Android Navigation Component - OnNavigateUpListener

Question

I would like to attach a callback to when the Android Navigation Controller navigates up from a specific fragment (findNavController().navigateUp()). How can I achieve this functionality?

I've already heard about requireActivity().onBackPressedDispatcher.addCallback(this). This only listens to the system's back button not the back arrow on the toolbar. I'd like to listen to the event where the user presses the back arrow on the toolbar in the top-left corner.

Answer 1

In your MainActivity that contains the NavHostFragment you can add onDestinationChanged to the NavController listener

navController = findNavController(MainActivity.this, R.id.navHostFragment);
navController.addOnDestinationChangedListener(this);

@Override
public void onDestinationChanged(@NonNull NavController controller, @NonNull NavDestination destination, @Nullable Bundle arguments) {
    int destinationId = destination.getId();
    if (destinationId == R.id.YOUR_DESIRED_FRAGMENT_ID) // your own behavior...
}

you can then add any behavior you desire.

I hope this help.

Answer 2

I have managed to find a solution. Here's what the onCreate() of my MainActivity looks like:

val navController = findNavController(R.id.nav_host_fragment)
val appBarConfiguration = AppBarConfiguration(navController.graph)
my_toolbar.setupWithNavController(navController, appBarConfiguration)

// The code below solved it for me. It overrides the toolbar back and then calls the OS back which is overridden in my fragment.
my_toolbar.setNavigationOnClickListener {
    onBackPressed()
}

And in my fragment, I do this in onActivityCreated()

requireActivity().onBackPressedDispatcher.addCallback(this) {
    // override custom back
}

Now I can override both toolbar back button and OS back button in a single handler.