递归函数，带完成块，检索多个 MKDirections - Swift

Question

我试图检索一个MKRoute包含多条路线的数组，这些路线都从同一个地方开始，但每条路线都有不同的目的地。

问题是我能想出的唯一方法是通过递归函数，但我找不到任何关于如何使用带有完成块的递归函数的信息。由于加载路由是异步完成的，因此需要一个完成块。

我如何获得以下功能但带有完成块？“添加到退货”功能？

func factorial(of num: Int) -> Int {
    if num == 1 {
        return 1
    } else {
        return num * factorial(of:num - 1)
    }
}

这是我的功能代码

func getDirections(originCoordinate: CLLocationCoordinate2D, destinationCoordinates: [CLLocationCoordinate2D], completion: @escaping(_ routes:[MKRoute]?, _ error: Error?) -> Void) {

    // Origin is the same for each route, what changes is the destination
    // A singular origin coordinate with an array of destination coordinates is passed in
    // The function would in theory be recursive, returning each route from the origin to each of the destinations.

    // Leave function if no more destination coordinates are passed
    if destinationCoordinates.count == 0 {return}

    // Origin
    let originPlacemark = MKPlacemark(coordinate: originCoordinate)
    let originItem = MKMapItem(placemark: originPlacemark)

    // Destination is made from the first element of the passed in destinationCoordinates array.
    let destinationPlacemark = MKPlacemark(coordinate: destinationCoordinates.first!)
    let destinationItem = MKMapItem(placemark: destinationPlacemark)

    // Direction Request setup
    let directionRequest = MKDirections.Request()
    directionRequest.source = originItem
    directionRequest.transportType = .automobile
    directionRequest.destination = destinationItem

    let directions = MKDirections(request: directionRequest)

    // Calculating directions
    // Heart of function
    directions.calculate { (response, err) in

        // Checking if a response is returned
        guard let response = response else {
            completion(nil, err)
            return
        }

        // Response is returned
        let route = response.routes[0]

        let tail = Array.dropFirst(destinationCoordinates)

        // Recursive part that I'm not sure how to properly implement
        completion([route].append(getDirections(originCoordinate, tail)), nil)

    }
    // If no response is retrieved, our guard let statement sends us here
}

Answer 1

对于带有完成处理程序的函数，在递归调用中，您需要为调用提供一个闭包，然后在该闭包中调用完成处理程序。

这是你如何做到这一点factorial：

func factorial(of num: Int, completion: (Int) -> ()) {
    if num == 1 {
        completion(1)
    } else {
        factorial(of: num - 1) { partial in
            completion(num * partial)
        }
    }
}

factorial(of: 8) { result in
    print(result)
}

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