MKDirections calculateETAWithCompletionHandler exe

Question

I'm trying to calculate the ETA between two coordinates in Swift using calculateETAWithCompletionHandler from the MKDirections class. I have the following code in my program,

MapViewController.swift

print("A")
calculateETAofClosestBus(closestBusStop)
print("B")

func calculateETAofClosestBus(destination: BusStop) {
    var shortestETA = 0

    print("C")
    var request = MKDirectionsRequest()
    var sourceItem = MKMapItem(placemark: MKPlacemark(coordinate: listBuses[0].pin.coordinate, addressDictionary: nil))
    request.source = sourceItem
    request.transportType = .Automobile
    let destinationCoordinates = CLLocationCoordinate2D(latitude: destination.coordinate.latitude, longitude: destination.coordinate.longitude)
    let destinationItem = MKMapItem(placemark: MKPlacemark(coordinate: destinationCoordinates, addressDictionary: nil))
    request.destination = destinationItem
    request.requestsAlternateRoutes = false
    var directions = MKDirections(request: request)
    print("D")
    directions.calculateETAWithCompletionHandler { (etaResponse, error) -> Void in
        print("E")
        if let error = error {
            print("Error while requesting ETA : \(error.localizedDescription)")
        } else {
            print("F")
            shortestETA = Int((etaResponse?.expectedTravelTime)!)
        }
    }
    print("G")
}

I included print statements to show the execution of the code. When I run the program, the output I get is

A
C
D
G
B
E
F

So, I've noticed that the calculateETAofClosestBus() function finishes executing (reaches G, then B), but then directions.calculateETAWithCompletionHandler executes, so 'E' and 'F' get printed AFTER we've returned from calculateETAofClosestBus().

I'm sure I don't understand how calculateETAWithCompletionHandler works, but I would like to calculate the ETA between two Annotations via an automobile and if there is a more intuitive way to accomplish this or if someone could help me understand the Handler better, that would be greatly appreciated.

Solution:

iOS - Swift - Function that returns asynchronously retrieved value helped me understand how to accomplish this best.

Answer 1

Please look ath this Link it clearly states calculateETAWithCompletionHandler works asynchronously .Since asynchronous, it works on different thread.But your main thread is executing on its own way and prints G

Begins calculating the requested travel-time information asynchronously.

If you would like to return in the asynchronous method..You can do using closure.Here is the link