ios - How would I create a callback around an XML request? -

i've been trying understand how callbacks work in swift. i've gone on quite few examples (like one) have helped me better understand callbacks, haven't had luck in implementing one.

i have function accepts url, downloads xml data web api , parses objects. @ same time have uilabel waiting data xml request.

below partial example of function i'd set callback. sake of clarity assume returns single data point which assigned uilabel later:

xmlutility.swift

// global var weekforecasts = [dayforecast]()  class xmlutility {      func retrievedatafromxml(myurl: string) {          if let url = nsurl(string: myurl) {             if let data = nsdata(contentsofurl: url) {                 var error: nserror?                 var cleaneddata = filterdata(data)                  if let doc = aexmldocument(xmldata: cleaneddata, error: &error) {                  //... work parsing xml ////                   day in date {                     //... work assigning values /////                     weekforecasts.append(thisday)                  }                      }            }       }  

the problem occurs in viewcontroller... have uilabels waiting values xml data request. when viewcontroller loads, xml hasn't processed yet , label failed receive value.

here's simplified example of doing in viewcontroller:

viewcontroller.swift

 @iboutlet weak var currenttemperaturelabel: uilabel!      override func viewdidload() {         super.viewdidload()        currenttemperaturelabel.text = // value out of [dayforecasts] } 

i understand why case, , have novice understanding of how solve problem. believe need use callback but, based on examples have seen far, not sure how implement one.

my question:

given example provided, how convert retrievedatafromxml method callback. additionally, how call function viewcontroller access data.

any on appreciated!

func retrievedatafromxml(myurl: string, completion: ((array<dayforecast>) -> void)) {      if let url = nsurl(string: myurl) {         if let data = nsdata(contentsofurl: url) {             var error: nserror?             var cleaneddata = filterdata(data)             var weekforecasts = [dayforecast]() //local variable              if let doc = aexmldocument(xmldata: cleaneddata, error: &error) {                  //... work creating objects xml                 day in date {                     //... work assigning values /////                     weekforecasts.append(thisday)                  }                     //pass local array completion block, takes                 //array<dayforecast> parameter                 completion(weekforecasts)              }         }     }   } 

called

//in example called in viewdidload func viewdidload() {     var urlstring = "urlstring"     retrievedatafromxml(urlstring, {(result) -> void in         //result weekforecasts          //ui elements can updated on main thread, main          //thread , update ui element on thread         dispatch_async(dispatch_get_main_queue(), {             self.currenttemperaturelabel.text = result[0] //or whatever index want             return         })     }) } 

is question asking for?