Function that fires after user's location has updated - ios

Question

Important Note: I am aware that I could simply call my callback function inside didUpdateLocations and achieve what I want. Unfortunately, this route cannot be taken because of some pre-existing design decisions that were made in this project (which I have no influence over).
I need to write a function that fires the first time the user's coordinates update, and then passes those coordinates to a completion handler. In my case, that completion handler is a function called fetchCountry(fromLocation: CLLocation) which returns the country corresponding to the CLLocation given.
In other words, I want to write a function similar to didUpdateLocations, with the capability of calling a completion handler after those updates have been received:
func getUserLocation(callback: #escaping (CLLocation?) -> Void) {
// wait until user's location has been retrieved by location manager, somehow
// prepare output for callback function and pass it as its argument
let latitude = manager.location!.coordinate.latitude
let longitude = manager.location!.coordinate.longitude
let location = CLLocation(latitude: latitude, longitude: longitude)
callback(location)
}
In short, getUserLocation is just a wrapper for didUpdateLocations but I am really not sure how I would go about writing this function so that it achieves what I want.
My greater goal here is to show the user a local notification only if they are in a certain country (e.g. United States) upon launching the app. It is a hard requirement for my application to make the decision of scheduling/not scheduling this notification inside AppDelegate.swift, but this decision cannot be made until the user's location has been retrieved. I plan to use getUserLocation inside the appDelegate like this:
I hope that I have conveyed clearly that I am looking to achieve this using a function with a completion handler. Here is what I would like my code to do (i.e. my use case), inside AppDelegate.swift:
// inside didFinishLaunchingWithOptions
UNUserNotificationCenter.current().requestAuthorization(options: [.badge, .alert, .sound]) { (granted, error) in
if granted {
// this is the use case of the function I am trying to write
LocationManager.shared.getLocation(completion: { location in
// fetches the country from given location using reverse geo-coding
LocationManager.shared.fetchCountry(from: location, completion: { country in
if country == "United States" {
let notification = LocalNotification()
notificationManager.schedule(notification: notification)
}
})
})
}
}

Answer 1

Edited the whole answer. You would need to use a synchronizing api (OperationQueue, DispatchQueue, etc) because your CLLocationManager is already fetching even before getUserLocation is called. Callbacks alone can't handle this that's why I removed that option already. For this case, I used DispatchQueue because I prefer using it, to each their own.
class LocationManager: NSObject, CLLocationManagerDelegate{
static let shared: LocationManager()
private let privateQueue = DispatchQueue.init("somePrivateQueue")
private var latestLocation: CLLocation!{
didSet{
privateQueue.resume()
}
}
func getUserLocation(queue: DispatchQueue, callback: #escaping (CLLocation?) -> Void) {
if latestLocation == nil{
privateQueue.suspend() //pause queue. wait until got a location
}
privateQueue.async{ //enqueue work. should run when latestLocation != nil
queue.async{ //use a defined queue. most likely mainQueue
callback(self.latestLocation)
//optionally clear self.latestLocation to ensure next call to this method will wait for new user location. But if you are okay with a cached userLocation, then no need to clear.
}
}
}
func fetchCountry(from currentLocation: CLLocation, completion: ) //you already have this right?
#objc func locationManager(_ manager: CLLocationManager,
didUpdateLocations locations: [CLLocation]){
latestLocation = locations.last! //API states that it is guaranteed to always have a value
}
}
I also agree on using a 3rd party library if possible. Those code would be guaranteed to be unit-tested and handle edge cases. Whereas, my above code could have a bug (none that I know of btw)

Answer 2

From your AppDelegate code I can assume that you are determining the country in the LocationManager class only. I would suggest to remove the call back from the getUserLocation() function and create a different function named postLocalNotification() in the AppDelegate to just post the local notification.
When you start fetching the user location the didUpdateLocation will be called in which you should call the fetchCountry() with the latest location. If the fetched country is proper and you want to post the local notification get the appelegate object and call the function which will post the notification as below
let appDelegate = UIApplication.shared.delegate as! AppDelegate
appDelegate.postLocalNotification()
Hope this helps.

Answer 3

Did you try to use PromiseKit+CoreLocation?
It provides CLLocationManager.requestLocation(authorizationType:satisfying:). If you don't want to import all the PromiseKit framework (which is great & avoid such completion chain), you can copy its code. They did exactly what you want: wrapping the CoreLocation request in a function.

Answer 4

I don't know what your special requirements are so this is general advice.
It sounds like you should set up a second location manager for your own use. Set it up in the app delegate and put its delegate callbacks in there, separate from the main location manager.
Don't try to delay willFinishLaunchingWithOptions from finishing. Depending on your requirements you might have to move any UI setup code to your own callback to set up the interface after the country is determined. I would even consider showing a different UI while you're doing this location and notification set up, then swap it out for the main UI when your have notification permission, location permission and the country.
Realise that the first location update you get through didUpdateLocations can be inaccurate. Check its distance accuracy but also check its timestamp and discard any update that's old. For your purposes (country accuracy) that probably means older than an hour. You're only really considering the use case where your app was opened for the first time after a user gets off a plane coming from another country. For accuracy, if the timestamp is recent, anything under 3000m or 5000m will be fine for that level.
Because the required accuracy is so low the location will be coming from cell tower triangulation. It should be fast (maybe within 2-5 seconds).
The one thing I'd be careful about is that your location manager will have to request location permissions while the main location manager does the same thing. I don't know how requesting permissions twice like that works.
I'd also separate fetching the country and getting location from the notification permissions.
Some more general advice: Your use case looks like it's handled in the WWDC session for Advanced NSOperations. The speaker handles cases where you need several things to be set up before the next part can move on. There's a location use case and a permissions use case in there too, one depending on the other.