New navigation component from arch with nested navigation graph

Question

I have one case and wish to implement it by arch navigation component. For example I have 2 Nav Graphs (main and nested). Can I call main graph from nested and how?

Answer 1

I created an answer with the info devrocca provided. It's a full answer from scratch, i didn't skip anything if anyone ever needs.

This is the main fragment for navigation. Camera is direct destination without any nested graph, Dashboard has it's own nested graph but it's added to same backstack camera fragment is added. Home has 3 fragments with it's own nav host

MainActivity
|- MainNavHost
   |- HomeNavHostFragment
   |  |- NestedNavHost
   |     |-HomeFragment1
   |     |-HomeFragment2
   |     |-HomeFragment3
   |  
   |- nav_graph_dashboard 
   |
   |- CameraFragment

Here is the navigation files

Main Navigation nav_graph.xml

Dashboard nested navigation graph

And nested navigation graph with it's own NavHost nav_graph_home

Layouts, i only add necessary ones, others are simple layouts with buttons, i add link for sample project with other navigation components samples included.

MainActivity





    


        

            

        

        

            

        

    

Main Fragment, this is first fragment that shown in the image used as start of main navigation

Layout that contains inner NavHostFragment for home navigation

MainActivity is for checking main navigation back stack, important thing here is

supportFragmentManager back stack is not updated as you navigate it's childFragmentManager even for main navigation, even if you only have one

class MainActivity : AppCompatActivity() {

    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        setContentView(R.layout.activity_main)

        // Get NavHostFragment
        val navHostFragment =
            supportFragmentManager.findFragmentById(R.id.main_nav_host_fragment)

        // ChildFragmentManager of NavHostFragment
        val navHostChildFragmentManager = navHostFragment?.childFragmentManager

        navHostChildFragmentManager?.addOnBackStackChangedListener {

            val backStackEntryCount = navHostChildFragmentManager.backStackEntryCount
            val fragments = navHostChildFragmentManager.fragments
        }
    }
}

Fragment that contains Home navigation's host

class HomeNavHostFragment : BaseDataBindingFragment() {
    override fun getLayoutRes(): Int = R.layout.fragment_home_navhost

    private var navController: NavController? = null

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

        val nestedNavHostFragment =
            childFragmentManager.findFragmentById(R.id.nested_nav_host_fragment) as? NavHostFragment
        navController = nestedNavHostFragment?.navController

        navController?.navigate(R.id.homeFragment1)

        listenBackStack()
    }

    private fun listenBackStack() {

        // Get NavHostFragment
        val navHostFragment =
            childFragmentManager.findFragmentById(R.id.nested_nav_host_fragment)

        // ChildFragmentManager of the current NavHostFragment
        val navHostChildFragmentManager = navHostFragment?.childFragmentManager

        navHostChildFragmentManager?.addOnBackStackChangedListener {

            val backStackEntryCount = navHostChildFragmentManager!!.backStackEntryCount
            val fragments = navHostChildFragmentManager!!.fragments

            Toast.makeText(
                requireContext(),
                "HomeNavHost backStackEntryCount: $backStackEntryCount, fragments: $fragments",
                Toast.LENGTH_SHORT
            ).show()
        }


        val callback = object : OnBackPressedCallback(true) {
            override fun handleOnBackPressed() {

                val backStackEntryCount = navHostChildFragmentManager!!.backStackEntryCount

                Toast.makeText(
                    requireContext(),
                    "HomeNavHost backStackEntryCount: $backStackEntryCount",
                    Toast.LENGTH_SHORT
                ).show()


        if (backStackEntryCount == 1) {
                OnBackPressedCallback@ this.isEnabled = false
                requireActivity().onBackPressed()
            } else {
                navController?.navigateUp()
            }
        }
        }

        requireActivity().onBackPressedDispatcher.addCallback(viewLifecycleOwner, callback)

    }
}

There is one thing i don't know if it's improved in graph or code with nested NavHostFragment

If you set start destination of nav_graph_home HomeFragment1 instead of HomeNavHostFragment it works as dashboard which ignores nested NavHost and added to main back stack of fragments.

Since you are in inner NavHostFragment findNavController() in any home fragment returns the inner one

class HomeFragment3 : BaseDataBindingFragment() {
    override fun getLayoutRes(): Int = R.layout.fragment_home3

    private var count = 0

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

        dataBinding.btnIncrease.setOnClickListener {
            dataBinding.tvTitle.text = "Count: ${count++}"
        }


        val mainNavController =
            Navigation.findNavController(requireActivity(), R.id.main_nav_host_fragment)

        dataBinding.btnGoToStart.setOnClickListener {

            //