Type Safe Nav Graph Organization

Organizing a type-safe navigation graph

Type-safe routes replace string paths with serializable objects. Each one is its own file, grouped into a package per area of the app, and holds nothing but its arguments:

@Serializable
data object HomeDestination

@Serializable
data class AlbumDestination(val browseId: String)

The host declares the transitions once and then does almost nothing else. What it registers directly is not "whatever was left over" — it is the five bottom-bar tabs plus the fullscreen player, which its own comment says out loud, and four of those six sit in packages that do have an area graph. The rule is how a destination is entered — from the app chrome, or from another screen:

NavHost(
    navController,
    startDestination = startDestination,
    enterTransition = { fadeIn() + slideInHorizontally { -it } },
    exitTransition = { fadeOut() + slideOutHorizontally { it } },
    popEnterTransition = { fadeIn() + slideInHorizontally { -it } },
    popExitTransition = { fadeOut() + slideOutHorizontally { it } },
) {
    // Bottom bar destinations
    composable<HomeDestination> { HomeScreen(navController = navController, /* … */) }
    // … the other four tabs, then the fullscreen player
    listScreenGraph(innerPadding = innerPadding, navController = navController)
    loginScreenGraph(innerPadding, navController, hideBottomBar = hideNavBar, showBottomBar = { showNavBar(false) })
    // … one call per remaining area
}

An area graph is an ordinary extension function on the builder, reading arguments off the entry:

fun NavGraphBuilder.listScreenGraph(innerPadding: PaddingValues, navController: NavController) {
    composable<AlbumDestination> { entry ->
        val data = entry.toRoute<AlbumDestination>()
        ForceDarkContent { AlbumScreen(browseId = data.browseId, navController = navController) }
    }
    // …
}

Traps

These are not nested navigation graphs, and the difference matters. The extension functions are a file-level grouping: each calls composable<…> directly, so every destination lands flat in the one host graph. No navigation<…> call exists anywhere, and therefore no per-area start destination, no nested back stack, no scoping of a view model to an area. Want any of those and you want a real nested graph; want none of them and this buys readability with no extra semantics to reason about.

Cross-cutting values arrive as function parameters, not through navigation. Window padding, the controller and the callbacks that hide or show the bottom bar are plain arguments on the area function, threaded from the host — which is why the login area's signature is the longest, being the only one needing the bar callbacks. Resist putting these on the route: a route is serialized into the back stack and restored across process death, so anything in it must still mean something an hour later. A lambda does not.

Theming is a property of the route, applied by wrapping the screen inside its own entry. Screens that sit over dark artwork are wrapped where they are registered:

composable<PlaylistDestination> { entry ->
    val data = entry.toRoute<PlaylistDestination>()
    ForceDarkContent { PlaylistScreen(playlistId = data.playlistId, /* … */) }
}

Wrapping the whole host instead forces the theme on every destination; putting the wrapper inside the screen makes the screen un-reusable anywhere that does not want it. Note which destinations get it and which do not — here the immersive list screens and the fullscreen player do, the plain list screen beside them does not — because that asymmetry is invisible from inside the screens.

One transition set, on the host. No destination overrides it: composable<X> { … } everywhere, never the overload taking its own enterTransition. Once one destination takes a private transition the next reviewer cannot tell whether the rest were deliberate or forgotten, so say why at any override.

Argument types stay serializable primitives; conversion is documented at the route. A route carrying what is conceptually an enum stores its string form and names the converter in KDoc, rather than registering a custom NavType:

/**
 * @param type Using LibraryDynamicPlaylistType.toStringParams
 */
@Serializable
data class LibraryDynamicPlaylistDestination(val type: String)

Constants that belong to a route live in that route's own companion object, next to the parameter they constrain, rather than in a shared constants file every screen imports.

A route may deliberately carry nothing, and that has to be written down. One login route is an argument-less object whose KDoc explains that the callback token reaches a view model instead — routing it would push a second copy of the login screen over the one the user started from. Without that note the empty route reads as an oversight and the next person adds the argument back.

Deep links are not on the destinations here. No composable<…>(deepLinks = …) exists; incoming links are parsed outside the graph and dispatched as an app-level intent. A fork in the road, not a default — deciding it explicitly is what stops half the links living on routes and half in a view model.

A route with no entry compiles. Nothing links a route to the graph until a composable<…> block names it, so a destination file plus a navigation call builds clean and fails at the tap — step 2.

Verifying it

# 1. Any route file that is not serializable — no output is the expected result.
find . -path "*/navigation/destination/*" -name "*.kt" -not -path "*/build/*" -exec grep -L "@Serializable" {} +

# 2. Declared routes that no graph registers — no output is the expected result. Count the left set
#    first (`| wc -l`, 22 here) and check it against the file count: a route declared as a plain
#    `object` rather than `data object` is legal and exists here exactly once, and a pattern that
#    matched only `data class|data object` dropped it — reaching "no output" by not looking.
comm -23 \
  <(find . -path "*/navigation/destination/*" -name "*.kt" -not -path "*/build/*" -exec sed -n 's/^\(data \)\?\(class\|object\) \([A-Za-z0-9_]*\).*/\3/p' {} + | sort -u) \
  <(grep -rho --include="*.kt" "composable<[A-Za-z0-9_]*>" . | sed 's/composable<\(.*\)>/\1/' | sort -u)

# 3. The area graphs, and the hosts. One host, and one extension function per area.
grep -rn --include="*.kt" "fun NavGraphBuilder\." . | grep -v "/build/"
grep -rn --include="*.kt" "NavHost(" . | grep -v "/build/"

# 4. Destinations that took a private transition — no output means the host's set is the only one.
grep -rn --include="*.kt" -E "composable<[A-Za-z0-9_]*> *\(" . | grep -v "/build/"

Then navigate deep, background the app, let the process be killed, and return: the back stack is rebuilt from the serialized routes, so an argument that was not really a value surfaces here and nowhere earlier. Watch the transitions in and out of one destination per area — a route that animates differently from its neighbours is an override step 4 missed.