ios - 递归函数,带有完成块,检索多个 MKDirections - Swift
问题描述
我正在尝试检索一个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。
问题是我能够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将递归函数与完成块一起使用的信息。由于加载路线是异步完成的,因此需要完成块。
如何获得以下功能但使用完成块?“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
}
解决方案
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。
以下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)
}
40320
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