Android navigation component: how save fragment state

Question

I use bottomNavigationView and navigation component. Please tell me how I don't destroy the fragment after switching to another tab and return to the old one? For example I have three tabs - A, B, C. My start tab is A. After I navigate to B, then return A. When I return to tab A, I do not want it to be re-created. How do it? Thanks

Answer 1

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.

Answer 2

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     
    }

Answer 3

just use navigation component version 2.4.0-alpha01 or above

Answer 4

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

Answer 5

Update 2021 use version 2.4.0-alpha05 or above. don't use this answer or other etc.

Answer 6

This can be achieved using Fragment show/hide logic.

private val bottomFragmentMap = hashMapOf<Int, Fragment>()
bottomFragmentMap[0] = FragmentA.newInstance()
bottomFragmentMap[1] = FragmentB.newInstance()
bottomFragmentMap[2] = FragmentC.newInstance()
bottomFragmentMap[3] = FragmentD.newInstance()


private fun loadFragment(fragmentIndex: Int) {
    val fragmentTransaction = childFragmentManager.beginTransaction()

    val bottomFragment = bottomFragmentMap[fragmentIndex]!!

    // first time case. Add to container
    if (!bottomFragment.isAdded) {
        fragmentTransaction.add(R.id.container, bottomFragment)
    }

    // hide remaining fragments
    for ((key, value) in bottomFragmentMap) {
        if (key == fragmentIndex) {
            fragmentTransaction.show(value)
        } else if (value.isVisible) {
            fragmentTransaction.hide(value)
        }
    }
    fragmentTransaction.commit()
}

Answer 7

Declare fragment on the activity & create fragment instance on onCreate method, then pass the fragment instance in updateFragment method. Create as many fragment instances as required corresponding to bottom navigation listener item id.

Fragment fragmentA;
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_home);

fragmentA = new Fragment();
updateFragment(fragmentA);
}

public void updateFragment(Fragment fragment) {
FragmentTransaction transaction = 
getSupportFragmentManager().beginTransaction();
transaction.add(R.id.layoutFragment, fragment);
transaction.commit();
}

Furthermore be sure you are using android.support.v4.app.Fragment and calling getSupportFragmentManager()