如何同步异步HealthKit查询的返回值？

Question

我写了一个从HKHealthStore检索最大心率和平均心率的函数。虽然两个HKStatisticQuery()调用都有效，但由于完成处理程序是异步的，因此函数的返回值（maxHRT，avgHRT）为（0,0）。

在更新返回值和退出函数之前，等待两个完成处理程序返回的优雅方法是什么？

代码：

func calcHeartRateStatistics() -> (Double, Double){

    var maxHRT:Double = 0
    var avgHRT:Double = 0


    // First specify type of sample we need, i.e. Heart Rate Type
    //
    let sampleType = HKQuantityType.quantityTypeForIdentifier(HKQuantityTypeIdentifierHeartRate)




    // Create query predicate so result only returns objects within the period between Start Date
    // and End Date
    //
    let predicate = HKQuery.predicateForSamplesWithStartDate(pvtStartDate, endDate: pvtEndDate, options: .None)

    // Query for Maximum Heart Rate that occurred between Start Date and End Date
    //
    let queryMaxHeartRate = HKStatisticsQuery(quantityType: sampleType!,
                                              quantitySamplePredicate: predicate,
                                              options: .DiscreteMax)
    { [unowned self] (query, result, error) in
        if let maxQuantity = result?.maximumQuantity() {
            let maxHeartRate = maxQuantity.doubleValueForUnit(HKUnit(fromString: "count/min"))
            maxHRT = round(maxHeartRate)
        }

    }

    // Query for Average Heart Rate that occurred between Start Date and End Date
    //
    let queryAverageHeartRate = HKStatisticsQuery(quantityType: sampleType!,
                                                  quantitySamplePredicate: predicate,
                                                  options: .DiscreteAverage)
    { [unowned self] (query, result, error) in
        if let averageQuantity = result?.averageQuantity(){
            let avgHeartRate = averageQuantity.doubleValueForUnit(HKUnit(fromString: "count/min"))
            avgHRT = round(avgHeartRate)
        }

    }

    pvtHealthStore.executeQuery(queryMaxHeartRate)
    pvtHealthStore.executeQuery(queryAverageHeartRate)
    return (maxHRT, avgHRT)
} // calcHeartRateStatistics