healthkit

# 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 calling other HealthKit APIs. Health data is
available on iOS, watchOS, visionOS, iPadOS 17+, and iOS apps running on
Vision Pro. It is unavailable on iPadOS 16 or earlier and may be restricted by
managed device policy.

```swift
import HealthKit

guard HKHealthStore.isHealthDataAvailable() else {
    // Health data is unavailable or restricted on this device.
    return
}

let healthStore = HKHealthStore()
```

Create a single `HKHealthStore` instance and reuse it throughout your app. It
is thread-safe. If HealthKit is optional, review Xcode's generated
`UIRequiredDeviceCapabilities` `healthkit` entry so unsupported devices are not
excluded unintentionally.

## 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

`authorizationStatus(for:)` reports write/share authorization. HealthKit does
not reveal whether read permission was granted or denied. If the user denies
read access, queries return only samples your app successfully saved, which may
look like empty or partial data.

```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