Refactor conditional navigation recipe to make it saveable.

[dturner]

Fixes #147

[gemini-code-assist]

Summary of Changes

Hello @dturner, I'm Gemini Code Assist¹! I'm currently reviewing this pull request and will post my feedback shortly. In the meantime, here's a summary to help you and other reviewers quickly get up to speed!

This pull request significantly refactors the conditional navigation recipe to ensure its state is fully saveable and restorable, addressing issue #147. By transitioning from a custom AppBackStack to a new Navigator class that leverages kotlinx.serialization for routes and rememberSaveable for state, the application's navigation flow, especially concerning login-protected routes, becomes robust against process death and configuration changes.

Highlights

• Serialization of Routes: Routes are now defined as a Serializable sealed class Route with a requiresLogin property, enabling them to be saved and restored across process death.
• New Navigator Class: A dedicated Navigator class has been introduced to encapsulate conditional navigation logic, replacing the previous custom AppBackStack implementation.
• Saveable State Management: The isLoggedIn state and the NavBackStack itself are now managed using rememberSaveable and rememberSerializable respectively, ensuring persistence across process death and configuration changes.
• rememberNavBackStack Utility: A new composable function rememberNavBackStack is provided to easily create a saveable NavBackStack for NavKey subtypes, leveraging NavBackStackSerializer and NavKeySerializer.

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[gemini-code-assist]

Code Review

This pull request refactors the conditional navigation logic to be saveable, which is a great improvement. The separation of concerns into a Navigator class is well done. However, there is a critical issue where a piece of navigation state (onLoginSuccessRoute) is not saved, which defeats the purpose of the refactor on process death. I've also identified a high-severity bug in the login logic and a medium-severity performance issue. My review provides detailed suggestions to address these points and make the implementation robust.

[gemini-code-assist]

The onLoginSuccessRoute property is not persisted across process death. This breaks the conditional navigation flow if the app is killed while on the login screen, defeating the purpose of this refactor.

To fix this, this state should be hoisted to ConditionalActivity and stored using rememberSaveable. The Navigator would then accept this state in its constructor.

In ConditionalActivity.kt:

val onLoginSuccessRoute = rememberSaveable { mutableStateOf<Route?>(null) }
val navigator = remember { // ...
    Navigator(
        // ...
        onLoginSuccessRouteState = onLoginSuccessRoute
    )
}

In Navigator.kt:

class Navigator(
    // ...
    onLoginSuccessRouteState: MutableState<Route?>,
) {
    // ...
    private var onLoginSuccessRoute by onLoginSuccessRouteState
    // ...
}

Since this change spans multiple files, I'm providing the explanation and examples here. Please apply these changes to make the navigation state fully saveable.

[gemini-code-assist]

The login() function has two issues:

1. If a user navigates to the login screen manually (so onLoginSuccessRoute is null), after a successful login, they remain on the login screen. The login screen should be popped from the back stack.
2. onLoginSuccessRoute is not cleared after being used. This could lead to unexpected redirection later if the user logs out and logs in again.

    fun login() {
        isLoggedIn = true
        onLoginSuccessRoute?.let { destination ->
            backStack.add(destination)
            backStack.remove(loginRoute)
            onLoginSuccessRoute = null
        } ?: run {
            // When login is successful, we should at least pop the login screen.
            if (backStack.lastOrNull() == loginRoute) {
                backStack.removeLast()
            }
        }
    }

[gemini-code-assist]

The Navigator instance is created on every recomposition. This is inefficient and can lead to subtle bugs. It should be wrapped in remember to ensure the same instance is used across recompositions.

            val navigator = remember(backStack, isLoggedInState) {
                Navigator(
                    backStack = backStack,
                    loginRoute = Login,
                    isLoggedInState = isLoggedInState
                )
            }

[claraf3]

Personally, I'm not really a fan of using "Route" in the context of nav3 because we don't use that term at all in nav3 library (kdocs or naming). I think sticking to "Key" helps clarify its role and where its used in the APIs.