not necessarily kotlin specific, but let's give it a try: I'm using Jetpack navigation to navigate between two fragments. One is a list of things, the other one shows a specific thing. On the list I'm getting an

Observable<[Thing]>

, and I'd really like to pass a

Observable<Thing>

to the second fragment. However, I cannot figure out how. I have the list observable in the first fragment's view model and my intuition is that when making the transition, there should be a way to prepare the view model for the second fragment and then it can find it via

ViewModelProvider

. Can't figure out how to do it, though. Maybe there's a totally different way. Any clues?

Your "intuition" is correct you can share a view model . If using navigation you can share viewmodel for a part of your graph out of the box. https://stackoverflow.com/questions/55137338/android-navigation-component-with-shared-view-models The key is this should be the same instance of the view model. You can always pass your data as arguments too

thanks. is it possible, though, to be a bit more fine grained? Fragment A has View Model A, it knows it opens Fragment B and has the data Fragment B needs. I'd rather have View Model B (so fragment B is reusable), but I'd like fragment A to prepare it before it opens Fragment B. As far as I can tell it's possible if I put fragment B in a separate Nav Graph, but not sure if I can do it when they are in the same graph

One fragment can have multiple view models. Fragment B can have two instances view model A and view model B but of you want to avoid references to viewmodel A from fragment B whatsoever then you have to pass the data to fragment B to be used 🤷

sure, but the data is an Observable here and I cannot pass it through args (since it's not serializable)

You can pass the data itself what you need to display (not the observable instance)

I kinda want to pass the observable as the new fragment can make a modification and I'd like to get the new value through the observable, not specifically handle it in the fragment

Then you have to provide access to that observable from fragment a and fragment b, so that observable can be in a shared viewmodel (I think this is a standard way now) or it can be global/static in a companion object for example (which we had to do once as there was a complicated set up) but the latter doesn't seem clean to me and smells a bit

yeah, so ideally, instead of sharing a view model, I'd like to setup a view model in fragment A to be used in fragment B. That way fragment B doesn't know about the view model of fragment A and can be invoked through multiple paths

I see what you mean, you can do it in reverse , give whichever fragment is invoking fragment b access to view model b to set the data before navigating to fragment b

sure. I tried that, but then

onCleared

doesn't get called

For that I would create viewmodel b as a Singleton if you are using dagger for example it is pretty easy

not using dagger. Unsure I want to use dagger

Yeah, I have faced a similar issue but the solution was to have a custom method in view model purgeData or something that will be called on Fragment b onViewDesroyed for example to reset

got it

Yeap, tie it to fragment b lifecycle and clear data in viewmodel

I was trying to set up a viewmodelstore in some way before the transition, but failed

I am not sure it is possible

technically if I put fragment B in a new graph, I can obtain it after I call

navigate

and tweak it

Why do you think it is necessary for it to be in a new graph?

yeah, but then

onCleared

doesn't get called since the view model store is shared for the whole graph. If I put it in a new graph, the new graph can have its own view model store, and it gets cleared automatically when it gets popped

Ok, I see what you mean , that situation smells a bit , I would report a bug and see what Google replies

thanks 🙂

What a long discussion and no mention of scoping

ViewModel

to a shared

NavGraph

, why is that?