The Duet Framework: One Shared Core, Two Native Apps

class MainActivity : ComponentActivity() {
  private lateinit var retained: RetainedRoot<RootMount>

  override fun onCreate(savedInstanceState: Bundle?) {
    super.onCreate(savedInstanceState)
    // Null on a fresh start and after a rotation; the saved spine after process death.
    val restored = decodeRouteSpine(savedInstanceState?.getString(SPINE_KEY))
    // A rotation returns the same instance, with every store still running.
    retained = instanceKeeper().getOrCreate {
      RetainedRoot(Dispatchers.Main.immediate, RootMount::teardown) { scope ->
        RootBuilder(dependency).buildRoot(scope, restored)
      }
    }
    setContent { AppRoot(retained.component) }
  }

  override fun onSaveInstanceState(outState: Bundle) {
    super.onSaveInstanceState(outState)
    outState.putString(SPINE_KEY, encodeRouteSpine(retained.component.routeSpine()))
  }
}

func scene(_ scene: UIScene, willConnectTo session: UISceneSession, options: UIScene.ConnectionOptions) {
  let saved = session.stateRestorationActivity?.userInfo?[spineKey] as? String
  let root = RootBuilder(dependency: SceneComponent()).buildRoot(restored: decodeRouteSpine(text: saved))
  window.rootViewController = UIHostingController(rootView: RootView(shell: root.shell))
  window.makeKeyAndVisible()
  root.shell.activate() // mounts the first child once the window is visible
  self.root = root
}

func stateRestorationActivity(for scene: UIScene) -> NSUserActivity? {
  let activity = NSUserActivity(activityType: spineActivityType)
  // The same Kotlin encoder the Android app calls.
  activity.addUserInfoEntries(from: [spineKey: encodeRouteSpine(spine: root.shell.routeSpine())])
  return activity
}

func sceneDidDisconnect(_ scene: UIScene) {
  root?.shell.deactivate() // dropping the reference alone cancels nothing on the Kotlin side
  root = nil
}

SplashAction.Appeared ->
  if (state.isArmed) Reduced(state) // a repeated Appeared writes nothing and emits nothing
  else Reduced(state.copy(isArmed = true), listOf(Effect.Run(ArmSafetyNet(SAFETY_NET_MILLIS))))

data class RootState(
  val phase: RootPhase = RootPhase.Splash,         // which child is mounted
  val entitlement: Entitlement = Entitlement.Free, // the root's own data
)                                                  // no SignInState, no MainState

sealed interface RootAction {
  data class SignIn(val event: SignInDelegateEvent) : RootAction // the child's outward events only
  data class Main(val event: MainDelegateEvent) : RootAction
  data class EntitlementChanged(val entitlement: Entitlement) : RootAction
}

// In the child: the event is returned as effect data, so the child's recording contains it.
is SignInOutcome.SignedIn ->
  Reduced(state.copy(isSigningIn = false), listOf(Effect.Run(NotifyHost(Completed(displayName)))))

val slot = host.adopt(ChildSlot<RootPhase, RootChildMount>(
  build = { phase ->
    when (phase) {
      RootPhase.SignIn -> RootChildMount.SignIn(
        SignInBuilder(component).buildSignIn(onDelegate = { rootStore.send(RootAction.SignIn(it)) }))
      RootPhase.Main -> RootChildMount.Main(
        MainBuilder(component).buildMain(displayName = rootStore.state.value.auth.displayNameOrGuest, entitlement = mount.entitlement))
      // …
    }
  },
  teardown = { it.teardown() },
))
host.adopt(StateTransitions(scope, rootStore.state) { _, state ->
  mount.project(state.entitlement) // publish the one value the tabs read
  slot.reconcile(state.phase)      // build the named child, tear down the previous one
  mount.publish(slot.activeHandle) // the child the composables render
})

// MainBuilder: the value reaches the tab as the tab's own action.
host.adopt(StateTransitions(scope, entitlement) { _, value ->
  homeStore.send(HomeAction.EntitlementChanged(value))
})

// AppRoot: collects the mounted child and renders its screen over the child's own store.
@Composable
fun AppRoot(root: RootMount) {
  when (val child = root.child.collectAsState().value) {
    is RootChildMount.SignIn -> SignInScreen(child.store)
    is RootChildMount.Main -> MainScreen(child.mount)
    // …
  }
}