I'm using `navigation` on a `compose desktop` app, and I have a main screen, which is the starting o...

# compose

[Pablo]

I'm using navigation on a compose desktop app, and I have a main screen, which is the starting one, and it haves some loading data on the init block of his viewmodel. The problem is that when I navigate to another screen and then go to the main screen again, the init block of the main screen is again called, doing the loading again. How can this be avoided?

NavHost(
    navController = appStateHolder.navController,
    startDestination = ScreenRoute.MainScreen.name,
    modifier = modifier
) {
    composable(ScreenRoute.MainScreen.name) {
        MainScreen(modifier = Modifier.padding(16.dp))
    }
    composable(ScreenRoute.StopsScreen.name) {
        StopsScreen(modifier = Modifier.padding(16.dp))
    }
    composable(ScreenRoute.LinesScreen.name) {
        LinesScreen(modifier = Modifier.padding(16.dp))
    }
}

To clarify, I'm navigating back to the main screen by calling navController.navigate(ScreenRoute.MainScreen.name) and not by pressing back, because back key is not present in compose desktop.

[Stylianos Gakis]

Call navController.popBackStack() instead of doing the navigation call which adds many copies of your main screen to your backstack

[Pablo]

I don't see that as a solution. Imagine the user do a more complex route like this:
Main Screen -> Screen 2 -> Screen 3 -> Screen 4 -> Screen 2

[Pablo]

how can I know that I must do a popbackstack to go from Screen 4 to Screen 2?

[Pablo]

is not supposed that the navigation api must know it and do that pop back internally and automatically without letting the developer to manually know if it is on the stack and pop it manually?

[Pablo]

like activities do in android

[Pablo]

Imagine that my app haves 100 screens, I can't see as a solution to manually check on every navigation if the screen is in the stack and do a different call if so

[Pablo]

that should be done automatically by navigation

[Pablo]

doesn't exist a way to achieve it? to tell navController that when he receives a navigate call, if a screen is already on the stack, it must pop it instead of creating a new one?

[Stylianos Gakis]

navController.popBackStack<ScreenRoute.MainScreen>(inclusive = false)

And your start destination must always be on your backstack, so this will always work.

[Stylianos Gakis]

NavOptions also give you the option to navigate to some destination while making sure it stays unique in your backstack and there are not many of them by using singleTop https://cs.android.com/androidx/platform/frameworks/support/+/androidx-main:navigati[…]igation/NavOptionsBuilder.android.kt;l=45?q=NavOptionsBuilder

[Stylianos Gakis]

Also while navigating, you can make sure to before doing the navigation call to pop the backstack up to some point, like

navController.navigate(SomeScreen()) {
  popUpTo<SomeOtherScreen> {  
    inclusive = true
  }
}

[Stylianos Gakis]

You got control over all this, it's your choice what does or does not happen

[Pablo]

ok, seems that that function is exactly what I'm searching for, so I did this:

navController.navigate(route) { popUpTo(route) { inclusive = true } }

but it's not working, the init block of the viewmodel is still being called when going back to main screen using that function

[Pablo]

I did it as explained here: https://developer.android.com/guide/navigation/backstack/circular

[Pablo]

but I can ensure is not working, or at least, if it's working, the init block of the viewmodel is still being called again when going back to main screen

[Stylianos Gakis]

How are you instantiating your ViewModel?

[Pablo]

vm: MainScreenViewModel = koinViewModel()

[Pablo]

I think I found the solution

[Pablo]

this works:

if (!navController.popBackStack(route, inclusive = false)) {
    navController.navigate(route)
}

[Pablo]

notice the inclusive = false

[Pablo]

if you put inclusive = true, it stops working, because it removes the destination from the backstack, and doing so, means that when you come back to it, it will be recreated from scratch recalling the init block of the viewmodel

[Pablo]

if you use navController.navigate(route) { popUpTo(route) { inclusive = false } } it doesnt work, doesn't matter if you put true or false on inclsuive, didn't work in either case

[Pablo]

I think that popUpTo removes the destination from the backstack even with inclusive = false

[Pablo]

maybe it's a bug

[Pablo]

what do you think?

[Stylianos Gakis]

If you navigate to ScreenA, while also doing popUpTo<ScreenA> { inclusive = false }, then you simply will have two instances of ScreenA in your backstack. Not a bug.
That's why you see two ViewModels too.

Print out your backstack information at that point to see what is happening.

[Pablo]

why the behaviour is the same with popUpTo(route) { inclusive = false } and with popUpTo(route) { inclusive = true }?

[Pablo]

in both cases, the init block of the viewmodel is called

[Pablo]

but with this, not, with this works correctly:

if (!navController.popBackStack(route, inclusive = false)) {
    navController.navigate(route)
}

[Pablo]

if it's a feature to have the same behaviour with popUpTo(route) { inclusive = false } and with popUpTo(route) { inclusive = true }, I don't understand it

[Stylianos Gakis]

Did you print out your backstack to check if you have two instances of the same destination, and therefore two instances of your ViewModel?

[Stylianos Gakis]

It's not the same behavior whatsoever, one pops the route it finds one does not.

[Pablo]

I'm checking this, Stylianos, and the backstack is increasing everytime even with this code:

if (!navController.popBackStack(route, inclusive = false)) {
    navController.navigate(route)
}

and also I noticed that it doesn't recall main screen viewmodel init block if I come back from other screen... but if I press "main screen" button when staying in main screen, then, it recalls the init block

[Pablo]

recap:

these three doesn't work, recalls main screen viewmodel init block when coming back to main screen from other:
navController.navigate(route) { popUpTo(route) { inclusive = true } }
navController.navigate(route) { popUpTo(route) { inclusive = false} }
if (!navController.popBackStack(route, inclusive = true)) { navController.navigate(route)}
This one doesn't call the init block in that case, but it does when I'm on main and press go to main button and also, increases constantly backstack
if (!navController.popBackStack(route, inclusive = false)) { navController.navigate(route)}

[Pablo]

do you know a complete solution for this issue?