完成处理程序在任务完成之前返回

Question

我的这个函数有一个完成处理程序，该处理程序将在任务完成后返回，但从我得到的输出来看，它表明完成处理程序被认为已完成并在任务完成之前返回.

正在调用的函数：

private func getSteps(completion: (Int) -> ()) {
    var val: Int = 0
    guard let sampleType = HKObjectType.quantityType(forIdentifier: .stepCount) else { return }
    let startDate = Calendar.current.startOfDay(for: Date())
    let predicate = HKQuery.predicateForSamples(withStart: startDate, end: Date(), options: .strictEndDate)
    var interval = DateComponents()
    
    interval.day = 1
    let query = HKStatisticsCollectionQuery(quantityType: sampleType, quantitySamplePredicate: predicate, options: [.cumulativeSum], anchorDate: startDate, intervalComponents: interval)
    query.initialResultsHandler = { query,result,error in
        if let myresult = result {
            myresult.enumerateStatistics(from: startDate, to: Date()) { (statistic, value) in
                if let count = statistic.sumQuantity() {
                    val = Int(count.doubleValue(for: HKUnit.count()))
                }
            }
        }
    }
    healthStore.execute(query)
    completion(val)
}

当我打印函数的完成处理程序时，它打印 0 而不是 Int(count.doubleValue(for: HKUnit.count())) ，函数内部设置了 val。任何关于为什么我得到 0 而不是 set val 的输入将不胜感激！

Answer 1

请注意 query.initialResultsHandler 是 异步的 并且您的代码将按以下顺序执行：

private func getSteps(completion: (Int) -> ()) {
    // #1
    var val: Int = 0
    ...
    // #2
    query.initialResultsHandler = { query,result,error in
        // #5
        if let myresult = result {
            myresult.enumerateStatistics(from: startDate, to: Date()) { (statistic, value) in
                if let count = statistic.sumQuantity() {
                    // #6
                    val = Int(count.doubleValue(for: HKUnit.count()))
                }
            }
        }
        // #7
    }
    // #3
    healthStore.execute(query)
    // #4
    completion(val)
}

注意 #4 在 #6 之前执行。

解决办法是把completion移到异步块里面：

private func getSteps(completion: (Int) -> ()) {
    ...
    query.initialResultsHandler = { query,result,error in
        if let myresult = result {
            myresult.enumerateStatistics(from: startDate, to: Date()) { (statistic, value) in
                if let count = statistic.sumQuantity() {
                    val = Int(count.doubleValue(for: HKUnit.count()))
                }
            }
        }
        completion(val) // <- move here
    }
    healthStore.execute(query)
}

完成处理程序在任务完成之前返回

Completion handler returning before task finishes

completionhandler

swift

healthkit