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Navigation

MyTwin uses AndroidX Navigation Compose with string-based routes. All routes are defined as constants in the Routes object inside AppNavGraph.kt.

Route Constants

object Routes {
    const val WELCOME    = "welcome"
    const val ONBOARDING = "onboarding"
    const val HOME       = "home"
    const val CHAT       = "chat"
    const val FOOD_SCAN  = "food_scan"
    const val SIMULATION = "simulation"
}

Flow Diagram

flowchart TD
    Start([App Launch])
    Splash[SplashScreen API<br/>holds on teal bg]
    Root{Onboarding<br/>complete?}
    Welcome[WelcomeScreen]
    Onboarding[OnboardingScreen<br/>5-step wizard]
    Home[HomeScreen]
    Chat[ChatScreen]
    Sim[SimulationScreen]
    Food[FoodScanScreen]

    Start --> Splash
    Splash --> Root
    Root -->|No| Welcome
    Root -->|Yes| Home
    Welcome -->|Begin / Skip| Onboarding
    Onboarding -->|Submit| Home
    Home -->|Talk to Twin| Chat
    Home -->|View Simulation| Sim
    Home -->|Scan Food| Food
    Chat -->|Back| Home
    Sim -->|Back| Home
    Food -->|Back| Home

Startup Gate

MainActivity delays rendering AppNavGraph until RootViewModel emits a non-null startDestination:

val startDestination by vm.startDestination.collectAsStateWithLifecycle()

if (startDestination != null) {
    AppNavGraph(startDestination = startDestination!!)
}

RootViewModel reads OnboardingDataSource via onboardingRepository.isOnboardingComplete(). The splash screen is held visible by the SplashScreen API until this resolves, so users never see a blank frame.

Back Stack Behavior

Transition Back Stack Note
Welcome → Onboarding Welcome is popped (inclusive = true)
Onboarding → Home Onboarding is popped
Home → Chat / Sim / Food Pushed onto stack; back returns to Home

Welcome and Onboarding are removed from the back stack on completion so the user cannot navigate back to them with the system Back gesture.

Screens never hold a NavController reference. They receive navigation actions as lambda callbacks:

HomeScreen(
    onChatClicked    = { navController.navigate(Routes.CHAT) },
    onFoodScanClicked = { navController.navigate(Routes.FOOD_SCAN) },
    onSimulationClicked = { navController.navigate(Routes.SIMULATION) },
)

This keeps screens fully testable in isolation — you can call HomeScreen with no-op lambdas in a preview or test.