Modern Concurrency: Beyond the Basics

Oct 20 2022 Swift 5.5, iOS 15, Xcode 13.4

Part 1: AsyncStream & Continuations

7. Wrapping Delegate With Continuation

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Previous episode: 6. Unit Testing Next episode: 8. Wrapping Callback With Continuation

At the time of recording, Xcode 14 flags a runtime error in the location delegate continuation. If this happens to you, use Xcode 13 for the rest of Part 1.

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Share Location

In ChatView, locate the first Button in the HStack.

Button(action: {
  Task {
    do {
      try await model.shareLocation()
    } catch {
      lastErrorMessage = error.localizedDescription
    }
  }
}, label: {
  Image(systemName: "location.circle.fill")
    .font(.title)
    .foregroundColor(Color.gray)
})

Managing authorizations

Jump to shareLocation() in BlabberModel. Add a location property:

let location: CLLocation = 
try await withCheckedThrowingContinuation { [weak self] continuation in

}

let location: CLLocation = 
try await withCheckedThrowingContinuation { [weak self] continuation in

}

ChatLocationDelegate

In the Utility group, open ChatLocationDelegate.

// First, create a type alias 
typealias LocationContinuation = CheckedContinuation<CLLocation, Error>
private var continuation: LocationContinuation?

init(continuation: LocationContinuation) {
  self.continuation = continuation
  // call super.init() so you can set self as the delegate 
  super.init()
  manager.delegate = self
  // then the location manager can request authorization
  manager.requestWhenInUseAuthorization()
}

self?.delegate = ChatLocationDelegate(continuation: continuation)

func locationManagerDidChangeAuthorization(_ manager: CLLocationManager) {
  switch manager.authorizationStatus {
  case .authorizedAlways, .authorizedWhenInUse:
    manager.startUpdatingLocation()
  case .notDetermined:
    break
  default:
    // TODO: resume continuation instead of break
    break
  }
}

func locationManagerDidChangeAuthorization(_ manager: CLLocationManager) {
  switch manager.authorizationStatus {
  case .authorizedAlways, .authorizedWhenInUse:
    manager.startUpdatingLocation()
  case .notDetermined:
    break
  default:
    🟩
    continuation?.resume(
      throwing: "The app isn't authorized to use location data"
    )
    continuation = nil
    🟥
  }
}

func locationManager(_ manager: CLLocationManager, didUpdateLocations locations: [CLLocation]) {
  guard let location = locations.first else { return }
  // TODO: resume continuation
}

func locationManager(
  _ manager: CLLocationManager,
  didUpdateLocations locations: [CLLocation]
) {
  guard let location = locations.first else { return }
  🟩
  continuation?.resume(returning: location)  // returns, resumes etc: read below
  continuation = nil  // read next sentence below
  🟥
}

func locationManager(_ manager: CLLocationManager, didFailWithError error: Error) {
  // TODO: resume continuation
  🟩
  continuation?.resume(throwing: error)
  continuation = nil
  🟥
}

Using your delegate

Now, the complete workflow is in place: Once you set up the location manager with the delegate, it will try to fetch the current location and will use the injected continuation to return either a location or an error.

print(location.description)