swift - MKDirections computeWithCompletionHandler

我试图在函数内部发出MKDirections请求，然后返回该值。
这就是我目前所拥有的

func requestETA(userCLLocation: CLLocation, coordinate: CLLocationCoordinate2D) -> String {
    let request = MKDirectionsRequest()

    // code setting up directions request, not important or relevant

    let directions = MKDirections(request: request)

    var travelTime = "Not Available"

        directions.calculate { response, error in
            if let route = response?.routes.first {
                travelTime = "\(route.expectedTravelTime/60)"
                print(travelTime)
            } else {
                travelTime = "0"
            }
        }
    print(travelTime)
    return travelTime
}

我知道MKDirections是异步运行的，所以当前在directions请求完成之前返回。如何使return语句等待某种完成处理程序？
同时也意识到这个问题在这里的答案相当模糊，非常笼统。我一直不知道如何把这个答案应用到这个具体的问题上。

最佳答案

可以添加完成处理程序以传递可选字符串和错误，如下所示：

func requestETA(userCLLocation: CLLocation, coordinate: CLLocationCoordinate2D, completion: @escaping (_ string: String?, _ error: Error?) -> () ) {
    let request = MKDirectionsRequest()
    // code setting up directions request
    let directions = MKDirections(request: request)
    var travelTime: String?
    directions.calculate { response, error in
        if let route = response?.routes.first {
            travelTime = String(route.expectedTravelTime/60)
        }
        completion(travelTime, error)
    }
}

使用

requestETA(userCLLocation: location, coordinate: coordinate) { (travelTime, error) in
    guard let travelTime = travelTime, error == nil else { return }
    print(travelTime)
}

关于swift - MKDirections computeWithCompletionHandler，我们在Stack Overflow上找到一个类似的问题：https://stackoverflow.com/questions/44533592/