Q:

How to wait for variable in Swift before executing function? (Swift)

below is my code. I want the value for Latitude and Longitude in my Poststring. But When he do the poststring my values are still nil because swift didn't update the location yet. So how can I wait for latitude and longitude before poststring gets the Values? I heard something of didset but I don't know how to use it and where I have to use it. 
import Foundation
import CoreLocation

protocol FeedmodelProtocol: class {
    func itemsDownloaded(items: NSArray)
}

class Feedmodel: NSObject, URLSessionDataDelegate, CLLocationManagerDelegate {

    weak var delegate: FeedmodelProtocol!
    let locationManager = CLLocationManager() // create Location Manager object
    var latitude : Double?
    var longitude : Double?

    func locationManager(_ manager: CLLocationManager, didUpdateLocations locations: [CLLocation]) {
        guard let location: CLLocationCoordinate2D = manager.location?.coordinate else { return }
        // set the value of lat and long
        latitude = location.latitude
        longitude = location.longitude

    }

    func downloadItems() {
        self.locationManager.requestAlwaysAuthorization()

        // For use in foreground
        // You will need to update your .plist file to request the authorization
        self.locationManager.requestWhenInUseAuthorization()

        if CLLocationManager.locationServicesEnabled() {
            locationManager.delegate = self
            locationManager.desiredAccuracy = kCLLocationAccuracyNearestTenMeters

            locationManager.startUpdatingLocation()

        }
        let myUrl = URL(string: "http://example.com/stock_service4.php");
        let defaultSession = Foundation.URLSession(configuration: URLSessionConfiguration.default)
        var request = URLRequest(url:myUrl!)
        request.setValue("application/x-www-form-urlencoded", forHTTPHeaderField: "Content-Type")
        request.httpMethod = "POST"
        var postString = ""
        if let lat = latitude, let long = longitude {
            locationManager.stopUpdatingLocation()
            postString = "lati=\(Int(lat))&longi=\(Int(long))"
            // do task here now that postString is not empty
        }
        request.httpBody = postString.data(using: .utf8)
        let task = defaultSession.dataTask(with: request) { data, response, error in

            guard let data = data, error == nil else {                                                 // check for fundamental networking error
                print("error=\(String(describing: error))")
                return

            }

            if let httpStatus = response as? HTTPURLResponse, httpStatus.statusCode != 200 {           // check for http errors
                print("statusCode should be 200, but is \(httpStatus.statusCode)")
                print("response = \(String(describing: response))")
                print("error=\(String(describing: error))")
            }

            let responseString = String(data: data, encoding: .utf8)
            print("responseString = \(String(describing: responseString))")
            print("error=\(String(describing: error))")
            self.parseJSON(data)

        }

        task.resume()

    }

    func parseJSON(_ data:Data) {

        var jsonResult = NSArray()

        do{
            jsonResult = try JSONSerialization.jsonObject(with: data, options:JSONSerialization.ReadingOptions.allowFragments) as! NSArray;
        } catch let error as NSError {
            print(error)

        }

        var jsonElement = NSDictionary()
        let stocks = NSMutableArray()

        for i in 0 ..< jsonResult.count
        {
            print(jsonResult)
            jsonElement = jsonResult[i] as! NSDictionary

            let stock = Stockmodel()

            //the following insures none of the JsonElement values are nil through optional binding
            if  let Datum = jsonElement["Datum"] as? String,
                let Tankstelle = jsonElement["Tankstelle"] as? String,
                let Kraftstoff1 = jsonElement["Kraftstoff1"] as? String,
                let Preis1 = jsonElement["Preis1"] as? String,
                let Kraftstoff2 = jsonElement["Kraftstoff2"] as? String,
                let Preis2 = jsonElement["Preis2"] as? String,
                let Notiz = jsonElement["Notiz"] as? String,
                let longitude = jsonElement["longitude"] as? String,
                let latitude = jsonElement["latitude"] as? String

            {
                print (Datum)
                print(Tankstelle)
                print(Kraftstoff1)
                print(Preis1)
                print(Kraftstoff2)
                print(Preis2)
                print(Notiz)
                print(longitude)
                print(latitude)
                stock.Datum = Datum
                stock.Tankstelle = Tankstelle
                stock.Kraftstoff1 = Kraftstoff1
                stock.Preis1 = Preis1
                stock.Kraftstoff2 = Kraftstoff2
                stock.Preis2 = Preis2
                stock.Notiz = Notiz
                stock.longitude = longitude
                stock.latitude = latitude

            }

            stocks.add(stock)

        }

        DispatchQueue.main.async(execute: { () -> Void in

            self.delegate.itemsDownloaded(items: stocks)

        })
    }
}

Thank You!

A:

The function locationManager(_:didUpdateLocations:) is called every time when the location is update. So you should add downloadItems() in the location manager like this : 
func locationManager(_ manager: CLLocationManager, didUpdateLocations locations: [CLLocation]) {
    guard let location: CLLocationCoordinate2D = manager.location?.coordinate else { return }

    // set the value of lat and long

    latitude = location.latitude
    longitude = location.longitude

    downloadItems()
}

See the documentation about locationManager(_:didUpdateLocations:)

