asynchronous - MKDirections calculateETAWithCompletionHandler executing with delay Swift 2.0 -

i'm trying calculate eta between 2 coordinates in swift using calculateetawithcompletionhandler mkdirections class. have following code in 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 show execution of code. when run program, output is

a c d g b e f 

so, i've noticed calculateetaofclosestbus() function finishes executing (reaches g, b), directions.calculateetawithcompletionhandler executes, 'e' , 'f' printed after we've returned calculateetaofclosestbus().

i'm sure don't understand how calculateetawithcompletionhandler works, calculate eta between 2 annotations via automobile , if there more intuitive way accomplish or if me understand handler better, appreciated.

solution:

ios - swift - function returns asynchronously retrieved value helped me understand how accomplish best.

please ath link states calculateetawithcompletionhandler works asynchronously .since asynchronous, works on different thread.but main thread executing on own way , prints g

begins calculating requested travel-time information asynchronously.

if return in asynchronous method..you can using closure.here link