I'm running into a problem with the `Navigation Co...

# android

[Bradleycorn]

I'm running into a problem with the Navigation Component and a ViewModel scoped to a nav graph ... I'm removing a few places in my app where I was using onActivityCreated in some fragments, since that callback is deprecated. I've been able to move most everything related to view work into onViewCreated, but I have a problem there because I need to do a few things with a nav graph scoped ViewModel. The problem is that when the containing activity is recreated (due to a configuration change, or process death, etc) the onViewCreated gets called before the containing Activity's onCreate is finished (in fact, it gets called as soon as the activity's onCreate is called). As a result, the Nav Graph has not been setup yet, and so it can't properly create the ViewModel that is scoped to the graph, and the app crashes.

What's the path forward here? Per the deprecation notice in the docs for onActivityCreated, in the fragment's onAttach we should register a LifecycleObserver on the activity, observe for on_create and do work there. But that dooesn't work for me either. One of the things I need to do with the ViewModel is observe a LiveData, and the viewLifecycleOwner is not yet available (it's not available until the Fragment's onCreate) ...

the problem I'm running into is that after a configuration change, the Activity's onCreate and the Frgment's onViewCreated are executed in parallel, as illustrated by @ppvi in this article: https://medium.com/androiddevelopers/the-android-lifecycle-cheat-sheet-part-iii-fragments-afc87d4f37fd#4e7a . As outlined there, the benefit of onActivityCreated is that it is guaranteed to be called AFTER the Activity's onCreate is finished.

[Ian Lake]

What does the activity finishing onCreate() have to do with Nav Graph scoped ViewModels? Your NavController should absolutely already be created and available before any onViewCreated() calls when you're using Navigation 2.3.5

[Ian Lake]

And it sounds like you're still using the <fragment> tag. You shouldn't be using that - using FragmentContainerView will mean your onViewCreated() will happen after onCreate() finishes anyways

[Bradleycorn]

Thanks @Ian Lake. I am indeed using the <fragment> tag. I went to switch over to the FragmentContainerView a while back and ran into some difficulties (I don't remember what exactly) . I'll look into that again.

To answer your question, I'm setting the nav graph "manually" using navController.setGraph in my activity's onCreate ... So, the onViewCreated in the fragment gets called as soon as the super.onCreate is called in my Activity (so, before the call to setGraph), and thus, the graph hasn't been set yet.

[Ian Lake]

Yeah, you'll want to avoid the <fragment> tag then. There's a reason there's a lint check explicitly telling you not to use it for over a year

[Ian Lake]

The docs all have been updated to use FragmentContainerView (note how to gets the NavController - that's how you need to do it in `onCreate()`: https://developer.android.com/guide/navigation/navigation-getting-started#navigate

[Bradleycorn]

Yep. Not sure what I ran into last time that made me not update then. Might have just been, "I don't have the me for this now, I'll get a defect filed and deal with it later", and well... I guess it's "later" now

[Bradleycorn]

Ah ha! I remember now! When I went to switch to FragmentContainerView before, this issue was still in it's infancy: https://issuetracker.google.com/issues/142847973
There was lively debate about what to do, and I thought, "I'll let the dust settle and then update" ...

[Bradleycorn]

and sure enough, I updated and everything works like a champ

[Bradleycorn]

so in the end .. RTFM ....

[Colton Idle]

Happens to all of us! Glad it worked out!