Multi-Subscriber Async Sequence Example

ContentView.swift

import SwiftUI

actor AccountManager {
    private var currentUsernameTriggers = Dictionary<UUID, AsyncStream<String?>.Continuation>()
    private(set) var currentUsername: String? {
        didSet {
            for (key, cont) in currentUsernameTriggers {
                if case .terminated = cont.yield(currentUsername) {
                    print("Continuation with key \(key) was terminated...")
                    removeObserver(forKey: key)
                }
            }
        }
    }

    var currentUsernameChanges: AsyncStream<String?> {
        .init {
            let key = UUID()
            currentUsernameTriggers[key] = $0
            // If the value rarely changes, but there is a lot of fluctuation in the number of subscribers,
            // adding this `onTermination` will keep the list of continuations smaller.
            $0.onTermination = { [weak self] _ in
                print("Stream with key \(key) was terminated...")
                guard let self else { return }
                Task {
                    await self.removeObserver(forKey: key)
                }
            }
        }
    }

    private func removeObserver(forKey key: UUID) {
        currentUsernameTriggers.removeValue(forKey: key)
    }

    private static nonisolated let usernameChars = "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789";
    func changeUsername() {
        currentUsername = String(Self.usernameChars.shuffled().prefix(10))
    }
}

struct ObservingView: View {
    let accountManager: AccountManager

    @State
    private var currentUsername: String?

    var body: some View {
        (currentUsername.map(Text.init) ?? Text("No User"))
            .contentTransition(.interpolate)
            .animation(.default, value: currentUsername)
            .task {
                currentUsername = await accountManager.currentUsername
                for await username in await accountManager.currentUsernameChanges {
                    currentUsername = username
                }
            }
    }
}

struct ContentView: View {
    // Just a little helper to use in SwiftUI's `ForEach` below.
    // Basically a `UUID` conforming to `Identifiable`.
    private struct ObserverViewID: Sendable, Hashable, Identifiable {
        let id = UUID()
    }

    private let accountManager = AccountManager()

    @State
    private var observerViewIDs = [ObserverViewID(), ObserverViewID()]

    var body: some View {
        VStack(spacing: 25) {
            HStack {
                Button("Change Username") { Task { await accountManager.changeUsername() } }
                Button("Add Observer") { observerViewIDs.append(.init()) }
            }
            .buttonStyle(.bordered)
            ScrollView {
                LazyVStack(spacing: 15) {
                    ForEach(observerViewIDs) { id in
                        HStack {
                            ObservingView(accountManager: accountManager)
                            Spacer()
                            Button(role: .destructive,
                                   action: { observerViewIDs.removeAll(where:  { $0 == id }) }) {
                                Image(systemName: "trash")
                            }
                        }
                    }
                }
            }
        }
        .padding()
    }
}