Navigation component keep state navigator sample

activity_main.xml

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

    <fragment
            app:layout_constraintTop_toTopOf="parent"
            app:layout_constraintStart_toStartOf="parent"
            app:layout_constraintEnd_toEndOf="parent"
            app:layout_constraintBottom_toTopOf="@id/bottomNav"
            android:id="@+id/nav_host_fragment"
            android:layout_width="0dp"
            android:layout_height="0dp"
            android:name="androidx.navigation.fragment.NavHostFragment"
            app:defaultNavHost="true"/>

    <com.google.android.material.bottomnavigation.BottomNavigationView
            android:id="@+id/bottomNav"
            android:layout_width="0dp"
            app:layout_constraintStart_toStartOf="parent"
            app:layout_constraintEnd_toEndOf="parent"
            app:layout_constraintTop_toBottomOf="@id/nav_host_fragment"
            app:layout_constraintBottom_toBottomOf="parent"
            android:layout_height="wrap_content"
            android:background="?android:attr/windowBackground"
            app:menu="@menu/menu_navigation"/>

</androidx.constraintlayout.widget.ConstraintLayout>

fragment1_flow.xml

<?xml version="1.0" encoding="utf-8"?>
<fragment
        xmlns:app="http://schemas.android.com/apk/res-auto"
        xmlns:android="http://schemas.android.com/apk/res/android"
        android:id="@+id/nav_host_fragment1"
        android:layout_width="match_parent"
        android:layout_height="match_parent"
        android:name="androidx.navigation.fragment.NavHostFragment"
        app:navGraph="@navigation/navigation1"
        app:defaultNavHost="true"/>

fragment2_flow.xml

<?xml version="1.0" encoding="utf-8"?>
<fragment
        xmlns:app="http://schemas.android.com/apk/res-auto"
        xmlns:android="http://schemas.android.com/apk/res/android"
        android:id="@+id/nav_host_fragment2"
        android:layout_width="match_parent"
        android:layout_height="match_parent"
        android:name="androidx.navigation.fragment.NavHostFragment"
        app:navGraph="@navigation/navigation2"
        app:defaultNavHost="true"/>

KeepStateNavigator.kt

import android.content.Context
import android.os.Bundle
import androidx.fragment.app.FragmentManager
import androidx.navigation.NavDestination
import androidx.navigation.NavOptions
import androidx.navigation.Navigator
import androidx.navigation.fragment.FragmentNavigator

@Navigator.Name("keep_state_fragment") // `keep_state_fragment` is used in navigation xml
class KeepStateNavigator(
    private val context: Context,
    private val manager: FragmentManager, // Should pass childFragmentManager.
    private val containerId: Int
) : FragmentNavigator(context, manager, containerId) {

    override fun navigate(
        destination: Destination,
        args: Bundle?,
        navOptions: NavOptions?,
        navigatorExtras: Navigator.Extras?
    ): NavDestination? {
        val tag = destination.id.toString()
        val transaction = manager.beginTransaction()

        val currentFragment = manager.primaryNavigationFragment
        if (currentFragment != null) {
            transaction.detach(currentFragment)
        }

        var fragment = manager.findFragmentByTag(tag)
        if (fragment == null) {
            val className = destination.className
            fragment = instantiateFragment(context, manager, className, args)
            transaction.add(containerId, fragment, tag)
        } else {
            transaction.attach(fragment)
        }

        transaction.setPrimaryNavigationFragment(fragment)
        transaction.setReorderingAllowed(true)
        transaction.commit()

        return null
    }
}

MainActivity.kt

import android.os.Bundle
import androidx.appcompat.app.AppCompatActivity
import androidx.navigation.NavController
import androidx.navigation.Navigation
import androidx.navigation.plusAssign
import androidx.navigation.ui.NavigationUI
import navigation.KeepStateNavigator
import kotlinx.android.synthetic.main.activity_main.*

class MainActivity : AppCompatActivity() {
    private val navController: NavController
        get() {
            return Navigation.findNavController(this, R.id.nav_host_fragment)
        }

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

    private fun setupNavigation() {
        supportFragmentManager.findFragmentById(R.id.nav_host_fragment)?.let { navHostFragment ->
            // setup custom navigator
            val navigator = KeepStateNavigator(this, navHostFragment.childFragmentManager, R.id.nav_host_fragment)
            navController.navigatorProvider += navigator

            // set navigation graph
            navController.setGraph(R.navigation.mobile_navigation)
            NavigationUI.setupWithNavController(bottomNav, navController)
        }
    }
}

menu_navigation.xml

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

    <item android:id="@+id/flowFragment1"
          android:icon="@drawable/ic_airplay_black_24dp"
          android:title="1"/>

    <item android:id="@+id/flowFragment2"
          android:icon="@drawable/ic_subscriptions_black_24dp"
          android:title="2"/>
</menu>

mobile_navigation.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/mobile_navigation" app:startDestination="@id/flowFragment1">

    <keep_state_fragment android:id="@+id/flowFragment1"
              android:name="FlowFragment1"
              android:label="FlowFragment1"
              tools:layout="@layout/fragment1_flow"/>

    <keep_state_fragment android:id="@+id/flowFragment2"
              android:name="flowFragment2"
              android:label="flowFragment2"
              tools:layout="@layout/fragment2_flow"/>

</navigation>