How to terminate Swift Combine response after first emit?

Question

I am requesting a single value from a publisher and would like to terminate after I get the response. Below I'm just deallocating the cancel token after the first time, is there a better way to do this?

extension MyInteractor {
    private static var locationPermissionToken: Cancellable?

    func requestLocationPermission(completion: @escaping (Result<Void, LocationError>) -> Void) {
        Self.locationPermissionToken = locationProxy.authorizationPublisher
            .sink { status in
                Self.locationPermissionToken = nil
                status ? completion(.success(())) : completion(.failure(.deniedLocationServices))
            }

        locationProxy.requestAuthorization()
    }
}

Answer 1

Considering that you have a custom Publisher, I would suggest modifying the implementation of that so that it sends a completion straight after it emitted a value.

Since you are using a PassthroughSubject rather than a custom type conforming to Publisher, you can create a custom method that sends both a value and a completion. Then you need to call this method instead of calling authorizationSubject.send from inside your type.

private func emitAndComplete(authorizationStatus: Bool) {
    Self.authorizationSubject.send(authorizationStatus)
    Self.authorizationSubject.send(completion: .finished)
}

Full modified code:

@available(OSX 10.15, iOS 13, tvOS 13, watchOS 6, *)
public class LocationProxy: NSObject {
    private lazy var manager = CLLocationManager()

    private static let authorizationSubject = PassthroughSubject<Bool, Never>()
    public private(set) lazy var authorizationPublisher: AnyPublisher<Bool, Never> = Self.authorizationSubject.eraseToAnyPublisher()

    var isAuthorized: Bool { CLLocationManager.isAuthorized }

    func requestAuthorization(for type: LocationAPI.AuthorizationType = .whenInUse) {
        // Handle authorized and exit
        guard !isAuthorized(for: type) else {
            emitAndComplete(authorizationStatus: true)
            return
        }

        // Request appropiate authorization before exit
        defer {
            #if os(macOS)
            if #available(OSX 10.15, *) {
                manager.requestAlwaysAuthorization()
            }
            #elseif os(tvOS)
            manager.requestWhenInUseAuthorization()
            #else
            switch type {
            case .whenInUse:
                manager.requestWhenInUseAuthorization()
            case .always:
                manager.requestAlwaysAuthorization()
            }
            #endif
        }

        // Handle mismatched allowed and exit
        guard !isAuthorized else {
            // Process callback in case authorization dialog not launched by OS
            // since user will be notified first time only and ignored subsequently
            emitAndComplete(authorizationStatus: false)
            return
        }

        // Handle denied and exit
        guard CLLocationManager.authorizationStatus() == .notDetermined else {
            emitAndComplete(authorizationStatus: false)
            return
        }
    }

    public func locationManager(_ manager: CLLocationManager, didChangeAuthorization status: CLAuthorizationStatus) {
        guard status != .notDetermined else { return }
        emitAndComplete(authorizationStatus: isAuthorized)
    }

    private func emitAndComplete(authorizationStatus: Bool) {
        Self.authorizationSubject.send(authorizationStatus)
        Self.authorizationSubject.send(completion: .finished)
    }
}