异步功能不更新变量

Question

        var endLat: Double = 0.0
        var endLong: Double = 0.0

              func forwardGeocodingStarting(address: String) {
            CLGeocoder().geocodeAddressString(address, completionHandler: { (placemarks, error) in
                if error != nil {
                    print(error)
                    return
                }

                let placemark = placemarks?[0]
                let location = placemark?.location
                let coordinate = location?.coordinate

                dispatch_async(dispatch_get_main_queue()) {
                    startLat = (coordinate?.latitude)!
                    startLong = (coordinate?.longitude)!
                    print("\(startLat) is lat and \(startLong) is long")
                }
            })
        }

Hello, here is a geocoding function that I created, that simply takes an address and returns the address's coordinates. My problem is, that at the end of the code section, when I do print(endLat) it prints out 0, however when I do it in the dispatch_async in the function, it comes out to the proper latitude.

I realize this is an asynchronous function, which is why I tried to use the dispatch_async to update the variable instantly.

My question is, how can I have these variables update instantly? Thanks in advance.

Answer 1

You cannot have these variables update instantaneously, as you are calling an asynchronous function. Thats an obvious contradiction :-)

That you dispatch back to the main queue is fine, but that doesn't change the asynchronous behaviour. In fact your are even issuing yet another dispatch_async ... (meaning the body of that code will get run at some later point in time)

You could wait for the results to be available, eg using a dispatch group , but quite likely that is not what you want here. I assume you want to use the values in some view controller? In this case you probably fill in some instance variables as the results arrive and update the UI state once both values are available.

Answer 2

You've got a little confused with what you mean by "Asynchronous function not updating variable" . forwardGeocodingStarting(_:) is NOT an Async function. geocodeAddressString is an async function designed by the API to allow you to use the following values you get after the operation is done within that function only. Hence you keep getting 0 as the it doesn't wait till the operation is done completely to display the value. dispatch_async(dispatch_get_main_queue) is used only to update UI on the Main thread. You've to use a completion handler in your function like so:

 typealias theDouble = (Double, Double) -> ()  

 func forwardGeocodingStarting(address: String, completion: theDouble){
//your code
let aVar: Double!
let bVar: Double!

let placemark = placemarks?[0]
let location = placemark?.location
let coordinate = location?.coordinate
aVar = (coordinate?.latitude)!
bVar = (coordinate?.longitude)!
completion(aVar, bVar)
}

Now when you call your function, update the variables startLat and startLong like so:

forwardGeocodingStarting(<your string address>) { firstVar, secondVar in

startLat = firstVar
startLong = secondVar
 print("\(startLat) is lat and \(startLong) is long") //You will get correct values

}