使用 MKDirections (Swift 4) 导出两点之间的行驶距离后，如何访问闭包外的距离值？ [复制]

Question

我写了这样的东西来计算 2 个点/位置之间的行驶距离。

方法实现：

第 1 类：

static func calculateDistance(_ location1 : CLLocationCoordinate2D, location2: CLLocationCoordinate2D, completion: @escaping (_ distance: CLLocationDistance?) -> ()) {

    let start = MKMapItem(placemark: MKPlacemark(coordinate: location1))
    let destination = MKMapItem(placemark: MKPlacemark(coordinate: location2))
    let request = MKDirectionsRequest()
    request.source = start
    request.destination = destination
    request.requestsAlternateRoutes = false
    let direction = MKDirections(request: request)
    var distanceInMiles: CLLocationDistance?

    direction.calculate { (response, error) in
        if let response = response, let route = response.routes.first {
            distanceInMiles = route.distance * 0.000621371
            completion(distanceInMiles)
        }
    }
}

使用问题

第 2 类： 如何访问不同类中的距离值？例如，我有一个参数化的 init，其中第三个参数 "dist" 的类型为 CLLocationDistance。我想要实现的是从 Class1 的 calculateDistance 方法访问距离值

let assigningDistValue = Class1(coordinate: location, secondParam: paramValue, dist:finalDistance!)

我已经阅读了与此问题相关的所有建议解决方案，但没有任何帮助。

Answer 1

你不能访问finalDistance 在闭包之后，因为代码是按这个顺序运行的：

var finalDistance: CLLocationDistance?
// 1:
let calculatedDistance = Class1.calculateDistance(location, location2: secondlocation) { (distance) in
    // 3:
    guard let distanceInMiles = distance else { return }
    print("This is to print distance in miles", distanceInMiles)
    finalDistance = calculatedDistance
}
// 2:
let assigningDistValue = Class1(coordinate: location, secondParam: paramValue, dist:finalDistance!)

只需将let 行移到异步素材的末尾即可：

let calculatedDistance = Class1.calculateDistance(location, location2: secondlocation) { (distance) in
    guard let distanceInMiles = distance else { return }
    print("This is to print distance in miles", distanceInMiles)
    finalDistance = calculatedDistance
    // put it here
    let assigningDistValue = Class1(coordinate: location, secondParam: paramValue, dist:finalDistance!) {
        // and on we go...
    }
}

或者，使用另一个完成块，就像您在显示的第一个代码中所做的那样。这在第一个代码中是正确的，但是在第二个代码中，所有关于异步意味着什么的知识似乎都被遗忘了。