I’m using the navigation component with compose and I want to reuse a class instance while the current screen/ navEntry is in the backstack. Basically I want to have viewModels that are not android viewModels and that are retained, which is not supported. I have achieved this through DI scopes (with Koin). To cancel the scope I use this code:

navBackStackEntry.lifecycle.addObserver(object: LifecycleEventObserver {
    override fun onStateChanged(source: LifecycleOwner, event: Lifecycle.Event) {
        if (event == Lifecycle.Event.ON_DESTROY) {
            koinScope.close()
        }
    }

})

The problem is that this is run on every recomposition, so I end up having a lot of observers. It works (the scope will be closed when it needs to be) but it is not ideal having several useless observers being called. What can I do to only add an observer the first time?

Sounds like you're looking for

onCommit(navBackStackEntry)

which will only run when the inputs (i.e., the

NavBackStackEntry

instance changes (which it doesn't, so this only runs once)

Thanks for you answer @Ian Lake. It did not work though. Despite getting the same NavBackStackEntry, the onCommit block is called every time (maybe something is different and equals returns false?). To clarify what I do: I start at screen A, navigate to B, navigate to C, and press back. I have the following inside NavHost:

composable(ScreenA) {…}

composable(ScreenB) {

onCommit(it) {…}

…

}

composable(ScreenC) {…}

With this navigation flow, the onCommit block is called twice.

If you log the

NavBackStackEntry

, are you getting a different instance? Or is it the same instance and there's something wrong with

onCommit

?

It is the same instance, I logged it and that’s what surprised me. Does onCommit compare contents (==) or instances (===)?

It doesn't matter for

NavBackStackEntry

since it intentionally doesn't override equals (object equality is enough), but what you're seeing is certainly unexpected - any call to

navigate

will create a new instance. Do you mind filing a bug with a reproduction project? https://issuetracker.google.com/issues/new?component=409828&template=1093757

Thanks, I've routed it over to the right person. We'll take a look