iOS8 Day-by-Day :: Day 12 :: HealthKit

Written by Sam Davies

Updated 9 Apr 2015: This post has been updated to Swift 1.2

This post is part of a daily series of posts introducing the most exciting new parts of iOS8 for developers – #iOS8DayByDay. To see the posts you’ve missed check out the index page, but have a read through the rest of this post first!

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One of the more consumer-oriented features introduced in iOS8 is that of the Health app. It was featured in the keynote at WWDC and has received a fair amount of hype since that point. In fact some of the world’s biggest names in healthcare have already put their names behind it as being a huge step forward in modern healthcare.

The underlying technology behind the health app is HealthKit, which is essentially a structured datastore designed specifically for health data. Not only is there an schema for health data, but extra concerns such as access permissions, querying and unit conversion. HealthKit allows app developers to interact with this datastore – to add and query data points and calculate statistics.

In today’s article you’ll get a whistle-stop tour of some of the features of HealthKit, and see how easy it is to create an app which can save data points and query the datastore. The accompanying app is called BodyTemple and is available in the github repo at

Data Structure Overview

The structure of data in HealthKit represents an ontology of pretty much everything related to personal well-being. This includes obvious measurements such as body mass, but also extends to more specialized readings such as blood oxygen saturation and dietary zinc. The underlying database is a set of time- series – consisting of independent time-indexed samples.

In terms of code these samples are represented by the HKSample class, which itself is a subclass of the immutable HKObject class. HKObject is the fundamental type used in HealthKit – and includes properties such as UIID, source and metadata. Since these objects are immutable, they are created using initializers on the HKQuantitySample and HKCategorySample subclasses. These are fairly self explanatory in their distinction between representing a categorical data type and a quantifiable type (such as body mass). They each have some kind of type property to specify what the value represents (categoryType and quantityType respectively). The following demonstrates creating a quantity sample for body mass:

let weightType = HKObjectType.quantityTypeForIdentifier(HKQuantityTypeIdentifierBodyMass)
let weightValue = HKQuantity(unit: HKUnit(fromString: "kg"), doubleValue: massNumber)
let metadata = [ HKMetadataKeyWasUserEntered : true ]
let sample = HKQuantitySample(type: weightType, quantity: weightValue,
                startDate:, endDate:, metadata: metadata)  
  • HKObjectType is a class which represents the aforementioned type property. It has methods for creating types such asquantityTypeForIdentifier() and categoryTypeForIdentifier(), which take a framework-defined string constant to specify exactly which type it represents (e.g. HKQuantityTypeIdentifierBodyMass).
  • A quantity sample obviously has an associated value, and this is provided by the HKQuantity class. This requires a double value and an HKUnit object which represents the units the double was measured in.
  • You can provide a metadata dictionary including details such as the measurement device – here the HKMetadataKeyWasUserEntered key shows that the data has been manually entered.
  • Finally the HKQuantitySample is constructed using the type, quantity and metadata objects you’ve created, along with a start and end date.

The HKUnit class is an incredibly powerful concept, in that it will cause HealthKit to convert between units (provided they represent the same physical quantity) implicitly. This goes both for simple units (such as those representing mass, volume and temperature) and for compound units (such as those representing density). The powerful string parsing of the HKUnit(fromString:) initializer is an excellent way to create these objects.

To see the selection of different values that can be recorded, check out the HKTypeIdentifiers file. The following represents a small selection:

  • Body mass
  • Body fat percentage
  • Height
  • Step count
  • Blood glucose
  • Blood alcohol content
  • Dietary energy consumed
  • Body temperature
  • Inhaler usage

In addition to quantity and categorical time-series types, you can also store characteristic types, that is those which are incredibly unlikely (or don’t) to change over time, for example date of birth, sex and blood type. You would expect that a user would set these in the health app and then they are available to you when reading data from the store.

As well as personal physical measurements, you can also store details of workout sessions through the HKWorkout subclass of HKSample. This has properties for recording the type of workout, the duration, the energy burned and the distance. Since HKWorkout is another subclass of HKObject it is also immutable, so these properties are all populated at initialization time. The energy burned and distance are again examples of HKQuantity objects, and the activity type is an enum (HKWorkoutActivityType) which includes over 50 types of exercise, including:

  • Archery
  • CrossTraining
  • Dance
  • Fishing
  • Hocket
  • SkatingSports
  • SnowSports
  • StairClimbing
  • Swimming
  • Yoga

You’ll notice that through out this run through of the data model, you’ve been dealing solely with HKObject subclasses. This is very much like how CoreData uses NSManagedObject, and allows the interface to the data store to be consistent, irrespective of whether you’re storing a workout or a blood pressure reading. We’ll take a look at this interface soon, but first there’s a rather important issue to take a look at – that of permissions.


By its nature, any data associated with health is extremely personal, and therefore incredibly sensitive. Whilst having this central store for all health data on a device is great – the security risk could be huge. For example, an ad network knowing the user’s body mass, and when they last ate is a huge concern for privacy. It can also be a lot more subtle than that – there is huge correlation between users recording blood sugar levels, and being diabetic – so even knowing that the data exist is a leak of personal data.

Before your app can use HealthKit, you need to enable it in the Capabilities pane of the project settings. This requires that you sign in to your Apple developer account, and then acquires the appropriate entitlements for your app.

HealthKit Capabilities HealthKit Capabilities Enabled

In order to protect users’ privacy, HealthKit includes a very granular permissions system, including a super-simple UI. In order for an app to read from or write to HealthKit it has to ask the user for specific permissions. The permissions are based on the types introduced in the last section – e.g. HKQuantityType or HKCharacteristicType. All access to the data is performed through a HKHealthStore object, and it is this that you request permissions from. The following method demonstrates requesting access to the data store:

private func requestAuthorisationForHealthStore() {
  let dataTypesToWrite = [
  let dataTypesToRead = [

  self.healthStore?.requestAuthorizationToShareTypes(Set<NSObject>(arrayLiteral: dataTypesToWrite),
    readTypes: Set<NSObject>(arrayLiteral: dataTypesToRead), completion: {
    (success, error) in
      if success {
        println("User completed authorisation request.")
      } else {
        println("The user cancelled the authorisation request. \(error)")

The method requestAuthorizationToShareTypes(_:, readTypes:, completion:) accepts NSSet objects containing the quantities that you require to share ( write) and read. The completion closure is called once the procedure is complete.

When this method is run, then the store checks to see whether it has already asked the user for this configuration of permissions. If it has then it will call the completion block immediately – irrespective of whether permission was granted or not. If it hasn’t yet asked the user about this set of permissions then it’ll display a modal request page like this:

Note that you cannot discover whether you have permissions to read a given type, and that if you attempt to write a type for which you do not have authorization then you’ll get an error back describing the problem.

You should only use one HKHealthStore throughout your app – this way meaning that you can consolidate you permission requests to one point. In BodyTemple this happens in the TabBarViewController, which since it is the top-level view controller owns and propagates the health store to its child view controllers, and requests the permissions required for use of the app.

Writing Data

You’ve already seen the HKHealthStore object in the context of requesting appropriate permissions, and this same object is used to both read and write to the data store. Saving data is actually really simple – and this is primarily because of the apparent complexity in the data model. Since every single thing that can be stored in HealthKit is a descendant of HKObject then saving is as simple as:

func saveSampleToHealthStore(sample: HKObject) {
  println("Saving weight")
  self.healthStore?.saveObject(sample, withCompletion: {
    (success, error) in
    if success {
      println("Weight saved successfully ")
    } else {
      println("Error: \(error)")

The saveObject(_:, withCompletion:) will attempt to save any HKObject to the datastore, and since it is an asynchronous method it has a callback for when it is complete. It is at this point that you would discover that your app doesn’t have permission to write (as in the screenshot in the previous section).

In BodyTemple hitting save on the following screen will create a HKQuantitySample object which is then saved on the screen dismissal:

Gaining Weight

Once this has completed you can see that it has worked in the Health app:

Health App Chart Health App DataPoint

Reading Data

There are several different approaches for requesting data from HealthKit, depending primarily on the type of data you want to retrieve, and whether you want HealthKit to perform any kind of processing on the data.

In the simplest case you might want to retrieve some characteristic data – such as the date of birth of the user:

func requestAgeAndUpdate() {
  var error: NSError?
  let dob = self.healthStore?.dateOfBirthWithError(&error)

  if error {
    println("There was an error requesting the date of birth: \(error)")

  // Calculate the age
  let now = NSDate()
  let age = NSCalendar.currentCalendar().components(.YearCalendarUnit, fromDate: dob,
                                                    toDate: now, options: .WrapComponents)

  self.ageLabel.text = "\(age.year)"

There are methods directly on the HKHealthStore to get hold of these. The above requestAgeAndUpdate() method populates a label with the user’s age:

User's Age

To request sampled data you need to create an HKQuery, which itself requires a sample type and predicate, upon which to filter. The following method demonstrates how to get a list of body mass samples for the last 2 months:

func perfromQueryForWeightSamples() {
  let endDate = NSDate()
  let startDate = NSCalendar.currentCalendar().dateByAddingUnit(.CalendarUnitMonth,
    value: -2, toDate: endDate, options: nil)

  let weightSampleType = HKSampleType.quantityTypeForIdentifier(HKQuantityTypeIdentifierBodyMass)
  let predicate = HKQuery.predicateForSamplesWithStartDate(startDate,
    endDate: endDate, options: .None)

  let query = HKSampleQuery(sampleType: weightSampleType, predicate: predicate,
    limit: 0, sortDescriptors: nil, resultsHandler: {
    (query, results, error) in
      if !results {
        println("There was an error running the query: \(error)")
      dispatch_async(dispatch_get_main_queue()) {
        self.weightSamples = results as! [HKQuantitySample]

Notice that the query is again asynchronous, and so you must provide a closure to handle the delivery of the results. Note that this will occur on a background queue and therefore you need to make sure that you marshal any UI updates back to the main queue.

Weight history

This represents a one-off query, but you might also want to perform a long- running query, which will notify you each time new samples appear in the database, by repeatedly calling the result handler closure. This can be useful if you are working with values which have a high sample rate, and are being created by a different app. This is exactly the functionality provided by HKObserverQuery, which looks very similar to the HKSampleQuery your saw above. Since it is a long-running query it must be canceled by calling the stopQuery(_:) method on HKHealthStore.

Another query type worth mentioning is the anchored query, represented by the HKAnchoredObjectQuery, which will provide you a pointer to the most recent result returned. You can then re-run the query at a later stage and only get newer results returned. This means that you don’t have to repeatedly process the result set to determine whether you have already seen each sample.

The final class worth mentioning with respect to HealthKit is HKStatistics, which, as the name suggests, allows you to perform basic statistical treatments of the dataset – i.e. finding the sum/mean. It’s important here to be aware of the fact that some measurements are cumulative (that is to say the a sum makes sense, e.g. energy intake), whereas some are discrete – where min and max might apply. These concepts are well-defined in HealthKit determined by the HKQuantityAggregationStyle enumeration.

You can also generate a collection of statistics (such as total energy consumed per day for the last week) using the HKStatisticsCollectionQuery. This is an expansion on a statistics query, but with the concept of time intervals and an anchor date.


On the surface of it the health app seems like a pretty cool idea for iOS8, but once you get down into HealthKit then you realise that this could be really quite powerful. In creating HealthKit Apple hasn’t attempted to create one healthcare app to rule them all, but instead has created a framework through which the important aspects of healthcare monitoring can interoperate. There is only one canonical set of body mass readings for a person, so why do they currently have to be entered for each app? It’s also a lot clearer to the end user exactly what healthcare data the app has access to – which is another easy win.

From the point of view of developers, having an easy-to-use pre-existing datastore is huge news in itself. It might not offer the perfect schema for your purposes, but the power of being able to share data entered from other apps is huge.

Admittedly HealthKit is only going affect developers of apps in this space, but it is certainly worth taking a look at it. It gives you an idea of how Apple’s APIs are modernizing as the ecosystem moves forward.

The code for the accompanying app is available on github at the usual place: Go grab it, play with it and then complain about it to me on twitter – I’m @iwantmyrealname