如何获得WatchOS4 heartRateVariabilitySDNN并使用它？

Question

我正在尝试使用heartRateVariabilitySDNN.我通过查询成功获取heartRate,并使用以下代码获取heartRateVariabilitySDNN但不知道它是如何工作的.此外,没有关于此查询或Apple示例代码的正确文档.

func createheartRateVariabilitySDNNStreamingQuery(_ workoutStartDate: Date) -> HKQuery? {

    let typeHeart = HKQuantityType.quantityType(forIdentifier: .heartRateVariabilitySDNN)
    let predicate: NSPredicate? = HKQuery.predicateForSamples(withStart: workoutStartDate, end: nil, options: HKQueryOptions.strictEndDate)

    let squery = HKStatisticsQuery(quantityType: typeHeart!, quantitySamplePredicate: predicate, options: .discreteAverage, completionHandler: {(query: HKStatisticsQuery,result: HKStatistics?, error: Error?) -> Void in
        DispatchQueue.main.async(execute: {() -> Void in

            print("got: \(String(describing: result))")
        })
    })

    return squery
}