Android Navigation Component load 2 nested Fragments into Parent Fragment

Question

I have a Comparator Screen which is a Fragment that is splitted into 2 sub-screens. Before using Navigation Component I could easily just:

 private void initializeFragments() {

        FoodComparatorFragment comparatorFragment1 = new FoodComparatorFragment();
        FoodComparatorFragment comparatorFragment2 = new FoodComparatorFragment();

        FragmentTransaction transaction = getSupportFragmentManager().beginTransaction();
        transaction.add(R.id.comparator_fragment_1, comparatorFragment1);
        transaction.add(R.id.comparator_fragment_2, comparatorFragment2);

        transaction.commit();
    }

However, I don't know how I should perform this operation with the Navigation Component WITHOUT using explicitly the FragmentManager.

Answer 1

By default with navigation components there is one NavHostFragment hosted by the main activity for one activity/many fragments model. For many activities/fragments model, each activity has its own NavHostFragment/NavGraph.

Here we'll consider one activity, many fragments model, where one of these fragments is the Comparator Screen (parent fragment) that is splitted into 2 sub-screens/fragments.

But the NavHostFragment can only host a single fragment at a time; so the idea here is to have two NavHostFragments within the comparator/parent fragment.

---

Considering the below example:

• We have a MainActivity that hosts two fragments (HomeFragment, ParentFragment).
• The ParentFragment is the comparator with two NavHostFragments, each can host a single fragment; the top one is TopFragment, the bottom is BottomFragment.
• There is an action to go from the HomeFragment to the ParentFragment
• There is int argument sent to each sub-screen, the Top/Bottom fragments, done by sending arguments to the sub-screens along with the action from the HomeFragment to the ParentFragment; they will be displayed on TextViews on the screen.

MainActivity (1 NavHostFragments/NavGraph) ->> nav_host_fragment_activity_main
    |
    |---- HomeFragment
    |
    |---- ParentFragment (2 NavHostFragments/NavGraphs) ->> top_nav_host_fragment & bottom_nav_host_fragment
    |       |
    |       |---- TopFragment
    |       |
    |       |---- BottomFragment

Now we have 3 NavGraphs:

Main (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/navigation_home">

    <fragment
        android:id="@+id/navigation_home"
        android:name="com.example....HomeFragment"
        android:label="@string/title_home"
        tools:layout="@layout/fragment_home">

        <action
            android:id="@+id/action_homeFragment_to_parentFragment"
            app:destination="@id/navigation_parent"
            app:popUpTo="@id/navigation_home"
            app:popUpToInclusive="false" />

    </fragment>

    <fragment
        android:id="@+id/navigation_parent"
        android:name="com.example....ParentFragment"
        android:label="@string/title_parent"
        tools:layout="@layout/fragment_parent">

        <argument
            android:name="top_arg"
            android:defaultValue="-1"
            app:argType="integer" />

        <argument
            android:name="bottom_arg"
            android:defaultValue="-1"
            app:argType="integer" />

    </fragment>

</navigation>

And in ParentFragment, another two NavGraphs, each for a sub-screen:

For the 1st sub-screen (top_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/top_navigation"
    app:startDestination="@+id/top_fragment">

    <fragment
        android:id="@+id/top_fragment"
        android:name="com.example.....TopFragment"
        tools:layout="@layout/fragment_top">

        <argument
            android:name="arg"
            android:defaultValue="-1"
            app:argType="integer" />

    </fragment>

</navigation>

For the 2nd sub-screen (bottom_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/bottom_navigation"
    app:startDestination="@+id/bottom_fragment">

    <fragment
        android:id="@+id/bottom_fragment"
        android:name="com.example.....BottomFragment"
        tools:layout="@layout/fragment_bottom">

        <argument
            android:name="arg"
            android:defaultValue="-1"
            app:argType="integer" />

    </fragment>

</navigation>

Layouts:

activity_main.xml

<?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">

    <androidx.fragment.app.FragmentContainerView
        android:id="@+id/nav_host_fragment_activity_main"
        android:name="androidx.navigation.fragment.NavHostFragment"
        android:layout_width="match_parent"
        android:layout_height="match_parent"
        app:defaultNavHost="true"
        app:layout_constraintBottom_toBottomOf="parent"
        app:layout_constraintLeft_toLeftOf="parent"
        app:layout_constraintRight_toRightOf="parent"
        app:layout_constraintTop_toTopOf="parent"
        app:navGraph="@navigation/mobile_navigation" />

</androidx.constraintlayout.widget.ConstraintLayout>

fragment_parent.xml

<?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"
    xmlns:tools="http://schemas.android.com/tools"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    tools:context=".ui.fragments.HomeFragment">

    <androidx.fragment.app.FragmentContainerView
        android:id="@+id/top_nav_host_fragment"
        android:name="androidx.navigation.fragment.NavHostFragment"
        android:layout_width="0dp"
        android:layout_height="0dp"
        android:background="@color/teal_200"
        app:layout_constraintBottom_toTopOf="@id/bottom_nav_host_fragment"
        app:layout_constraintLeft_toLeftOf="parent"
        app:layout_constraintRight_toRightOf="parent"
        app:layout_constraintTop_toTopOf="parent" />

    <androidx.fragment.app.FragmentContainerView
        android:id="@+id/bottom_nav_host_fragment"
        android:name="androidx.navigation.fragment.NavHostFragment"
        android:layout_width="0dp"
        android:layout_height="0dp"
        android:background="@color/purple_200"
        app:layout_constraintBottom_toBottomOf="parent"
        app:layout_constraintLeft_toLeftOf="parent"
        app:layout_constraintRight_toRightOf="parent"
        app:layout_constraintTop_toBottomOf="@+id/top_nav_host_fragment" />


</androidx.constraintlayout.widget.ConstraintLayout>

fragment_top/bottom.xml are straightforward

fragment_top.xml:

<?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"
    xmlns:tools="http://schemas.android.com/tools"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    tools:context=".ui.fragments.ParentFragment">

    <TextView
        android:id="@+id/textview"
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:layout_marginStart="8dp"
        android:layout_marginTop="8dp"
        android:layout_marginEnd="8dp"
        android:textAlignment="center"
        android:text="top fragment"
        android:textSize="20sp"
        app:layout_constraintBottom_toBottomOf="parent"
        app:layout_constraintEnd_toEndOf="parent"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintTop_toTopOf="parent" />

</androidx.constraintlayout.widget.ConstraintLayout>

The NavGraph of the Top & Bottom fragments will be set programmatically in order to allow passing the arguments from the main NavGraph to the sub-screens' NavGraphs.

Now in the HomeFragment, there is a button to go to the ParentFragment with the action defined in the NavGraph, which sends the arguments needed for both sub-screens:

findNavController().navigate(
    HomeFragmentDirections.actionHomeFragmentToParentFragment()
    .setTopArg(100)
    .setBottomArg(200))

Then receive the arguments at the ParentFragment with the SafeArgs:

// get fragment arguments through SafeArgs
val args = ParentFragmentArgs.fromBundle(requireArguments()) 
val bottomBundle = Bundle()
bottomBundle.putInt("arg", args.bottomArg)
val topBundle = Bundle()
topBundle.putInt("arg", args.topArg)
// setting the navGraph of the top/bottom NavHostFragments, 
// and pass the argument along as a bundle.
val bottomNavHostFragment =
    childFragmentManager.findFragmentById(R.id.bottom_nav_host_fragment)
            as NavHostFragment
bottomNavHostFragment.navController.setGraph(R.navigation.bottom_navigation, bottomBundle)

val topNavHostFragment = childFragmentManager.findFragmentById(R.id.top_nav_host_fragment)
        as NavHostFragment
topNavHostFragment.navController.setGraph(R.navigation.top_navigation, topBundle)

Once more at the destination top/bottom sub-screens, get the argument with safeArgs, and set it to the TextView:

// TopFragment
textView.text = TopFragmentArgs.fromBundle(
    requireArguments()
).arg.toString()

// BottomFragment
textView.text = BottomFragmentArgs.fromBundle(
    requireArguments()
).arg.toString()