Android navigation component: how save fragment state

Update: Using last version of fragment navigation component, handle fragment states itself. see this sample

Old:

class BaseViewModel : ViewModel() {

    val bundleFromFragment = MutableLiveData<Bundle>()
}


class HomeViewModel : BaseViewModel () {

   ... HomeViewModel logic
}

inside home fragment (tab of bottom navigation)

private var viewModel: HomeViewModel by viewModels()

override fun onViewCreated(view: View, savedInstanceState: Bundle?) {
    super.onViewCreated(view, savedInstanceState)

    viewModel.bundleFromFragment.observe(viewLifecycleOwner, Observer {
      
        val message = it.getString("ARGUMENT_MESSAGE", "")
       binding.edtName.text = message
    })
}

override fun onDestroyView() {
    super.onDestroyView()
    viewModel.bundleFromFragment.value = bundleOf(
        "ARGUMENT_MESSAGE" to binding.edtName.text.toString(),
        "SCROLL_POSITION" to binding.scrollable.scrollY
    )
}

You can do this pattern for all fragments inside bottom navigation

just use navigation component version 2.4.0-alpha01 or above

In case you can deal with destroying fragment, but want to save ViewModel, you can scope it into the Navigation Graph:

private val viewModel: FavouritesViewModel by 
    navGraphViewModels(R.id.mobile_navigation) {
        viewModelFactory
    }

Read more here

EDIT

As @SpiralDev noted, using Hilt simplifies a bit:

private val viewModel: MainViewModel by 
    navGraphViewModels(R.id.mobile_navigation) {
         defaultViewModelProviderFactory     
    }

As per the open issue, Navigation does not directly support multiple back stacks - i.e., saving the state of stack B when you go back to B from A or C since Fragments do not support multiple back stacks.

As per this comment:

The NavigationAdvancedSample is now available at https://github.com/googlesamples/android-architecture-components/tree/master/NavigationAdvancedSample

This sample uses multiple NavHostFragments, one for each bottom navigation tab, to work around the current limitations of the Fragment API in supporting multiple back stacks.

We'll be proceeding with the Fragment API to support multiple back stacks and the Navigation API to plug into it once created, which will remove the need for anything like the NavigationExtensions.kt file. We'll continue to use this issue to track that work.

Therefore you can use the NavigationAdvancedSample approach in your app right now and star the issue so that you get updates for when the underlying issue is resolved and direct support is added to Navigation.