swift - How to perform task completion - ios

Question

I'm trying to do reverse geocoding for multiple locations at the same time. So I create a function performReverseGeoLocation. The problem is, that since CLGeocoder().reverseGeocodeLocation a closure, the completionHandlerLocations will get executed first. How do I change these functions so that the caller will get completion handler after all CLGeocoder().reverseGeocodeLocation inside the for loop is done?
Code I have tried:
private func getImageLocation() {
performReverseGeoLocation(completionHandlerLocations: { (cities, countries) in
print("***** This is executed before the reverse geo code location is done")
})
}
private func performReverseGeoLocation(completionHandlerLocations: #escaping (_ cities: [String], _ countries: [String]) -> Void) {
var cities = [String]()
var countries = [String]()
for image in self.images {
let longitude = image.longitude
let latitude = image.latitude
let location = CLLocation(latitude: latitude, longitude: longitude)
CLGeocoder().reverseGeocodeLocation(location, completionHandler: {(placemarks, error) -> Void in
print("***** This is executed after completionHandlerLocations is done")
if error != nil {
self.alertError("Reverse geocoder failed with error" + (error?.localizedDescription)!)
return
}
if placemarks!.count > 0 {
let pm = placemarks![0]
let country = pm.country
let city = pm.locality
if (!cities.contains(city!)) {
cities.append(city!)
}
if (!countries.contains(country!)) {
countries.append(country!)
}
}
else {
self.alertError("Fail to perform reverse geo location")
}
})
}
// THIS IS WILL EXECUTED FIRST
completionHandlerLocations(cities, countries)
}

Answer 1

You can do something like this:
var count = 0
for image in self.images {
...
CLGeocoder().reverseGeocodeLocation(location) {
// get result
counter ++
if count == self.images.count { // finish all requests
completionHandlerLocations(cities, countries)
}
}
}
That's the most simple way to do.

Answer 2

As i-am-jorf mentioned, you can create a DispatchGroup and wait for the notification when all reverse geocoding tasks are complete:
private func performReverseGeoLocation(completionHandlerLocations: #escaping (_ cities: [String], _ countries: [String]) -> Void) {
let group = DispatchGroup()
var cities = [String]()
var countries = [String]()
self.images.forEach { (location) in
group.enter()
let longitude = image.longitude
let latitude = image.latitude
let location = CLLocation(latitude: latitude, longitude: longitude)
CLGeocoder().reverseGeocodeLocation(location, completionHandler: { (placemark, error) in
// do all your checks...
if placemark != nil && placemark!.count > 0 {
cities.append(placemark!.first!.locality!)
countries.append(placemark!.first!.country!)
}
group.leave()
})
}
group.notify(queue: DispatchQueue.main) {
completionHandlerLocations(cities, countries)
}
}