BackgroundDelivery.swift

private var myAnchor: HKQueryAnchor?

/// Sets up the observer queries for background health data delivery.
///
/// - parameter types: Set of `HKObjectType` to observe changes to.
private func setUpBackgroundDeliveryForDataTypes(types: Set<HKObjectType>) {
  for type in types {
    guard let sampleType = type as? HKSampleType else { print("ERROR: \(type) is not an HKSampleType"); continue }

    if let anchorData = UserDefaults.standard.object(forKey: "anchor") as? Data {
      myAnchor = NSKeyedUnarchiver.unarchiveObject(with: anchorData) as? HKQueryAnchor
    }

    let query = HKAnchoredObjectQuery(type: sampleType,
                      predicate: nil,
                      anchor: myAnchor,
                      limit: HKObjectQueryNoLimit) { [weak self] (_, samplesOrNil, _, newAnchor, _)  in
      debugPrint("observer query update handler called for type \(sampleType)")

      guard let samples = samplesOrNil else {
        // Handle the error here.
        return
      }

      guard samples.last != nil else {
        return
      }

      guard let strongSelf = self else { return }

      strongSelf.myAnchor = newAnchor

      if let newAnchor = newAnchor {
        UserDefaults.standard.setValue(NSKeyedArchiver.archivedData(withRootObject: newAnchor), forKey: "anchor")
      }
    }

    // Optionally, add an update handler.
    query.updateHandler = { [weak self] (query, samplesOrNil, deletedObjectsOrNil, newAnchor, errorOrNil) in

      guard let samples = samplesOrNil else {
        // Handle the error here.
        fatalError("*** An error occurred during an update: \(errorOrNil!.localizedDescription) ***")
      }

      print(samples.count)

      guard let lastSample = samples.last else {
        return
      }

      guard let strongSelf = self else { return }

      strongSelf.myAnchor = newAnchor

      if let newAnchor = newAnchor {
        UserDefaults.standard.setValue(NSKeyedArchiver.archivedData(withRootObject: newAnchor), forKey: "anchor")
      }

      strongSelf.handleSample(lastSample)
    }

    healthStore.execute(query)
    healthStore.enableBackgroundDelivery(for: type, frequency: .immediate) { (success: Bool, error: Error?) in
      debugPrint("enableBackgroundDeliveryForType handler called for \(type) - success: \(success), error: \(error)")
    }
  }
}