ios - 使用 MKDirections 实例的计算方法

Question

我在 for 循环中使用 MKDirections 对象的计算方法时遇到一个非常奇怪的问题。我的代码计算从用户当前位置到 3 个不同露营地位置的距离，但并非所有距离都是在代码中的同一点计算的。下面是我的代码；

        for siteIndex in 0...countySites.count - 1 {

            var selectedSite = countySites[siteIndex]
            let destinationPlacemark = MKPlacemark(coordinate: selectedSite.locationCoordinate)

            request.destination = MKMapItem(placemark: destinationPlacemark)

            let distanceAndDirections = MKDirections(request: request)

            if currentCoordinate != nil {

                distanceAndDirections.calculate { (response, error) in
                    if error == nil {

                        let distanceInMetres = response?.routes.first?.distance
                        let distanceInMiles = distanceInMetres! / 1610
                        let roundedDistanceInMiles = Int(distanceInMiles.rounded())
                        distanceToSite = roundedDistanceInMiles

                        selectedSite.distance = distanceToSite
                        self.countySites[siteIndex] = selectedSite

                        print("distanceToSite = \(distanceToSite)")

                        if self.countySites.count > 1 {
                            if siteIndex == self.countySites.count - 1 {

                                print("BEFORE: countySites[0].distance = \(self.countySites[0].distance)")
                                print("BEFORE: countySites[1].distance = \(self.countySites[1].distance)")
                                print("BEFORE: countySites[2].distance = \(self.countySites[2].distance)")

                                self.countySites = self.sortByDistance(sites: self.countySites)

                                print("AFTER: countySites[0].distance = \(self.countySites[0].distance)")
                                print("AFTER: countySites[1].distance = \(self.countySites[1].distance)")
                                print("AFTER: countySites[2].distance = \(self.countySites[2].distance)")

                                self.tableView.reloadData()
                            }
                        }

                    } else {

                        print("Distance error: \(error.debugDescription)")

                    }
                }

            } else {
                print("currentCoordinate is nil")
            }
        }

当我在我的设备上运行代码时，我得到以下输出；

Got here!
distanceToSite = 224
distanceToSite = 235
BEFORE: countySites[0].distance = 224
BEFORE: countySites[1].distance = -1
BEFORE: countySites[2].distance = 235
AFTER: countySites[0].distance = -1
AFTER: countySites[1].distance = 224
AFTER: countySites[2].distance = 235
distanceToSite = 235

谁能解决这个问题吗？谢谢。

Answer 1

自 calculate(completionHandler:)以异步方式返回值，不能保证在到达列表中的最后一个元素后计算所有距离。要实现您的需要，您必须执行以下操作:

let dispatchGroup = DispatchGroup()

for siteIndex in 0 ..< countySites.count {
    var selectedSite = countySites[siteIndex]

    if let currentCoordinate = currentCoordinate {
        let request = MKDirections.Request()
        request.source = MKMapItem(placemark: MKPlacemark(coordinate: currentCoordinate))
        request.destination = MKMapItem(placemark: MKPlacemark(coordinate: selectedSite.locationCoordinate))
        let distanceAndDirections = MKDirections(request: request)

        dispatchGroup.enter()
        distanceAndDirections.calculate { (response, error) in
            if error == nil {

                let distanceInMetres = response?.routes.first?.distance
                let distanceInMiles = distanceInMetres! / 1610
                let roundedDistanceInMiles = Int(distanceInMiles.rounded())
                let distanceToSite = roundedDistanceInMiles

                selectedSite.distance = distanceToSite
                countySites[siteIndex] = selectedSite

                print("distanceToSite = \(distanceToSite)")

            } else {

                print("Distance error: \(error.debugDescription)")

            }
            dispatchGroup.leave()
        }

    } else {
        print("currentCoordinate is nil")
    }
}

dispatchGroup.notify(queue: .main) {
    print("BEFORE: countySites[0].distance = \(self.countySites[0].distance)")
    print("BEFORE: countySites[1].distance = \(self.countySites[1].distance)")
    print("BEFORE: countySites[2].distance = \(self.countySites[2].distance)")

    self.countySites = self.sortByDistance(sites: self.countySites)

    print("AFTER: countySites[0].distance = \(self.countySites[0].distance)")
    print("AFTER: countySites[1].distance = \(self.countySites[1].distance)")
    print("AFTER: countySites[2].distance = \(self.countySites[2].distance)")

    self.tableView.reloadData()
}

您必须创建一个DispatchGroup并调用dispatchGroup.enter()在调用calculate(completionHandler:)之前。在同一方法的completionHandler中，您必须调用dispatchGroup.leave() 。一旦enter和 leave调用计数具有相同的值，dispatchGroup.notify 的闭包被执行。请注意，我替换了您的封闭范围 0...countySites.count - 1与半开范围 0 ..< countySites.count .

亲切的问候