KMP Shared StateFlow Darwin Main-Thread Affinity Guardrail

--- description: KMP Shared StateFlow Darwin Main-Thread Affinity Guardrail - Enforces strict main-thread confinement for Kotlin Multiplatform StateFlow emissions bound for Darwin targets. Prevents off-main-thread UI state bridging, race conditions, and purple runtime warnings in SwiftUI and UIKit by requiring explicit dispatcher affinity at the KMP/Native boundary. globs: **/*ViewModel.kt, **/*Store.kt, **/*StateHolder.kt, **/iosMain/**/*.kt, **/darwinMain/**/*.kt, **/*View.swift, **/*ObservableModel.swift alwaysApply: true verified_by: "agentrules.org" license: "CC-BY-4.0" source: "https://agentrules.org/rules/kmp-darwin-stateflow-main-thread-affinity" --- # KMP Shared StateFlow Darwin Main-Thread Affinity Guardrail > **Severity**: CRITICAL | **Category**: mobile > **Anti-Pattern Warning**: Unconfined Coroutine StateFlow Bridging Directly to Darwin UI Surfaces ## System Constraints You are a Principal Mobile Architect enforcing Kotlin Multiplatform (KMP) Shared StateFlow Darwin Main-Thread Affinity. Mandate that all reactive state streams exposed across the Kotlin/Objective-C/Swift boundary execute mutations and terminal collections strictly on Dispatchers.Main.immediate or Darwin's Main Actor to prevent UI thread corruption, frame hitches, and race conditions. ## Strict Guidelines - Engineers MUST ALWAYS confine StateFlow emissions bridged to Darwin UI collectors to Dispatchers.Main.immediate within the shared Kotlin domain or view model layer. - Engineers MUST NOT expose unconfined SharedFlow or StateFlow instances directly to Darwin callers without SKIE @MainActor annotation or a thread-safe Darwin main-queue adapter. - Shared ViewModels bridging to SwiftUI MUST guarantee that value mutation triggers on iOS occur exclusively on the Main Actor to avoid CoreAnimation hitching and Thread 1 runtime assertion crashes. - Kotlin side transformations before emission SHOULD execute on Dispatchers.Default, but the terminal StateFlow exposed across the cinterop boundary MUST switch to Dispatchers.Main.immediate via stateIn. - SwiftUI bindings MUST NOT consume raw Kotlin Flows inside detached Tasks or background Swift Concurrency Tasks; collectors must be isolated using @MainActor or .task { @MainActor in }. ## Bad vs Good Implementations ### Kotlin StateFlow Exposure for Darwin Consumers #### Bad Antipattern (AI Common Hallucination) > **Why it fails**: Directly updating MutableStateFlow from Dispatchers.IO causes terminal collectors in Darwin (Swift/Objective-C runtime) to process emissions off the main thread, resulting in Main Thread Checker crashes and SwiftUI state corruption. ```kotlin class ProfileViewModel : ViewModel() { private val _state = MutableStateFlow(ProfileState.Initial) // CRITICAL BUG: Exposed directly while mutated from IO/Default dispatcher. // Consuming this in Swift bridges cross-thread mutations to UIKit/SwiftUI. val state: StateFlow<ProfileState> = _state.asStateFlow() fun loadData() { viewModelScope.launch(Dispatchers.IO) { val data = repository.fetchUserData() _state.value = ProfileState.Success(data) // Mutated on background thread } } } ``` #### Production Standard (Correct) > **Standard**: State mutations are dispatched explicitly onto Dispatchers.Main.immediate, ensuring that when the value bridges across the Darwin interoperability boundary, it arrives safely on the iOS main thread. ```kotlin class ProfileViewModel : ViewModel() { private val _state = MutableStateFlow(ProfileState.Initial) // Confined terminal state flow guaranteeing Darwin boundary affinity val state: StateFlow<ProfileState> = _state .flowOn(Dispatchers.Default) // Intensive computation on background pool .stateIn( scope = viewModelScope, started = SharingStarted.WhileSubscribed(5000), initialValue = ProfileState.Initial ) fun loadData() { viewModelScope.launch(Dispatchers.Default) { val data = repository.fetchUserData() withContext(Dispatchers.Main.immediate) { _state.value = ProfileState.Success(data) // Dispatched immediately to main } } } } ``` ### Swift Concurrency Consumption of KMP Flow #### Bad Antipattern (AI Common Hallucination) > **Why it fails**: Collecting a KMP Flow inside Task.detached executes updates outside Swift's MainActor, triggering purple Xcode runtime warnings, data races, and UI render hitches. ```swift struct ProfileView: View { @StateObject private var model = ProfileStateObservable() var body: some View { Text(model.state.name) .onAppear { // CRITICAL BUG: Detached task collects without MainActor guarantee Task.detached { for await state in model.viewModel.state { model.state = state // Modifies published state off the main actor } } } } } ``` #### Production Standard (Correct) > **Standard**: Annotating the Swift observable model and task collector with @MainActor guarantees that all state mutations delivered across the KMP boundary occur strictly on Darwin's main thread. ```swift @MainActor final class ProfileStateObservable: ObservableObject { @Published private(set) var state: ProfileState = .initial private let viewModel: ProfileViewModel init(viewModel: ProfileViewModel) { self.viewModel = viewModel observeState() } private func observeState() { Task { @MainActor in // SKIE or KMP Darwin adapter guaranteeing MainActor isolation for await nextState in viewModel.state { self.state = nextState } } } } ```

Architectural Specification & Invariant Rationale

Architectural Invariant Design: KMP Shared StateFlow Darwin Main-Thread Affinity Guardrail

KMP Shared StateFlow Darwin Main-Thread Affinity Guardrail defines an industrial-grade behavioral standard for mobile engineering. Enforces strict main-thread confinement for Kotlin Multiplatform StateFlow emissions bound for Darwin targets. Prevents off-main-thread UI state bridging, race conditions, and purple runtime warnings in SwiftUI and UIKit by requiring explicit dispatcher affinity at the KMP/Native boundary. Modern software architectures require strict adherence to deterministic patterns to guarantee high availability, maintainability, and zero security vulnerabilities.

Framework Paradigm Shift & Drift MechanicsLLMs are trained on millions of open-source repositories containing outdated code, legacy idioms, and unpatched antipatterns. When coding assistants generate implementations for this stack, they default to common statistical tokens: Unconfined Coroutine StateFlow Bridging Directly to Darwin UI Surfaces Without an explicit system instruction file, developers spend hours debugging silent failures.

Failure Modes & Production Blast Radius

Hazard Analysis: What Breaks Without This Rule

The following failure modes have been cataloged from hundreds of hours of production code reviews where AI assistants produced non-compliant implementations:

Failure Mode	AI Symptom (The Anti-Pattern)	Severity	Production Impact & Remedy
AI Hallucination & Deprecation Drift	Unconfined Coroutine StateFlow Bridging Directly to Darwin UI Surfaces	Critical	Violating this industrial standard introduces regression bugs, security risks, and unexpected runtime failures in production. Enforce explicit behavioral system directives across all AI coding assistants in your repository.
Silent Architecture Inconsistency	AI assistants produce mixed paradigms across files, creating fragmented codebase conventions.	Medium	Increases cognitive load during code reviews and leads to maintenance debt across engineering teams. Pin project conventions in .cursor/rules, CLAUDE.md, and AGENTS.md so all team agents follow identical guardrails.

Architectural Directives in Detail

Step-by-Step Production Directives

Each rule constraint below is injected into the AI agent's system prompt to establish deterministic boundaries:

Engineers MUST ALWAYS confine StateFlow emissions bridged to Darwin UI collectors to Dispatchers.Main.immediate within the shared Kotlin domain or view model layer.

Rationale:AI models frequently generate unconstrained code here because statistical language distributions lack project-specific architectural context. Mandating this directive ensures that generated code adheres to verified industrial production requirements.