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

The point is to get the right NavController to navigate in the right graph. Let's take this scenario as an example:

MainActivity
|- MainNavHost
   |- NavBarFragment
   |  |- NestedNavHost
   |  |  |-NestedContentFragment1
   |  |  |-NestedContentFragment2
   |  |
   |  |- BottomNavigationView
   |
   |- LoginFragment

The main graph and the nested graph are in separate xml files: this is required, as far as I understood, because the navigations target different layout areas, so they require two different NavHosts. Each Navhost will need to reference its graph by id, which requires them to be in different resource files.

The point is that to navigate in a specific graph, we must get a reference to the right graph's owner: to do this, when calling Navigation.findNavController(view), the view argument is crucial.

Docs say that

NavHostFragments register their navigation controller at the root of their view subtree such that any descendant can obtain the controller instance through the Navigation helper class's methods

So for example, if inside NavBarFragment we write

override fun onViewCreated(view: View, savedInstanceState: Bundle?) {
    navController = Navigation.findNavController(view)
}

here view is a parent of the NestedNavHost (that is the nested NavHostFragment), not a descendant, meaning that findNavController will search upstream in the tree and will return the MainNavHost's NavController.

If instead we write

override fun onViewCreated(view: View, savedInstanceState: Bundle?) {
    val nestedNavHostFragment = childFragmentManager.findFragmentById(R.id.nestedNavHostFragment) as? NavHostFragment
    navController = nestedNavHostFragment?.navController
}

where nestedNavHostFragment is the id of the FragmentContainerView in the layout, we get a reference to the correct NestedNavHost. Note the use of childFragmentManager, not parentFragmentManager.

In case you're still using the deprecated xml <fragment> tag, you can write

override fun onViewCreated(view: View, savedInstanceState: Bundle?) {
    val fragmentContainer = view.findViewById<View>(R.id.nestedNavHostFragment)
    navController = Navigation.findNavController(fragmentContainer)
}

where nestedNavHostFragment is the id of the <fragment> tag. We get a reference to the correct NestedNavHost now, because the view we pass to findNavController belongs to the NestedNavHost's subtree.

Similarly, if you need to get a reference to the main NavController from inside a NestedContentFragment, here's what we can do:

override fun onViewCreated(view: View, savedInstanceState: Bundle?) {
    // we can get the innermost NavController using this view,
    // because we are inside its subtree:
    nestedNavController = Navigation.findNavController(view)

    // we can find the outer NavController passing the owning Activity
    // and the id of a view associated to that NavController,
    // for example the NavHostFragment id:
    mainNavController = Navigation.findNavController(activity!!, R.id.mainNavHostFragment)
}

Answer 2

Actually you could use Global actions to navigate from a nested nav graph destination to a main nav graph destination.

Create a global action from nested nav graph to desired destination in main nav graph (highlighted in the image below)

example:

<navigation android:id="@+id/main_nav_graph"
     ... >
     <fragment android:id="@+id/fragStart" .../>
     <fragment .../>
     <fragment .../>

     <navigation  android:id="@+id/nested_nav_graph">
           ...

     <!-- Global Action -->
     <action
         android:id="@+id/action_global_start"
         app:destination="@id/fragStart" />
     </navigation>

</navigation>

To navigate to main graph destination use

findNavController().navigate(R.id.action_global_start)

Answer 3

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

<?xml version="1.0" encoding="utf-8"?>
<navigation xmlns:android="http://schemas.android.com/apk/res/android"
        xmlns:app="http://schemas.android.com/apk/res-auto"
        xmlns:tools="http://schemas.android.com/tools"
        android:id="@+id/nav_graph"
        app:startDestination="@id/main_dest">

    <!-- MainFragment-->
    <fragment
            android:id="@+id/main_dest"
            android:name="com.smarttoolfactory.tutorial1_3navigation_nestednavhost.blankfragment.MainFragment"
            android:label="MainFragment"
            tools:layout="@layout/fragment_main">

        <!-- Camera -->
        <action
                android:id="@+id/action_main_dest_to_cameraFragment"
                app:destination="@id/cameraFragment" />

        <!-- Home NavGraph -->
        <action
                android:id="@+id/action_main_dest_to_nav_graph_home"
                app:destination="@id/nav_graph_home" />

        <!-- Dashboard  NavGraph-->
        <action
                android:id="@+id/action_main_dest_to_nav_graph_dashboard"
                app:destination="@id/nav_graph_dashboard" />

    </fragment>

    <!-- Camera -->
    <fragment
            android:id="@+id/cameraFragment"
            android:name="com.smarttoolfactory.tutorial1_3navigation_nestednavhost.blankfragment.CameraFragment"
            android:label="CameraFragment" />


    <!-- Home-->
    <include app:graph="@navigation/nav_graph_home" />

    <!-- Dashboard-->
    <include app:graph="@navigation/nav_graph_dashboard" />


    <!-- Global Action Start -->
    <action
            android:id="@+id/action_global_start"
            app:destination="@id/main_dest"
            app:popUpTo="@id/main_dest"
            app:popUpToInclusive="true" />


</navigation>

Dashboard nested navigation graph

<?xml version="1.0" encoding="utf-8"?>
<navigation xmlns:android="http://schemas.android.com/apk/res/android"
        xmlns:app="http://schemas.android.com/apk/res-auto"
        xmlns:tools="http://schemas.android.com/tools"
        android:id="@+id/nav_graph_dashboard"
        app:startDestination="@id/dashboard_dest">

    <fragment
            android:id="@+id/dashboard_dest"
            android:name="com.smarttoolfactory.tutorial1_3navigation_nestednavhost.blankfragment.DashboardFragment1"
            android:label="DashboardFragment1"
            tools:layout="@layout/fragment_dashboard1">
        <action
                android:id="@+id/action_dashboardFragment1_to_dashboardFragment2"
                app:destination="@id/dashboardFragment2" />
    </fragment>

    <fragment
            android:id="@+id/dashboardFragment2"
            android:name="com.smarttoolfactory.tutorial1_3navigation_nestednavhost.blankfragment.DashboardFragment2"
            android:label="DashboardFragment2"
            tools:layout="@layout/fragment_dashboard2">
    </fragment>

</navigation>

And nested navigation graph with it's own NavHost nav_graph_home

<?xml version="1.0" encoding="utf-8"?>
<navigation xmlns:android="http://schemas.android.com/apk/res/android"
        xmlns:app="http://schemas.android.com/apk/res-auto"
        xmlns:tools="http://schemas.android.com/tools"
        android:id="@+id/nav_graph_home"
        app:startDestination="@id/home_dest">

    <fragment
            android:id="@+id/home_dest"
            android:name="com.smarttoolfactory.tutorial1_3navigation_nestednavhost.blankfragment.HomeNavHostFragment"
            android:label="HomeHost"
            tools:layout="@layout/fragment_home_navhost" />

    <fragment
            android:id="@+id/homeFragment1"
            android:name="com.smarttoolfactory.tutorial1_3navigation_nestednavhost.blankfragment.HomeFragment1"
            android:label="HomeFragment1"
            tools:layout="@layout/fragment_home1">
        <action
                android:id="@+id/action_homeFragment1_to_homeFragment2"
                app:destination="@id/homeFragment2" />
    </fragment>

    <fragment
            android:id="@+id/homeFragment2"
            android:name="com.smarttoolfactory.tutorial1_3navigation_nestednavhost.blankfragment.HomeFragment2"
            android:label="HomeFragment2"
            tools:layout="@layout/fragment_home2">
        <action
                android:id="@+id/action_homeFragment2_to_homeFragment3"
                app:destination="@id/homeFragment3" />
    </fragment>

    <fragment
            android:id="@+id/homeFragment3"
            android:name="com.smarttoolfactory.tutorial1_3navigation_nestednavhost.blankfragment.HomeFragment3"
            android:label="HomeFragment3"
            tools:layout="@layout/fragment_home3" />

</navigation>

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


<?xml version="1.0" encoding="utf-8"?>
<layout xmlns:android="http://schemas.android.com/apk/res/android"
        xmlns:app="http://schemas.android.com/apk/res-auto">

    <androidx.coordinatorlayout.widget.CoordinatorLayout
            android:layout_width="match_parent"
            android:layout_height="match_parent">


        <com.google.android.material.appbar.AppBarLayout
                android:id="@+id/appbar"
                android:layout_width="match_parent"
                android:layout_height="wrap_content"
                android:theme="@style/ThemeOverlay.AppCompat.Dark.ActionBar">

            <androidx.appcompat.widget.Toolbar
                    android:id="@+id/toolbar"
                    android:layout_width="match_parent"
                    android:layout_height="?attr/actionBarSize"
                    app:popupTheme="@style/ThemeOverlay.AppCompat.ActionBar" />

        </com.google.android.material.appbar.AppBarLayout>

        <androidx.constraintlayout.widget.ConstraintLayout
                android:layout_width="match_parent"
                android:layout_height="match_parent"
                app:layout_behavior="@string/appbar_scrolling_view_behavior">

            <androidx.fragment.app.FragmentContainerView
                    android:id="@+id/main_nav_host_fragment"
                    android:name="androidx.navigation.fragment.NavHostFragment"
                    android:layout_width="0dp"
                    android:layout_height="0dp"
                    app:layout_constraintLeft_toLeftOf="parent"
                    app:layout_constraintRight_toRightOf="parent"
                    app:layout_constraintTop_toTopOf="parent"
                    app:layout_constraintBottom_toBottomOf="parent"

                    app:defaultNavHost="true"
                    app:navGraph="@navigation/nav_graph"/>

        </androidx.constraintlayout.widget.ConstraintLayout>

    </androidx.coordinatorlayout.widget.CoordinatorLayout>

</layout>

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

<?xml version="1.0" encoding="utf-8"?>
<layout xmlns:android="http://schemas.android.com/apk/res/android"
        xmlns:app="http://schemas.android.com/apk/res-auto">

    <androidx.constraintlayout.widget.ConstraintLayout
            android:id="@+id/parentLayout"
            android:layout_width="match_parent"
            android:layout_height="match_parent">

        <Button
                android:id="@+id/btnDestCam"
                android:layout_width="wrap_content"
                android:layout_height="wrap_content"
                android:text="Destination Camera"
                app:layout_constraintBottom_toTopOf="@+id/btnNavGraphHome"
                app:layout_constraintHorizontal_bias="0.5"
                app:layout_constraintLeft_toRightOf="parent"
                app:layout_constraintRight_toLeftOf="parent"
                app:layout_constraintTop_toTopOf="parent" />

        <Button
                android:id="@+id/btnNavGraphHome"
                android:layout_width="wrap_content"
                android:layout_height="wrap_content"
                android:text="Nested NavHost Graph Home"
                app:layout_constraintBottom_toTopOf="@+id/btnNavGraphDashboard"
                app:layout_constraintHorizontal_bias="0.5"
                app:layout_constraintLeft_toRightOf="parent"
                app:layout_constraintRight_toLeftOf="parent"
                app:layout_constraintTop_toBottomOf="@+id/btnDestCam" />

        <Button
                android:id="@+id/btnNavGraphDashboard"
                android:layout_width="wrap_content"
                android:layout_height="wrap_content"
                android:text="Nested Graph Dashboard"
                app:layout_constraintBottom_toBottomOf="parent"
                app:layout_constraintHorizontal_bias="0.5"
                app:layout_constraintLeft_toRightOf="parent"
                app:layout_constraintRight_toLeftOf="parent"
                app:layout_constraintTop_toBottomOf="@+id/btnNavGraphHome" />


    </androidx.constraintlayout.widget.ConstraintLayout>


</layout>

Layout that contains inner NavHostFragment for home navigation

<?xml version="1.0" encoding="utf-8"?>
<layout xmlns:android="http://schemas.android.com/apk/res/android"
        xmlns:app="http://schemas.android.com/apk/res-auto">

    <androidx.constraintlayout.widget.ConstraintLayout
            android:layout_width="match_parent"
            android:layout_height="match_parent">

        <androidx.fragment.app.FragmentContainerView
                android:id="@+id/nested_nav_host_fragment"
                android:name="androidx.navigation.fragment.NavHostFragment"
                android:layout_width="0dp"
                android:layout_height="0dp"
                app:layout_constraintBottom_toBottomOf="parent"
                app:layout_constraintLeft_toLeftOf="parent"
                app:layout_constraintRight_toRightOf="parent"
                app:layout_constraintTop_toTopOf="parent"

                app:defaultNavHost="false"
                app:navGraph="@navigation/nav_graph_home" />

    </androidx.constraintlayout.widget.ConstraintLayout>

</layout>

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<FragmentHomeNavhostBinding>() {
    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<FragmentHome3Binding>() {
    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 {

            // Using destination belong to main_nav_host with nested navHost causes app to crash
//            findNavController().navigate(R.id.action_global_start)

            mainNavController.navigate(R.id.action_global_start)/**/
        }
    }
}

You can also use global action but it's not required since back navigation in inner navHost directly moves you back to main navigation if you don't use OnBackPressed.

Link for full example and the other nav component samples if you are interested.

Answer 4

Actually is working, using

val host: NavHostFragment? = (childFragmentManager.findFragmentById(R.id.main_app_fragment_container)  as NavHostFragment?)

I can navigate from main fragment

Answer 5

I found a temporary solution to the problem of inner NavController being covered. You can use custom NavHostFragment which provides you with desired navController. My code:

<androidx.fragment.app.FragmentContainerView
        ...
        android:name="MyNavHostFragment"
        app:defaultNavHost="false"
        app:navGraph="@navigation/inner_nav">
        ...
    </androidx.fragment.app.FragmentContainerView>

...

class MyNavHostFragment: NavHostFragment() {

    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        MainFragment.innerNavController = navController
    }
}

...

class MainFragment : Fragment() {
    companion object{
        lateinit var innerNavController: NavController
    }

    override fun onViewCreated(view: View, savedInstanceState: Bundle?) {
        super.onViewCreated(view, savedInstanceState)
        val bottomNavigationView = 
             view!!.findViewById<BottomNavigationView>(R.id.bottom_navigation_view)
        bottomNavigationView.setupWithNavController(innerNavController)
    }
}

Answer 6

we can achieve it by finding the root navhost controller and then navigating through root nav host controller

val Fragment.findRootNavHost: NavController?
get() = this.activity?.let { 
    Navigation.findNavController(it, your_root_fragment_id) 
}

findRootNavHost?.navigate(`your_destination_fragment_id`)

Kindly check the medium article link Github repo for the same