Reverse geocoding explanation

Question

I found this great piece of code for Reverse Geocoding in Swift 4 here: Reverse geocoding in Swift 4

I cannot understand what is going on here:

func geocode(latitude: Double, longitude: Double, completion: @escaping (CLPlacemark?, Error?) -> ())  {
CLGeocoder().reverseGeocodeLocation(CLLocation(latitude: latitude, longitude: longitude)) { completion($0?.first, $1) }
}

After you call it from here:

geocode(latitude: -22.963451, longitude: -43.198242) { placemark, error in
 guard let placemark = placemark, error == nil else { return }
// you should always update your UI in the main thread
DispatchQueue.main.async {
    //  update UI here
    print("address1:", placemark.thoroughfare ?? "")
    print("address2:", placemark.subThoroughfare ?? "")
    print("city:",     placemark.locality ?? "")
    print("state:",    placemark.administrativeArea ?? "")
    print("zip code:", placemark.postalCode ?? "")
    print("country:",  placemark.country ?? "")       
 }
}

Can anyone offer an explanation.

cekisakurek · Answer 1 · 2018-12-19T16:36:22.267

# Gets latitude, longitude and a completion block
func geocode(latitude: Double, longitude: Double, completion: @escaping (CLPlacemark?, Error?) -> ()) 
{
    # Calls system function to resolve the coordinates
    CLGeocoder().reverseGeocodeLocation(CLLocation(latitude: latitude, longitude: longitude))
    {
        # $0 is the placemarks array so this returns the first value of the placemarks array
        # $1 is the error 
        completion($0?.first, $1)

    }
}

Geocode function gets latitude and longitude and a completion block. It passes the coordinates to CLGeocoder().reverseGeocodeLocation function and returns the first placemark with the completion block.

Method signature for reverseGeocodeLocation is

func reverseGeocodeLocation(_ location: CLLocation, completionHandler: @escaping CLGeocodeCompletionHandler)

And the completion handler is defined as

typealias CLGeocodeCompletionHandler = ([CLPlacemark]?, Error?) -> Void

as you can see [CLPlacemark]? is an optional array of CLPlacemarks.

Thank you :) so what happens if the completion block returns a $1. Do I need to add in an error handler here? — Man James, Dec 18 '18 at 13:53
You can check the error in func geocode() where it returns the error with proper name. — cekisakurek, Dec 19 '18 at 16:34
Actually you are already doing it with guard let placemark = placemark, error == nil else { return } line. — cekisakurek, Dec 19 '18 at 16:35

Reverse geocoding explanation

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