Replace Combine with @Observable and AsyncStream

[g-enius]

Summary

Replaces all Combine usage with two independent iOS 17 technologies — @Observable (Observation framework) for ViewModel → View reactivity, and AsyncStream + custom StreamBroadcaster for service event streams — achieving zero import Combine across the entire codebase.

• 63 files changed — 773 lines added, 637 removed
• Minimum deployment target: iOS 16 → iOS 17
• import Combine remaining: 0

Depends on: PR #3 — Migrate UIKit navigation to SwiftUI NavigationStack

Why iOS 17?

AsyncSequence/AsyncStream are iOS 13 APIs. The iOS 17 requirement comes from:

Technology	Replaces	Why iOS 17
@Observable	ObservableObject + @Published	Observation framework requires iOS 17
AsyncStream.makeStream(of:) (SE-0388)	Closure-based AsyncStream { continuation in }	Eager continuation used in StreamBroadcaster

Neither depends on the other; they coincidentally share the same deployment target.

Key Changes

Reactive State

Combine	AsyncStream
AnyPublisher<Set<String>, Never>	AsyncStream<Set<String>>
CurrentValueSubject / PassthroughSubject	StreamBroadcaster (custom, in Core)
.sink { }.store(in: &cancellables)	Task { for await value in stream { } }
Set<AnyCancellable> cleanup	task.cancel()
.debounce(for:scheduler:)	didSet + Task.sleep with cancellation

ViewModel Observation

Combine	@observable
ObservableObject + @Published	@Observable macro
@ObservedObject	@Bindable (two-way) or plain property
@StateObject	@State
Per-object invalidation	Per-property tracking

StreamBroadcaster

Custom @MainActor-isolated multi-consumer broadcaster replacing Combine subjects. Each consumer gets an independent AsyncStream — cleanup is automatic on Task cancellation.

// Service (producer)
private let broadcaster = StreamBroadcaster<Set<String>>()
var favoritesStream: AsyncStream<Set<String>> { broadcaster.makeStream() }
func update() { broadcaster.yield(newValue) }

// ViewModel (consumer) — guard INSIDE loop to avoid retain cycle
let stream = favoritesService.favoritesStream
observation = Task { [weak self] in
    for await value in stream {
        guard let self else { break }
        self.favoriteIds = value
    }
}

What Stays the Same

• SwiftUI NavigationStack (from PR Migrate UIKit navigation to SwiftUI NavigationStack #3)
• MVVM + Coordinator architecture
• Session-scoped DI, all service protocols
• Navigation closures pattern

Migration Pitfalls

1. guard let self before for await = retain cycle. The strong capture persists across the entire loop suspension.

2. AsyncStream doesn't emit current value on subscribe. Unlike @Published, must read current state at init time, then observe future changes via stream.

3. @Observable conflicts with property wrappers. Service properties need @ObservationIgnored @Service(.network).

4. @StateObject requires ObservableObject. Use @State for ownership with @Observable classes.

Test plan

• All 74 unit tests pass
• Build and run on iOS 17+ simulator
• Zero import Combine statements
• Home → Detail → back navigation
• Profile modal present/dismiss
• Favorites sync across screens
• Search debounce works
• Feature toggle changes propagate
• Pull-to-refresh works
• Deep links work

🤖 Generated with Claude Code

[g-enius]

The new PR finally LGTM. Now create tags for each branch! @claude

[claude]

Claude finished @g-enius's task in 2m 40s —— View job

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Creating Git Tags for Each Branch

Following the existing vX.Y.Z[-branch-name] tag pattern. All 3 branches had new commits since their v4.1.0 tags, so tagged as v4.2.0.

• Gather context: existing tags use vX.Y.Z[-branch-name] pattern; all 3 branches ahead of their last tags
• Create v4.2.0 for main → ebfc4fe (rename async-sequence refs, fix DefaultFeatureToggleService)
• Create v4.2.0-navigation-stack for feature/navigation-stack → 3a7a545 (fix stale docs, Duration API, routing table)
• Create v4.2.0-observation for feature/observation → 0257d83 (replace Combine with @observable + AsyncStream — LGTM)
• Push all tags to remote

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All 3 tags are now live on the remote. You can view them at:

• v4.2.0
• v4.2.0-navigation-stack
• v4.2.0-observation