
end: Date(),
options: .strictEndDate)
/// Set sorting by date.
let sortDescriptor = NSSortDescriptor(
key: HKSampleSortIdentifierStartDate,
ascending: false)
/// Create the query
let query = HKSampleQuery(
sampleType: sampleType,
predicate: predicate,
limit: Int(HKObjectQueryNoLimit),
sortDescriptors: [sortDescriptor]) { (_, results, error) in
guard error == nil else {
print("Error: \(error!.localizedDescription)")
completion(results?[0] as? HKQuantitySample)
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