如何等待异步函数Swift

Question

我有一个收集步骤数据的函数，但是我需要一种方法让它在循环之前等待查询完成。 我意识到有关此问题还有其他问题，但我不知道如何解决。 该功能如下：

func stepsAllTime(completion: (Double, NSError?) -> () ) {
    var stopStart = true
    while stopStart {

        x += -1
        // The type of data we are requesting
        let sampleType = HKSampleType.quantityTypeForIdentifier(HKQuantityTypeIdentifierStepCount)
        var daysAgo = x
        var daysSince = x + 1
        var daysSinceNow = -1 * daysAgo
        checker = allTimeSteps.count

        // Our search predicate which will fetch data from now until a day ago
        let samplePredicate = HKQuery.predicateForSamplesWithStartDate(NSCalendar.currentCalendar().dateByAddingUnit(.CalendarUnitDay, value: daysAgo, toDate: NSDate(), options: nil), endDate: NSCalendar.currentCalendar().dateByAddingUnit(.CalendarUnitDay, value: daysSince, toDate: NSDate(), options: nil), options: .None)

        // The actual HealthKit Query which will fetch all of the steps and sub them up for us.
        let stepQuery = HKSampleQuery(sampleType: sampleType, predicate: samplePredicate, limit: 0, sortDescriptors: nil) { query, results, error in
            var steps: Double = 0


            if results?.count > 0 {
                for result in results as! [HKQuantitySample] {
                    steps += result.quantity.doubleValueForUnit(HKUnit.countUnit())
                }
            }

            completion(steps, error)
            self.allTimeStepsSum += steps
            self.allTimeSteps.append(steps)
            println("New Sum:")
            println(self.allTimeStepsSum)
            println("Days Since Today:")
            println(daysSinceNow)
            if !(self.allTimeStepsTotal > self.allTimeStepsSum) {
                stopStart = false
            }
        }
        if allTimeStepsTotal > allTimeStepsSum {

            self.healthKitStore.executeQuery(stepQuery)

        }

    }

}

如何才能做到这一点？ Swift中是否有某种“完成时”功能？

Answer 1

您可以通过实现回调使程序等待查询完成。

您可以在以下博客文章http://www.charles-socrates-chandler.com/blog/2015/2/10/callbacks-in-swift中了解更多

Answer 2

我假设我的评论是正确的。 您可能想看一看递归模式，如下所示：

import HealthKit

class Person {

    var nbSteps: Double = 0

    func fetchInfo() -> Void {
        let sampleType = HKSampleType()
        let samplePredicate: NSPredicate? = nil // Your predicate
        let stepQuery = HKSampleQuery(
            sampleType: sampleType,
            predicate: samplePredicate,
            limit: 0,
            sortDescriptors: nil) { (sampleQuery, object, error) -> Void in
                self.nbSteps += 1 // or the value you're looking to add

                dispatch_async(dispatch_get_main_queue(), { () -> Void in // Dispatch in order to keep things clean
                    self.fetchInfo()
                })
        }
    }

}