Swift async/await in a for loop or map

Question

I have this model:

struct ExactLocation {
    var coordinates: CLLocationCoordinate2D? {
        get async throws {
            let geocoder = CLGeocoder()
            let placemark = try await geocoder.geocodeAddressString(address)
            let mark = MKPlacemark(placemark: placemark.first!)
            return mark.coordinate
        }
    }
}
struct Property {
    let exactLocation: ExactLocation?
}

Answer 1

You can use a TaskGroup<\/code> here, which is a kind of AsyncSequence<\/code> . "Getting the coordinates" for each property are the tasks that you should add to a task group. You can then transform the tasks in whatever way you like (eg compactMap<\/code> ). Finally, you can convert the task group to an array using reduce<\/code> , and await<\/code> that:

private func addAnnotations(for properties: [Property]) async {
    let coordinates = await withTaskGroup(
        of: CLLocationCoordinate2D?.self) { group -> [CLLocationCoordinate2D] in
        for property in properties {
            group.addTask { try? await property.exactLocation?.coordinates }
        }
        return await group.compactMap { $0 }
        .reduce(into: []) { $0.append($1) }
    }
}

Answer 2

First, be careful about the number of requests that you perform. The docs say :

• Send at most one geocoding request for any one user action.

• If the user performs multiple actions that involve geocoding the same location, reuse the results from the initial geocoding request instead of starting individual requests for each action.

• When you want to update the user's current location automatically (such as when the user is moving), issue new geocoding requests only when the user has moved a significant distance and after a reasonable amount of time has passed. For example, in a typical situation, you should not send more than one geocoding request per minute.

And the old Location and Maps Programming Guide says:

The same CLGeocoder object can be used to initiate any number of geocoding requests but only one request at a time may be active for a given geocoder.

So, the whole idea of rapidly issuing a series of geolocation requests may be imprudent, and even if you were to do just a few, I would be inclined to avoid performing them concurrently. So, I would consider a simple for loop, eg:

func addAnnotations(for addresses: [String]) async throws {
    let geocoder = CLGeocoder()
    
    for address in addresses {
        if 
            let placemark = try await geocoder.geocodeAddressString(address).first,
            let coordinate = placemark.location?.coordinate
        {
            let annotation = MKPointAnnotation()
            annotation.title = placemark.name
            annotation.coordinate = coordinate
            
            // ...
            
            // you might even want to throttle your requests, e.g.
            //
            // try await Task.sleep(nanoseconds: nanoseconds) 
        }
    }
}

---

Technically, you could do the computed property approach. Now, I did not see address in your model anywhere, but let's imagine:

struct Property {
    let address: String
    
    var coordinate: CLLocationCoordinate2D? {
        get async throws {
            let geocoder = CLGeocoder()
            return try await geocoder
                .geocodeAddressString(address)
                .first?.location?.coordinate
        }
    }
}

(Note the elimination of the forced unwrapping operator and the unnecessary instantiating of another placemark.)

Then you could do:

func addAnnotations(for properties: [Property]) async throws {
    for property in properties {
        if let coordinate = try await property.coordinate {
            let annotation = MKPointAnnotation()
            annotation.coordinate = coordinate
            ...
        }
    }
}

I am not crazy about that approach (as we are hiding rate-limited CLGeocoder requests with all sorts of constraints inside a computed property; if you access the same property repeatedly, duplicate geocoder requests will be issued, which Apple explicitly advises that we avoid). But the async property technically works, too.