Bdtjtk

You don't really need a ViewPager to work with BottomNavigation and the new Navigation architecture component. I have been working in a sample app that uses exactly the two, see here.

The basic concept is this, you have the main activity that will host the BottomNavigationView and that is the Navigation host for your navigation graph, this is how the xml for it look like:

activity_main.xml

<?xml version="1.0" encoding="utf-8"?>

<android.support.constraint.ConstraintLayout 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/container"

    android:layout_width="match_parent"

    android:layout_height="match_parent"

    tools:context=".main.MainActivity">



    <fragment

        android:id="@+id/my_nav_host_fragment"

        android:name="androidx.navigation.fragment.NavHostFragment"

        android:layout_width="match_parent"

        android:layout_height="0dp"

        app:defaultNavHost="true"

        app:layout_constraintBottom_toTopOf="@+id/navigation"

        app:layout_constraintEnd_toEndOf="parent"

        app:layout_constraintStart_toStartOf="parent"

        app:layout_constraintTop_toTopOf="parent"

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



    <android.support.design.widget.BottomNavigationView

        android:id="@+id/navigation"

        android:layout_width="0dp"

        android:layout_height="wrap_content"

        android:layout_marginStart="0dp"

        android:layout_marginEnd="0dp"

        android:background="?android:attr/windowBackground"

        app:layout_constraintBottom_toBottomOf="parent"

        app:layout_constraintLeft_toLeftOf="parent"

        app:layout_constraintRight_toRightOf="parent"

        app:menu="@menu/navigation" />



</android.support.constraint.ConstraintLayout>

The navigation Menu (tabs menu) for the BottomNavigationView looks like this:

navigation.xml

<?xml version="1.0" encoding="utf-8"?>

<menu xmlns:android="http://schemas.android.com/apk/res/android">



    <item

        android:id="@+id/navigation_home"

        android:icon="@drawable/ic_home"

        android:title="@string/title_home" />



    <item

        android:id="@+id/navigation_people"

        android:icon="@drawable/ic_group"

        android:title="@string/title_people" />



    <item

        android:id="@+id/navigation_organization"

        android:icon="@drawable/ic_organization"

        android:title="@string/title_organization" />



    <item

        android:id="@+id/navigation_business"

        android:icon="@drawable/ic_business"

        android:title="@string/title_business" />



    <item

        android:id="@+id/navigation_tasks"

        android:icon="@drawable/ic_dashboard"

        android:title="@string/title_tasks" />



</menu>

All of this is just the BottomNavigationView setup. Now to make it work with the Navigation Arch Component you need to go into the navigation graph editor, add all your fragment destinations (in my case I have 5 of them, one for each tab) and set the id of the destination with the same name as the one in the navigation.xml file:

This will tell android to make a link between the tab and the fragment, now every time the user clicks the "Home" tab android will take care of loading up the correct fragment.
There is also one piece of kotlin code that needs to be added to your NavHost (the main activity) to wire things up with the BottomNavigationView:

You need to add in your onCreate:

bottomNavigation.setupWithNavController(Navigation.findNavController(this, R.id.my_nav_host_fragment))

This tells android to do the wiring between the Navigation architecture component and the BottomNavigationView. See more in the docs.

To get the same beahvior you have when you use youtube, just add this:

navigation.setOnNavigationItemSelectedListener {item ->



            onNavDestinationSelected(item, Navigation.findNavController(this, R.id.my_nav_host_fragment))



        }

This will make destinations go into the backstack so when you hit the back button, the last visited destination will be popped up.

You have to set host navigation like below xml:

<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"

    xmlns:app="http://schemas.android.com/apk/res-auto"

    android:layout_width="match_parent"

    android:layout_height="match_parent"

    android:orientation="vertical">



    <android.support.v7.widget.Toolbar

        android:id="@+id/toolbar"

        android:layout_width="match_parent"

        android:layout_height="wrap_content"

        android:background="@color/colorPrimary" />



    <fragment

        android:id="@+id/navigation_host_fragment"

        android:name="androidx.navigation.fragment.NavHostFragment"

        android:layout_width="match_parent"

        android:layout_height="0dp"

        android:layout_weight="1"

        app:defaultNavHost="true"

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



    <android.support.design.widget.BottomNavigationView

        android:id="@+id/bottom_navigation_view"

        android:layout_width="match_parent"

        android:layout_height="wrap_content"

        app:itemIconTint="@drawable/color_state_list"

        app:itemTextColor="@drawable/color_state_list"

        app:menu="@menu/menu_bottom_navigation" />

</LinearLayout>

Setup With Navigation Controller :

NavHostFragment navHostFragment = (NavHostFragment) getSupportFragmentManager().findFragmentById(R.id.navigation_host_fragment);

NavigationUI.setupWithNavController(bottomNavigationView, navHostFragment.getNavController());

menu_bottom_navigation.xml :

<?xml version="1.0" encoding="utf-8"?>

<menu xmlns:android="http://schemas.android.com/apk/res/android">

    <item

        android:id="@id/tab1"  // Id of navigation graph 

        android:icon="@mipmap/ic_launcher"

        android:title="@string/tab1" />

    <item

        android:id="@id/tab2" // Id of navigation graph

        android:icon="@mipmap/ic_launcher"

        android:title="@string/tab2" />



    <item

        android:id="@id/tab3" // Id of navigation graph

        android:icon="@mipmap/ic_launcher"

        android:title="@string/tab3" />

</menu>

nav_graph.xml :

<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/tab1">

    <fragment

        android:id="@+id/tab1"

        android:name="com.navigationsample.Tab1Fragment"

        android:label="@string/tab1"

        tools:layout="@layout/fragment_tab_1" />



    <fragment

        android:id="@+id/tab2"

        android:name="com.navigationsample.Tab2Fragment"

        android:label="@string/tab2"

        tools:layout="@layout/fragment_tab_2"/>



    <fragment

        android:id="@+id/tab3"

        android:name="com.simform.navigationsample.Tab3Fragment"

        android:label="@string/tab3"

        tools:layout="@layout/fragment_tab_3"/>

</navigation>

By setting up the same id of "nav_graph" to "menu_bottom_navigation" will handle the click of bottom navigation.

You can handle back action using popUpTo property in action tag.

You can have a viewpager setup with bottom navigation view. Each fragment in the viewpager will be a container fragment, it will have child fragments with its own backstack. You can maintain backstack for each tab in viewpager this way

I have made an app like this (still not published on PlayStore) that has the same navigation, maybe its implementation is different from what Google does in their apps, but the functionality is the same.

the structure involves I have Main Activity that I switch the content of it by showing/hiding fragments using:

public void switchTo(final Fragment fragment, final String tag /*Each fragment should have a different Tag*/) {



// We compare if the current stack is the current fragment we try to show

if (fragment == getSupportFragmentManager().getPrimaryNavigationFragment()) {

  return;

}



// We need to hide the current showing fragment (primary fragment)

final Fragment currentShowingFragment = getSupportFragmentManager().getPrimaryNavigationFragment();



final FragmentTransaction fragmentTransaction = getSupportFragmentManager().beginTransaction();

if (currentShowingFragment != null) {

  fragmentTransaction.hide(currentShowingFragment);

}



// We try to find that fragment if it was already added before

final Fragment alreadyAddedFragment = getSupportFragmentManager().findFragmentByTag(tag);

if (alreadyAddedFragment != null) {

  // Since its already added before we just set it as primary navigation and show it again

  fragmentTransaction.setPrimaryNavigationFragment(alreadyAddedFragment);

  fragmentTransaction.show(alreadyAddedFragment);

} else {

  // We add the new fragment and then show it

  fragmentTransaction.add(containerId, fragment, tag);

  fragmentTransaction.show(fragment);

  // We set it as the primary navigation to support back stack and back navigation

  fragmentTransaction.setPrimaryNavigationFragment(fragment);

}



fragmentTransaction.commit();

}

If you have a bottomNavigationView with 3 items corresponding to 3 Fragments: FragmentA, FragmentB and FragmentC where FragmentA is the startDestination in your navigation graph, then when you're on FragmentB or FragmentC and you click back, you're going to be redirected to FragmentA, that's the behavior recommended by Google and that's implemented by default.

If however you wish to alter this behavior, you'll need to either use a ViewPager as was suggested by some of the other answers, or manually handle the fragments backStack and back transactions yourself -which in a way would undermine the use of the Navigation component altogether-.

First, let me clarify how Youtube and Instagram handles fragment navigation.

• When the user is on a detail fragment, back or up pop the stack once, with the state properly restaured. A second click on the already selected bottom bar item pop all the stack to the root, refreshing it


• When the user is on a root fragment, back goes to the last menu selected on the bottom bar, displaying the last detail fragment, with the state properly restaured (JetPack doesn't)


• When the user is on the start destination fragment, back finishes activity

None of the other answers above solve all this problems using the jetpack navigation.

JetPack navigation has no standard way to do this, the way that I found more simple is to dividing the navigation xml graph into one for each bottom navigation item, handling the back stack between the navigation items myself using the activity FragmentManager and use the JetPack NavController to handle the internal navigation between root and detail fragments (its implementation uses the childFragmentManager stack).

Suppose you have in your navigation folder this 3 xmls:

res/navigation/

    navigation_feed.xml

    navigation_explore.xml

    navigation_profile.xml

Have your destinationIds inside the navigation xmls the same of your bottomNavigationBar menu ids. Also, to each xml set the app:startDestination to the fragment that you want as the root of the navigation item.

Create a class BottomNavController.kt:

class BottomNavController(

        val context: Context,

        @IdRes val containerId: Int,

        @IdRes val appStartDestinationId: Int

) {

    private val navigationBackStack = BackStack.of(appStartDestinationId)

    lateinit var activity: Activity

    lateinit var fragmentManager: FragmentManager

    private var listener: OnNavigationItemChanged? = null

    private var navGraphProvider: NavGraphProvider? = null



    interface OnNavigationItemChanged {

        fun onItemChanged(itemId: Int)

    }



    interface NavGraphProvider {

        @NavigationRes

        fun getNavGraphId(itemId: Int): Int

    }



    init {

        var ctx = context

        while (ctx is ContextWrapper) {

            if (ctx is Activity) {

                activity = ctx

                fragmentManager = (activity as FragmentActivity).supportFragmentManager

                break

            }

            ctx = ctx.baseContext

        }

    }



    fun setOnItemNavigationChanged(listener: (itemId: Int) -> Unit) {

        this.listener = object : OnNavigationItemChanged {

            override fun onItemChanged(itemId: Int) {

                listener.invoke(itemId)

            }

        }

    }



    fun setNavGraphProvider(provider: NavGraphProvider) {

        navGraphProvider = provider

    }



    fun onNavigationItemReselected(item: MenuItem) {

        // If the user press a second time the navigation button, we pop the back stack to the root

        activity.findNavController(containerId).popBackStack(item.itemId, false)

    }



    fun onNavigationItemSelected(itemId: Int = navigationBackStack.last()): Boolean {



        // Replace fragment representing a navigation item

        val fragment = fragmentManager.findFragmentByTag(itemId.toString())

                ?: NavHostFragment.create(navGraphProvider?.getNavGraphId(itemId)

                        ?: throw RuntimeException("You need to set up a NavGraphProvider with " +

                                "BottomNavController#setNavGraphProvider")

                )

        fragmentManager.beginTransaction()

                .setCustomAnimations(

                        R.anim.nav_default_enter_anim,

                        R.anim.nav_default_exit_anim,

                        R.anim.nav_default_pop_enter_anim,

                        R.anim.nav_default_pop_exit_anim

                )

                .replace(containerId, fragment, itemId.toString())

                .addToBackStack(null)

                .commit()



        // Add to back stack

        navigationBackStack.moveLast(itemId)



        listener?.onItemChanged(itemId)



        return true

    }



    fun onBackPressed() {

        val childFragmentManager = fragmentManager.findFragmentById(containerId)!!

                .childFragmentManager

        when {

            // We should always try to go back on the child fragment manager stack before going to

            // the navigation stack. It's important to use the child fragment manager instead of the

            // NavController because if the user change tabs super fast commit of the

            // supportFragmentManager may mess up with the NavController child fragment manager back

            // stack

            childFragmentManager.popBackStackImmediate() -> {

            }

            // Fragment back stack is empty so try to go back on the navigation stack

            navigationBackStack.size > 1 -> {

                // Remove last item from back stack

                navigationBackStack.removeLast()



                // Update the container with new fragment

                onNavigationItemSelected()

            }

            // If the stack has only one and it's not the navigation home we should

            // ensure that the application always leave from startDestination

            navigationBackStack.last() != appStartDestinationId -> {

                navigationBackStack.removeLast()

                navigationBackStack.add(0, appStartDestinationId)

                onNavigationItemSelected()

            }

            // Navigation stack is empty, so finish the activity

            else -> activity.finish()

        }

    }



    private class BackStack : ArrayList<Int>() {

        companion object {

            fun of(vararg elements: Int): BackStack {

                val b = BackStack()

                b.addAll(elements.toTypedArray())

                return b

            }

        }



        fun removeLast() = removeAt(size - 1)

        fun moveLast(item: Int) {

            remove(item)

            add(item)

        }

    }

}



// Convenience extension to set up the navigation

fun BottomNavigationView.setUpNavigation(bottomNavController: BottomNavController, onReselect: ((menuItem: MenuItem) -> Unit)? = null) {

    setOnNavigationItemSelectedListener {

        bottomNavController.onNavigationItemSelected(it.itemId)

    }

    setOnNavigationItemReselectedListener {

        bottomNavController.onNavigationItemReselected(it)

        onReselect?.invoke(it)

    }

    bottomNavController.setOnItemNavigationChanged { itemId ->

        menu.findItem(itemId).isChecked = true

    }

}

Do your layout main.xml like this:

<androidx.constraintlayout.widget.ConstraintLayout 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:layout_width="match_parent"

    android:layout_height="match_parent">



    <FrameLayout

        android:id="@+id/container"

        android:layout_width="match_parent"

        android:layout_height="0dp"

        app:layout_constraintBottom_toTopOf="@id/bottomNavigationView"

        app:layout_constraintTop_toTopOf="parent" />



    <com.google.android.material.bottomnavigation.BottomNavigationView

        android:id="@+id/bottomNavigationView"

        android:layout_width="match_parent"

        android:layout_height="wrap_content"

        android:layout_marginStart="0dp"

        android:layout_marginEnd="0dp"

        app:layout_constraintBottom_toBottomOf="parent"

        app:menu="@menu/navigation" />



</androidx.constraintlayout.widget.ConstraintLayout>

Use on your activity like this:

class MainActivity : AppCompatActivity(),

        BottomNavController.NavGraphProvider  {



    private val navController by lazy(LazyThreadSafetyMode.NONE) {

        Navigation.findNavController(this, R.id.container)

    }



    private val bottomNavController by lazy(LazyThreadSafetyMode.NONE) {

        BottomNavController(this, R.id.container, R.id.navigation_feed)

    }



    override fun onCreate(savedInstanceState: Bundle?) {

        super.onCreate(savedInstanceState)

        setContentView(R.layout.main)



        bottomNavController.setNavGraphProvider(this)

        bottomNavigationView.setUpNavigation(bottomNavController)

        if (savedInstanceState == null) bottomNavController

                .onNavigationItemSelected()



        // do your things...

    }



    override fun getNavGraphId(itemId: Int) = when (itemId) {

        R.id.navigation_feed -> R.navigation.navigation_feed

        R.id.navigation_explore -> R.navigation.navigation_explore

        R.id.navigation_profile -> R.navigation.navigation_profile

        else -> R.navigation.navigation_feed

    }



    override fun onSupportNavigateUp(): Boolean = navController

            .navigateUp()



    override fun onBackPressed() = bottomNavController.onBackPressed()

}