Don’t call us - we’ll call you: Modern SwiftUI callbacks using AsyncStream

Mobile apps have to permanently deal with asynchronous events - notifications, network status, database updates, user input, and more. The way how we have dealt with asynchronous events has changed significantly over time - from callback delegates to closures, and now AsyncSequence.

In this talk, we will take a deep dive into how AsyncStream works, how to use it in your apps, and how to use it to build ergonomic APIs. We will also take a trip down memory lane to understand where we’re coming from, and how asynchronous APIs have evolved over the years.

You will learn:

* All the ways to listen to asynchronous events
* How AsyncStream helps to make your apps more robust
* What’s the difference between AsyncSequence and AsyncStream
* How to use AsyncStream to implement modern APIs that are easy to use

I will provide real-world examples to explain the underlying concepts, and we’ll take a look at how the Firebase team implemented asynchronous APIs over the span of almost a decade and several iterations of Objective-C and Swift.

Transcript

1. SwiftLeeds 2025 @peterfriese.dev
2. SwiftLeeds 2025 @peterfriese.dev Don’t call us - we’ll call you Peter Friese, Developer Relations Engineer, Google
4. s BASIC
5. 10 PRINT "Hello world"
6. 10 PRINT "Hello world" 20 GOTO 10
7. 10 PRINT "Hello world" 05 LET I = 1 20 GOTO 10
8. 10 PRINT "Hello world" 20 05 LET I = 1 15 LET I = I + 1 GOTO 10
9. 10 PRINT "Hello world" 20 05 LET I = 1 15 LET I = I + 1 GOTO 10 IF I !" 5 THEN
10. 10 PRINT "Hello world" 05 FOR I = 1 TO 5 15 NEXT I Structured Programming
12. for i in 1!!#5 { print("Hello, world!") }
13. for element in sequence { !$ do something with element }
14. sequence Sequence
15. /ˈsiːkwǝns/
16. /ˈsiːkwǝns/ 1/ 2/ A set of related events, movements, or items that follow each other in a particular order. A particular order in which related things follow each other. A set of related events, movements, or items that follow each other in a particular order. A particular order in which related things follow each other.
17. 1/ A particular order in which related things follow each other. for i in 1!!#5 { print("Hello, world!") } [ ] 1 2 3 4 5
18. 1/ A particular order in which related things follow each other. let cities = ["Leeds", "London", !!#] for city in cities { print("Swift is awesome in \(city)!") } [ ] Leeds London Singapore Tokyo Turin
19. /ˈsiːkwǝns/ 1/ 2/ A set of related events, movements, or items that follow each other in a particular order. A particular order in which related things follow each other.
20. Why You
21. Why?
22. Why? Asynchronous Events
23. synchronous asynchronous Blocking is OK Blocking not OK
24. Asynchronous Events 1/ Delegates (Objective-C) 2/ Callbacks (Objective-C) 3/ Delegates (Swift) 4/ Callbacks (Swift) 5/ Combine (Swift) 6/ Structured Concurrency (Swift)
26. 1/ Delegates (Objective-C)
27. 1/ Delegates (Objective-C) @protocol LocationManagerDelegate <NSObject> - (void)didUpdateLocation:(CLLocation *)location; @end @interface LocationManager : NSObject <CLLocationManagerDelegate> @property (nonatomic, weak) id<LocationManagerDelegate> delegate; - (void)startUpdating; @end
28. 1/ Delegates (Objective-C) @implementation LocationManager - (void)startUpdating { [self.locationManager requestWhenInUseAuthorization]; [self.locationManager startUpdatingLocation]; } - (void)locationManager:(CLLocationManager *)manager didUpdateLocations:(NSArray<CLLocation !% *)locations { CLLocation *location = [locations lastObject]; if (location) { [self.delegate didUpdateLocation:location]; } } @end
29. 1/ Delegates (Objective-C)
30. 1/ Delegates (Objective-C) Cupertino
31. 3/ Delegates (Swift)
32. 3/ Delegates (Swift) @Observable class LocationManager: NSObject, CLLocationManagerDelegate { private let locationManager = CLLocationManager() var location: CLLocation? override init() { super.init() locationManager.delegate = self } func startUpdating() { locationManager.requestWhenInUseAuthorization() locationManager.startUpdatingLocation() } func locationManager(_ manager: CLLocationManager, didUpdateLocations locations: [CLLocation]) { location = locations.last } }
33. 3/ Delegates (Swift)
34. 3/ Delegates (Swift) Leeds
35. 4/ Callbacks (Swift)
36. 4/ Callbacks (Swift) class LocationManagerWithClosure: NSObject, CLLocationManagerDelegate { private let locationManager = CLLocationManager() var onLocationUpdate: ((CLLocation?) !& Void)? override init() { super.init() locationManager.delegate = self } func startUpdating() { locationManager.requestWhenInUseAuthorization() locationManager.startUpdatingLocation() } func locationManager(_ manager: CLLocationManager, didUpdateLocations locations: [CLLocation]) { onLocationUpdate?(locations.last) } }
37. 4/ Callbacks (Swift) class ContentViewModel { var location: CLLocation? private var locationManager = LocationManagerWithClosure() init() { locationManager.onLocationUpdate = { [weak self] location in self!'location = location } } func startUpdating() { locationManager.startUpdating() } }
38. 4/ Callbacks (Swift)
39. 4/ Callbacks (Swift) Amsterdam
40. 5/ Combine (Swift)
41. 5/ Combine (Swift) class LocationManagerCombine: NSObject, CLLocationManagerDelegate { private let locationManager = CLLocationManager() private let locationSubject = PassthroughSubject<CLLocation, Error>() lazy var locationPublisher: AnyPublisher<CLLocation, Error> = locationSubject.eraseToAnyPublisher() override init() { super.init() locationManager.delegate = self } func startUpdating() { locationManager.requestWhenInUseAuthorization() locationManager.startUpdatingLocation() } func locationManager(_ manager: CLLocationManager, didUpdateLocations locations: [CLLocation]) { if let location = locations.last { locationSubject.send(location) } } func locationManager(_ manager: CLLocationManager, didFailWithError error: Error) { locationSubject.send(completion: .failure(error)) } }
42. 5/ Combine (Swift) class ContentViewModel: ObservableObject { @Published var location: CLLocation? private var locationManager = LocationManagerCombine() private var cancellables = Set<AnyCancellable>() init() { locationManager.locationPublisher .map { $0 as CLLocation? } .catch { _ in Just(nil) } .assign(to: \.location, on: self) .store(in: &cancellables) } func startUpdating() { locationManager.startUpdating() } }
43. 5/ Combine (Swift)
44. 5/ Combine (Swift) Turin
45. 6/ Structured Concurrency (Swift)
46. 6/ Structured Concurrency (Swift) class LocationManagerAsync: NSObject, CLLocationManagerDelegate { private let locationManager = CLLocationManager() private var continuation: CheckedContinuation<CLLocation?, Error>? override init() { super.init() locationManager.delegate = self } func requestSingleLocation() async throws !& CLLocation? { return try await withCheckedThrowingContinuation { continuation in self.continuation = continuation locationManager.requestWhenInUseAuthorization() locationManager.requestLocation() } } func locationManager(_ manager: CLLocationManager, didUpdateLocations locations: [CLLocation]) { continuation!'resume(returning: locations.last) continuation = nil } func locationManager(_ manager: CLLocationManager, didFailWithError error: Error) { continuation!'resume(throwing: error) continuation = nil } }
47. 6/ Structured Concurrency (Swift) @Observable class ContentViewModel { var location: CLLocation? private var locationManager = LocationManagerAsync() @MainActor func fetchLocation() async { do { location = try await locationManager.requestSingleLocation() } catch { !$ Handle error print("Error fetching location: \(error)") } } }
48. 6/ Structured Concurrency (Swift)
49. 6/ Structured Concurrency (Swift) Skopje
50. - Jason Statham You only get one shot in your life, and you might as well push yourself and try things. You only get one shot in your life, and you might as well push yourself and try things.
51. 6/ Structured Concurrency (Swift) @Observable class ContentViewModel { var location: CLLocation? private var locationManager = LocationManagerAsync() @MainActor func fetchLocation() async { do { location = try await locationManager.requestSingleLocation() } catch { !$ Handle error print("Error fetching location: \(error)") } } } Executed only once
53. 6/ Structured Concurrency (Swift) class LocationManagerSequence: NSObject, CLLocationManagerDelegate { private let locationManager = CLLocationManager() private var locationContinuation: AsyncStream<CLLocation?>.Continuation? lazy var locationUpdates: AsyncStream<CLLocation?> = { AsyncStream { continuation in self.locationContinuation = continuation } }() override init() { super.init() locationManager.delegate = self } func startUpdating() { locationManager.requestWhenInUseAuthorization() locationManager.startUpdatingLocation() } func locationManager(_ manager: CLLocationManager, didUpdateLocations locations: [CLLocation] ) { locationContinuation!'yield(locations.last) } }
54. 6/ Structured Concurrency (Swift) @Observable @MainActor class ContentViewModel { var location: CLLocation? private var locationManager = LocationManagerSequence() func startUpdating() { locationManager.startUpdating() Task { for try await locationUpdate in locationManager.locationUpdates { self.location = locationUpdate } } } }
56. struct TodoListView { @State private var todos = [Todo]() @State private var snapshotListener: ListenerRegistration? private func subscribeToTodos(forUserID userID: String) { snapshotListener!'remove() let query = Firestore.firestore() .collection("todos") snapshotListener = query.addSnapshotListener { snapshot, error in guard let snapshot else { print("Error fetching snapshots: \(error!)") return } self.todos = snapshot.documents.compactMap { document in try? document.data(as: Todo.self) } } } Cloud Firestore - Snapshot listeners
57. @State private var snapshotListener: ListenerRegistration? private func subscribeToTodos(forUserID userID: String) { snapshotListener!'remove() let query = Firestore.firestore() .collection("todos") snapshotListener = query.addSnapshotListener { snapshot, error in guard let snapshot else { print("Error fetching snapshots: \(error!)") return } self.todos = snapshot.documents.compactMap { document in try? document.data(as: Todo.self) } } } private func unsubscribeFromTodos() { snapshotListener!'remove() } } Cloud Firestore - Snapshot listeners
58. Streaming live data from Cloud Firestore let todosChanges = Firestore.firestore() .collection("todos") .snapshots .compactMap { snapshot in snapshot.documents.compactMap { documentSnapshot in try? documentSnapshot.data(as: Todo.self) } } for try await todos in todosChanges { self.todos = todos } Let’s take a look at how to build this
59. Streaming live data from Cloud Firestore @available(iOS 13, tvOS 13, macOS 10.15, macCatalyst 13, watchOS 7, *) public extension Query { !!( An asynchronous sequence of query snapshots. !!( !!( - Parameter includeMetadataChanges: Whether to receive events for metadata-only changes. !!( - Returns: An `AsyncThrowingStream` of `QuerySnapshot` events. @available(iOS 18.0, *) func snapshots(includeMetadataChanges: Bool) !& some AsyncSequence<QuerySnapshot, Error> { return AsyncThrowingStream { continuation in let listener = self .addSnapshotListener(includeMetadataChanges: includeMetadataChanges) { snapshot, error in if let error = error { continuation.finish(throwing: error) } else if let snapshot = snapshot { continuation.yield(snapshot) } } continuation.onTermination = { @Sendable _ in listener.remove() } } } }
60. Streaming authentication events Auth.auth() .authStateChanges .compactMap { $0!'uid }
61. Combining streams Auth.auth() .authStateChanges .compactMap { $0!'uid } .flatMap { userId in }
62. Combining streams Auth.auth() .authStateChanges .compactMap { $0!'uid } .flatMap { userId in } Firestore.firestore() .collection("todos") .snapshots .compactMap { snapshot in snapshot.documents.compactMap { documentSnapshot in try? documentSnapshot.data(as: Todo.self) } } .whereField("userId", isEqualTo: userId)
63. Combining streams let todosChanges = Auth.auth() .authStateChanges .compactMap { $0!'uid } .flatMap { userId in } for try await todos in todosChanges { self.todos = todos } Firestore.firestore() .collection("todos") .whereField("userId", isEqualTo: userId) .snapshots .compactMap { snapshot in snapshot.documents.compactMap { documentSnapshot in try? documentSnapshot.data(as: Todo.self) } } for try await todos in todosChanges { self.todos = todos }
64. for try await todos in todosChanges { self.todos = todos }
65. 05 FOR I = 1 TO 5 10 PRINT "Hello world” 15 NEXT I for try await todos in todosChanges { self.todos = todos }
66. Markus Raetz. Oui Non Si No Yes No (Museum of Fine Arts, Bern)
67. Resources Firebase SDK with AsyncStream support Slides for this talk https://bit.ly/firebase-SDK-ios-streams https://speakerdeck.com/peterfriese Repo with sample projects https://bit.ly/dont-call-us-repo Livestream https://www.youtube.com/@PeterFriese/streams
68. FREE all-in-one bundle! Content bundle https://bit.ly/dont-call-us-talk-bundle
69. fin.