Get fragment instance nested inside nav controller from activity

Question

My activity has bottom navigation and I would like to get the instance of one of the fragments of the navigation in the activity.

I am trying to get the instance of the com.myapp.ui.MapFragment inside the onCreate of the activity (after setContentView) by first getting an instance of the nav_host_fragment and then the instance of the com.myapp.ui.MapFragment using childFragmentManager:

val navFragment = supportFragmentManager.findFragmentById(R.id.nav_host_fragment)
val mapFragment = navFragment!!.childFragmentManager.findFragmentById(R.id.navigation_map)

It finds the instance of navFragment but mapFragment is null.

This is the activity layout:

<?xml version="1.0" encoding="utf-8"?>
<androidx.constraintlayout.widget.ConstraintLayout
xmlns:android="http://schemas.android.com/apk/res/android"
xmlns:app="http://schemas.android.com/apk/res-auto"
android:id="@+id/container"
android:layout_width="match_parent"
android:layout_height="match_parent">

<LinearLayout
    android:id="@+id/twitter_activity_container"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:orientation="vertical"
    android:weightSum="100">

    <fragment
        android:id="@+id/nav_host_fragment"
        android:name="androidx.navigation.fragment.NavHostFragment"
        android:layout_width="match_parent"
        android:layout_height="match_parent"
        app:defaultNavHost="true"
        app:layout_constraintBottom_toTopOf="@id/nav_view"
        app:layout_constraintLeft_toLeftOf="parent"
        app:layout_constraintRight_toRightOf="parent"
        app:layout_constraintTop_toTopOf="parent"
        app:navGraph="@navigation/mobile_navigation"
        android:layout_weight="90"/>

    <com.google.android.material.bottomnavigation.BottomNavigationView
        android:id="@+id/nav_view"
        android:layout_width="fill_parent"
        android:layout_height="wrap_content"
        android:background="?android:attr/windowBackground"
        app:layout_constraintBottom_toBottomOf="parent"
        app:layout_constraintLeft_toLeftOf="parent"
        app:layout_constraintRight_toRightOf="parent"
        android:layout_weight="10"
        app:menu="@menu/bottom_nav_menu" />

</LinearLayout>

</androidx.constraintlayout.widget.ConstraintLayout>

This is the navigation layout:

<?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/mobile_navigation"
app:startDestination="@+id/navigation_map">

<fragment
    android:id="@+id/navigation_map"
    android:name="com.myapp.ui.map.MapFragment"
    android:label="@string/title_map"
    tools:layout="@layout/fragment_map" />

<fragment
    android:id="@+id/navigation_places"
    android:name="com.myapp.ui.places.PlacesContainerFragment"
    android:label="@string/title_places"
    tools:layout="@layout/fragment_places_container" />

<fragment
    android:id="@+id/navigation_profile"
    android:name="com.myapp.ui.profile.ProfileFragment"
    android:label="@string/title_profile"
    tools:layout="@layout/fragment_profile" />
</navigation>

This is the activity's onCreate:

override fun onCreate(savedInstanceState: Bundle?) {
    super.onCreate(savedInstanceState)

    supportActionBar?.hide()
    setTheme(preferencesUtils.appTheme)

    setContentView(R.layout.activity_main)

    val navController = findNavController(R.id.nav_host_fragment)
    val appBarConfiguration = AppBarConfiguration(
        setOf(R.id.navigation_map, R.id.navigation_places, R.id.navigation_profile)
    )

    setupActionBarWithNavController(navController, appBarConfiguration)

    nav_view.setupWithNavController(navController)

    val navFragment = supportFragmentManager.findFragmentById(R.id.nav_host_fragment)
    val mapFragment = navFragment!!.childFragmentManager.findFragmentById(R.id.navigation_map)
}

Answer 1

As per the FragmentNavigator documentation (the underlying API that does FragmentTransactions in NavController):

The current Fragment from FragmentNavigator's perspective can be retrieved by calling FragmentManager.getPrimaryNavigationFragment() with the FragmentManager passed to this FragmentNavigator.

val navFragment = supportFragmentManager.findFragmentById(R.id.nav_host_fragment)
val mapFragment = navFragment!!.childFragmentManager.getPrimaryNavigationFragment()