healthkit

---
name: healthkit
description: "Read, write, and query Apple Health data using HealthKit. Covers HKHealthStore authorization, sample queries, statistics queries, statistics collection queries for charts, saving HKQuantitySample data, background delivery, workout sessions with HKWorkoutSession and HKLiveWorkoutBuilder, HKUnit, and HKQuantityTypeIdentifier values. Use when integrating with Apple Health, displaying health metrics, recording workouts, or enabling background health data delivery."
---

# HealthKit

Read and write health and fitness data from the Apple Health store. Covers authorization, queries, writing samples, background delivery, and workout sessions. Targets Swift 6.3 / iOS 26+.

## Contents

- [Setup and Availability](#setup-and-availability)
- [Authorization](#authorization)
- [Reading Data: Sample Queries](#reading-data-sample-queries)
- [Reading Data: Statistics Queries](#reading-data-statistics-queries)
- [Reading Data: Statistics Collection Queries](#reading-data-statistics-collection-queries)
- [Writing Data](#writing-data)
- [Background Delivery](#background-delivery)
- [Workout Sessions](#workout-sessions)
- [Common Data Types](#common-data-types)
- [HKUnit Reference](#hkunit-reference)
- [Common Mistakes](#common-mistakes)
- [Review Checklist](#review-checklist)
- [References](#references)

## Setup and Availability

### Project Configuration

1. Enable the HealthKit capability in Xcode (adds the entitlement)
2. Add `NSHealthShareUsageDescription` (read) and `NSHealthUpdateUsageDescription` (write) to Info.plist
3. For background delivery, enable the "Background Delivery" sub-capability

### Availability Check

Always check availability before accessing HealthKit. iPad and some devices do not support it.

```swift
import HealthKit

let healthStore = HKHealthStore()

guard HKHealthStore.isHealthDataAvailable() else {
    // HealthKit not available on this device (e.g., iPad)
    return
}
```

Create a single `HKHealthStore` instance and reuse it throughout your app. It is thread-safe.

## Authorization

Request only the types your app genuinely needs. App Review rejects apps that over-request.

```swift
func requestAuthorization() async throws {
    let typesToShare: Set<HKSampleType> = [
        HKQuantityType(.stepCount),
        HKQuantityType(.activeEnergyBurned)
    ]

    let typesToRead: Set<HKObjectType> = [
        HKQuantityType(.stepCount),
        HKQuantityType(.heartRate),
        HKQuantityType(.activeEnergyBurned),
        HKCharacteristicType(.dateOfBirth)
    ]

    try await healthStore.requestAuthorization(
        toShare: typesToShare,
        read: typesToRead
    )
}
```

### Checking Authorization Status

The app can only determine if it has **not yet requested** authorization. If the user denied access, HealthKit returns empty results rather than an error -- this is a privacy design.

```swift
let status = healthStore.authorizationStatus(
    for: HKQuantityType(.stepCount)
)

switch status {
case .notDetermined:
    // Haven't requested yet -- safe to call requestAuthorization
    break
case .sharingAuthorized:
    // User granted write access
    break
case .sharingDenied:
    // User denied write access (read denial is indistinguishable from "no data")
    break
@unknown default:
    break
}
```

## Reading Data: Sample Queries

Use `HKSampleQueryDescriptor` (async/await) for one-shot reads. Prefer descriptors over the older callback-based `HKSampleQuery`.

```swift
func fetchRecentHeartRates() async throws -> [HKQuantitySample] {
    let heartRateType = HKQuantityType(.heartRate)

    let descriptor = HKSampleQueryDescriptor(
        predicates: [.quantitySample(type: heartRateType)],
        sortDescriptors: [SortDescriptor(\.endDate, order: .reverse)],
        limit: 20
    )

    let results = try await descriptor.result(for: healthStore)
    return results
}

// Extracting values from samples:
for sample in results {
    let bpm = sample.quantity.doubleValue(
        for: HKUnit.count().unitDivided(by: .minute())
    )
    print("\(bpm) bpm at \(sample.endDate)")
}
```

## Reading Data: Statistics Queries

Use `HKStatisticsQueryDescriptor` for aggregated single-value stats (sum, average, min, max).

```swift
func fetchTodayStepCount() async throws -> Double? {
    let calendar = Calendar.current
    let startOfDay = calendar.startOfDay(for: Date())
    let endOfDay = calendar.date(byAdding: .day, value: 1, to: startOfDay)!

    let predicate = HKQuery.predicateForSamples(
        withStart: startOfDay, end: endOfDay
    )
    let stepType = HKQuantityType(.stepCount)
    let samplePredicate = HKSamplePredicate.quantitySample(
        type: stepType, predicate: predicate
    )

    let query = HKStatisticsQueryDescriptor(
        predicate: samplePredicate,
        options: .cumulativeSum
    )

    let result = try await query.result(for: healthStore)
    return result?.sumQuantity()?.doubleValue(for: .count())
}
```

**Options by data type:**
- Cumulative types (steps, calories): `.cumulativeSum`
- Discrete types (heart rate, weight): `.discreteAverage`, `.discreteMin`, `.discreteMax`

## Reading Data: Statistics Collection Queries

Use `HKStatisticsCollectionQueryDescriptor` for time-series data grouped into intervals -- ideal for charts.

```swift
func fetchDailySteps(forLast days: Int) async throws -> [(date: Date, steps: Double)] {
    let calendar = Calendar.current
    let endDate = calendar.startOfDay(
        for: calendar.date(byAdding: .day, value: 1, to: Date())!
    )
    let startDate = calendar.date(byAdding: .day, value: -days, to: endDate)!

    let predicate = HKQuery.predicateForSamples(
        withStart: startDate, end: endDate
    )
    let stepType = HKQuantityType(.stepCount)
    let samplePredicate = HKSamplePredicate.quantitySample(
        type: stepType, predicate: predicate
    )

    let query = HKStatisticsCollectionQueryDescriptor(
        predicate: samplePredicate,
        options: .cumulativeSum,
        anchorDate: endDate,
        intervalComponents: DateComponents(day: 1)
    )

    let collection = try await query.result(for: healthStore)
    var dailySteps: [(date: Date, steps: Double)] = []

    collection.statisticsCollection.enumerateStatistics(
        from: startDate, to: endDate
    ) { statistics, _ in
        let steps = statistics.sumQuantity()?
            .doubleValue(for: .count()) ?? 0
        dailySteps.append((date: statistics.startDate, steps: steps))
    }

    return dailySteps
}
```

### Long-Running Collection Query

Use `results(for:)` (plural) to get an `AsyncSequence` that emits updates as new data arrives:

```swift
let updateStream = query.results(for: healthStore)

Task {
    for try await result in updateStream {
        // result.statisticsCollection contains updated data
    }
}
```

## Writing Data

Create `HKQuantitySample` objects and save them to the store.

```swift
func saveSteps(count: Double, start: Date, end: Date) async throws {
    let stepType = HKQuantityType(.stepCount)
    let quantity = HKQuantity(unit: .count(), doubleValue: count)

    let sample = HKQuantitySample(
        type: stepType,
        quantity: quantity,
        start: start,
        end: end
    )

    try await healthStore.save(sample)
}

```

Your app can only delete samples it created. Samples from other apps or Apple Watch are read-only.

## Background Delivery

Register for background updates so your app is launched when new data arrives. Requires the background delivery entitlement.

```swift
func enableStepCountBackgroundDelivery() async throws {
    let stepType = HKQuantityType(.stepCount)

    try await healthStore.enableBackgroundDelivery(
        for: stepType,
        frequency: .hourly
    )
}
```

**Pair with an `HKObserverQuery`** to handle notifications. Always call the completion handler:

```swift
let observerQuery = HKObserverQuery(
    sampleType: HKQuantityType(.stepCount),
    predicate: nil
) { query, completionHandler, error in
    defer { completionHandler() }  // Must call to signal done
    guard error == nil else { return }
    // Fetch new data, update UI, etc.
}
healthStore.execute(observerQuery)
```

**Frequencies:** `.immediate`, `.hourly`, `.daily`, `.weekly`

Call `enableBackgroundDelivery` once (e.g., at app launch). The system persists the registration.

## Workout Sessions

Use `HKWorkoutSession` and `HKLiveWorkoutBuilder` to track live workouts. Available on watchOS 2+ and iOS 17+.

```swift
func startWorkout() async throws {
    let configuration = HKWorkoutConfiguration()
    configuration.activityType = .running
    configuration.locationType = .outdoor

    let session = try HKWorkoutSession(
        healthStore: healthStore,
        configuration: configuration
    )
    session.delegate = self

    let builder = session.associatedWorkoutBuilder()
    builder.dataSource = HKLiveWorkoutDataSource(
        healthStore: healthStore,
        workoutConfiguration: configuration
    )

    session.startActivity(with: Date())
    try await builder.beginCollection(at: Date())
}

func endWorkout(
    session: HKWorkoutSession,
    builder: HKLiveWorkoutBuilder
) async throws {
    session.end()
    try await builder.endCollection(at: Date())
    try await builder.finishWorkout()
}
```

For full workout lifecycle management including pause/resume, delegate handling, and multi-device mirroring, see [references/healthkit-patterns.md](references/healthkit-patterns.md).

## Common Data Types

### HKQuantityTypeIdentifier

| Identifier | Category | Unit |
|---|---|---|
| `.stepCount` | Fitness | `.count()` |
| `.distanceWalkingRunning` | Fitness | `.meter()` |
| `.activeEnergyBurned` | Fitness | `.kilocalorie()` |
| `.basalEnergyBurned` | Fitness | `.kilocalorie()` |
| `.heartRate` | Vitals | `.count()/.minute()` |
| `.restingHeartRate` | Vitals | `.count()/.minute()` |
| `.oxygenSaturation` | Vitals | `.percent()` |
| `.bodyMass` | Body | `.gramUnit(with: .kilo)` |
| `.bodyMassIndex` | Body | `.count()` |
| `.height` | Body | `.meter()` |
| `.bodyFatPercentage` | Body | `.percent()` |
| `.bloodGlucose` | Lab | `.gramUnit(with: .milli).unitDivided(by: .literUnit(with: .deci))` |

### HKCategoryTypeIdentifier

Common category types: `.sleepAnalysis`, `.mindfulSession`, `.appleStandHour`

### HKCharacteristicType

Read-only user characteristics: `.dateOfBirth`, `.biologicalSex`, `.bloodType`, `.fitzpatrickSkinType`

## HKUnit Reference

```swift
// Basic units
HKUnit.count()                              // Steps, counts
HKUnit.meter()                              // Distance
HKUnit.mile()                               // Distance (imperial)
HKUnit.kilocalorie()                        // Energy
HKUnit.joule(with: .kilo)                   // Energy (SI)
HKUnit.gramUnit(with: .kilo)                // Mass (kg)
HKUnit.pound()                              // Mass (imperial)
HKUnit.percent()                            // Percentage

// Compound units
HKUnit.count().unitDivided(by: .minute())   // Heart rate (bpm)
HKUnit.meter().unitDivided(by: .second())   // Speed (m/s)

// Prefixed units
HKUnit.gramUnit(with: .milli)               // Milligrams
HKUnit.literUnit(with: .deci)               // Deciliters
```

## Common Mistakes

### 1. Over-requesting data types

DON'T -- request everything:
```swift
// App Review will reject this
let allTypes: Set<HKObjectType> = [
    HKQuantityType(.stepCount),
    HKQuantityType(.heartRate),
    HKQuantityType(.bloodGlucose),
    HKQuantityType(.bodyMass),
    HKQuantityType(.oxygenSaturation),
    // ...20 more types the app never uses
]
```

DO -- request only what you use:
```swift
let neededTypes: Set<HKObjectType> = [
    HKQuantityType(.stepCount),
    HKQuantityType(.activeEnergyBurned)
]
```

### 2. Not handling authorization denial

DON'T -- assume data will be returned:
```swift
func getSteps() async throws -> Double {
    let result = try await query.result(for: healthStore)
    return result!.sumQuantity()!.doubleValue(for: .count()) // Crashes if denied
}
```

DO -- handle nil gracefully:
```swift
func getSteps() async throws -> Double {
    let result = try await query.result(for: healthStore)
    return result?.sumQuantity()?.doubleValue(for: .count()) ?? 0
}
```

### 3. Assuming HealthKit is always available

DON'T -- skip the check:
```swift
let store = HKHealthStore() // Crashes on iPad
try await store.requestAuthorization(toShare: types, read: types)
```

DO -- guard availability:
```swift
guard HKHealthStore.isHealthDataAvailable() else {
    showUnsupportedDeviceMessage()
    return
}
```

### 4. Running heavy queries on the main thread

DON'T -- use old callback-based queries on main thread. DO -- use async descriptors:
```swift
// Bad: HKSampleQuery with callback on main thread
// Good: async descriptor
func loadAllData() async throws -> [HKQuantitySample] {
    let descriptor = HKSampleQueryDescriptor(
        predicates: [.quantitySample(type: stepType)],
        sortDescriptors: [SortDescriptor(\.endDate, order: .reverse)],
        limit: 100
    )
    return try await descriptor.result(for: healthStore)
}
```

### 5. Forgetting to call completionHandler in observer queries

DON'T -- skip the completion handler:
```swift
let query = HKObserverQuery(sampleType: type, predicate: nil) { _, handler, _ in
    processNewData()
    // Forgot to call handler() -- system won't schedule next delivery
}
```

DO -- always call it:
```swift
let query = HKObserverQuery(sampleType: type, predicate: nil) { _, handler, _ in
    defer { handler() }
    processNewData()
}
```

### 6. Using wrong statistics options for the data type

DON'T -- use cumulative sum on discrete types:
```swift
// Heart rate is discrete, not cumulative -- this returns nil
let query = HKStatisticsQueryDescriptor(
    predicate: heartRatePredicate,
    options: .cumulativeSum
)
```

DO -- match options to data type:
```swift
// Use discrete options for discrete types
let query = HKStatisticsQueryDescriptor(
    predicate: heartRatePredicate,
    options: .discreteAverage
)
```

## Review Checklist

- [ ] `HKHealthStore.isHealthDataAvailable()` checked before any HealthKit access
- [ ] Only necessary data types requested in authorization
- [ ] `Info.plist` includes `NSHealthShareUsageDescription` and/or `NSHealthUpdateUsageDescription`
- [ ] HealthKit capability enabled in Xcode project
- [ ] Authorization denial handled gracefully (nil results, not crashes)
- [ ] Single `HKHealthStore` instance reused (not created per query)
- [ ] Async query descriptors used instead of callback-based queries
- [ ] Heavy queries not blocking main thread
- [ ] Statistics options match data type (cumulative vs. discrete)
- [ ] Background delivery paired with `HKObserverQuery` and `completionHandler` called
- [ ] Background delivery entitlement enabled if using `enableBackgroundDelivery`
- [ ] Workout sessions properly ended and builder finalized
- [ ] Write operations only for sample types the app created

## References

- Extended patterns (workouts, anchored queries, SwiftUI integration): [references/healthkit-patterns.md](references/healthkit-patterns.md)
- [HealthKit framework](https://sosumi.ai/documentation/healthkit)
- [HKHealthStore](https://sosumi.ai/documentation/healthkit/hkhealthstore)
- [HKSampleQueryDescriptor](https://sosumi.ai/documentation/healthkit/hksamplequerydescriptor)
- [HKStatisticsQueryDescriptor](https://sosumi.ai/documentation/healthkit/hkstatisticsquerydescriptor)
- [HKStatisticsCollectionQueryDescriptor](https://sosumi.ai/documentation/healthkit/hkstatisticscollectionquerydescriptor)
- [HKWorkoutSession](https://sosumi.ai/documentation/healthkit/hkworkoutsession)
- [HKLiveWorkoutBuilder](https://sosumi.ai/documentation/healthkit/hkliveworkoutbuilder)
- [Setting up HealthKit](https://sosumi.ai/documentation/healthkit/setting-up-healthkit)
- [Authorizing access to health data](https://sosumi.ai/documentation/healthkit/authorizing-access-to-health-data)