Type of HealthKit unit for measurements (HKUnit)

Question

While working with Apple's HealthKit to read or write health data, I've frequently come across one issue of unit conversion for different type of HKSampleObject.

After fetching healthData in HKQuantitySample type, how to convert it in double or any other type?

Answer 1

In Apple's doc for HKUnit they mention that you can convert it through any SI or non-SI units functionality.

here's some HKQuantitySample with unit conversion:

var data: Double? = nil
// here sample is of type HKQuantitySample
switch type {
        case HKSampleType.quantityType(forIdentifier: .environmentalAudioExposure),
            HKSampleType.quantityType(forIdentifier: .headphoneAudioExposure):
            data = sample.quantity.doubleValue(for: HKUnit.decibelAWeightedSoundPressureLevel())
        case HKSampleType.quantityType(forIdentifier: .heartRate):
            data = sample.quantity.doubleValue(for: HKUnit(from: "count/min"))
        case HKObjectType.quantityType(forIdentifier: .oxygenSaturation):
            data = sample.quantity.doubleValue(for: HKUnit(from: "%")) * 100
        case HKObjectType.quantityType(forIdentifier: .bodyMass),
            HKObjectType.quantityType(forIdentifier: .leanBodyMass):
            data = sample.quantity.doubleValue(for: HKUnit.gramUnit(with: .kilo))
        case HKObjectType.quantityType(forIdentifier: .stepCount),
            HKObjectType.quantityType(forIdentifier: .flightsClimbed),
            HKObjectType.quantityType(forIdentifier: .bodyMassIndex),
            HKObjectType.quantityType(forIdentifier: .uvExposure):
            data = sample.quantity.doubleValue(for: HKUnit.count())
        case HKObjectType.quantityType(forIdentifier: .heartRateVariabilitySDNN):
            data = sample.quantity.doubleValue(for: HKUnit.secondUnit(with: .milli))
        case HKObjectType.quantityType(forIdentifier: .appleStandTime):
            data = sample.quantity.doubleValue(for: HKUnit.second())
        case HKObjectType.quantityType(forIdentifier: .walkingHeartRateAverage),
            HKObjectType.quantityType(forIdentifier: .restingHeartRate):
            data = sample.quantity.doubleValue(for: HKUnit(from: "count/s"))
        case HKObjectType.quantityType(forIdentifier: .distanceWalkingRunning),
            HKObjectType.quantityType(forIdentifier: .distanceCycling):
            data = sample.quantity.doubleValue(for: HKUnit.meter())
        case HKObjectType.quantityType(forIdentifier: .basalBodyTemperature),
            HKObjectType.quantityType(forIdentifier: .bodyTemperature):
            data = sample.quantity.doubleValue(for: HKUnit.degreeCelsius())
        case HKObjectType.quantityType(forIdentifier: .bloodPressureSystolic),
            HKObjectType.quantityType(forIdentifier: .bloodPressureDiastolic):
            data = sample.quantity.doubleValue(for: HKUnit.millimeterOfMercury())
        case HKObjectType.quantityType(forIdentifier: .peakExpiratoryFlowRate):
            data = sample.quantity.doubleValue(for: HKUnit(from: "L/min"))
        case HKObjectType.quantityType(forIdentifier: .vo2Max):
            data = sample.quantity.doubleValue(for: HKUnit(from: "ml/kg*min"))
        default:
            break
        }

Note (thanks to ramddy's answer) : You can construct various types of HKUnits using some common class methods made available through HealthKit.

for example you can use:

let unit = HKUnit(from: "ml/kg*min"

or:

let kiloMin = HKUnit.gramUnit(with: .kilo).unitMultiplied(by: .minute())
let milliLit = HKUnit.literUnit(with: .milli)
let unit = milliLit.unitDivided(by: kiloMin)