Navigation best practices for multi-module projects

A navigation graph can consist of any combination of the following:

• A singular destination, such as a <fragment> destination.
• A nested graph that encapsulates a set of related destinations.
• An <include> element, which allows you to embed another navigation graph file as though it was nested.

This flexibility allows you to combine smaller navigation graphs together to form your app's complete navigation graph, even if those smaller navigation graphs are provided by separate library modules.

For the examples in this topic, each library module is focused around one feature and provides a single navigation graph that encapsulates all of the destinations needed to implement that feature. In a production app, you might have many submodules at a lower level that are implementation details of this higher-level library module. Each of these library modules are included, either directly or indirectly, into your app module. The example multi-module application used in this doc has the following structure:

Each library module is a self-contained unit with its own navigation graph and destinations. The app module depends on each, adding them as implementation details in its build.gradle file, as shown:

dependencies {
    ...
    implementation project(":list")
    implementation project(":favorites")
    implementation project(":settings")

The role of the app module

The app module is responsible for providing the complete graph for your app and adding the NavHost to your UI. Within the app module's navigation graph, you can reference the library graphs by using <include>. While using <include> is functionally the same as using a nested graph, <include> supports graphs from other project modules or from library projects, as shown in the following example:

<?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/list_nav_graph">

    <include app:graph="@navigation/list_navigation" />
    <include app:graph="@navigation/favorites_navigation" />
    <include app:graph="@navigation/settings_navigation" />
</navigation>

Once a library is included in the top-level navigation graph, you can navigate to the library graphs as needed. For example, you could create an action to navigate to the settings graph from a fragment in your navigation graph, as shown:

<?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/list_nav_graph">

    <include app:graph="@navigation/list_navigation" />
    <include app:graph="@navigation/favorites_navigation" />
    <include app:graph="@navigation/settings_navigation" />

    <fragment
        android:id="@+id/random_fragment"
        android:name="com.example.android.RandomFragment"
        android:label="@string/fragment_random" >
        <!-- Launch into Settings Navigation Graph -->
        <action
            android:id="@+id/action_random_fragment_to_settings_nav_graph"
            app:destination="@id/settings_nav_graph" />
    </fragment>
</navigation>

When multiple library modules need to reference a common set of destinations, such as a login graph, you should not include those common destinations into each library module's navigation graph. Instead, add those common destinations to your app module's navigation graph. Each library module can then navigate across library modules to navigate to those common destinations.

In the previous example, the action specifies a navigation destination of @id/settings_nav_graph. This ID refers to a destination that is defined within the included graph @navigation/settings_navigation.

Top-level navigation in app module

The Navigation component includes a NavigationUI class. This class contains static methods that manage navigation with the top app bar, the navigation drawer, and bottom navigation. If your app's top-level destinations are composed of UI elements provided by library modules, the app module is a natural place to put the top-level navigation and UI elements. Since the app module depends on the collaborating library modules, all of their destinations are accessible from code defined within your app module. This means that you can use NavigationUI to tie destinations to menu items if the ID of the item matches the ID of a destination.

In figure 2, the example app module defines a BottomNavigationView in its main activity. The menu item IDs in the menu match the navigation graph IDs of the library graphs:

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

    <item
        android:id="@id/list_nav_graph"
        android:icon="@drawable/ic_list"
        android:title="List"
        app:showAsAction="ifRoom"/>

    <item
        android:id="@id/favorites_nav_graph"
        android:icon="@drawable/ic_favorite"
        android:title="Favorites"
        app:showAsAction="ifRoom"/>

    <item
        android:id="@id/settings_nav_graph"
        android:icon="@drawable/ic_settings"
        android:title="Settings"
        app:showAsAction="ifRoom" />
</menu>

To let NavigationUI handle the bottom navigation, call setupWithNavController() from onCreate() in your main activity class, as shown in the following example:

override fun onCreate(savedInstanceState: Bundle?) {
    super.onCreate(savedInstanceState)
    setContentView(R.layout.activity_main)
    val navHostFragment =
        supportFragmentManager.findFragmentById(R.id.nav_host_fragment) as NavHostFragment
    val navController = navHostFragment.navController

    findViewById<BottomNavigationView>(R.id.bottom_nav)
            .setupWithNavController(navController)
}

@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_main);
    NavHostFragment navHostFragment =
            (NavHostFragment) supportFragmentManager.findFragmentById(R.id.nav_host_fragment);
    NavController navController = navHostFragment.getNavController();
    BottomNavigationView bottomNav = findViewById(R.id.bottom_nav);

    NavigationUI.setupWithNavController(bottomNav, navController);
}

With this code in place, NavigationUI navigates to the appropriate library graph when the user clicks on a bottom navigation item.

Keep in mind that it's generally bad practice for your app module to have a hard dependency on a specific destination embedded deeply within your library modules' navigation graph. In most cases, you want your app module to know only about the entry point to any embedded or included navigation graphs (this applies outside of library modules too). If you need to link to a destination deep within your library's navigation graph, the preferred way to do this is by using a deep link. Deep linking is also the only way for a library to navigate to a destination in another library's navigation graph.

Navigating across library modules

At compile time, independent library modules cannot see each other, so you can't use IDs to navigate to destinations in other modules. Instead, use a deep link to navigate directly to a destination that is associated with an implicit deep link.

Continuing the previous example, imagine you need to navigate from a button in the list module to a destination nested in the settings module. You can do this by adding a deep link to the destination in the settings navigation graph, as shown:

<?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/settings_nav_graph"
    app:startDestination="@id/settings_fragment_one">

    ...

    <fragment
        android:id="@+id/settings_fragment_two"
        android:name="com.example.google.login.SettingsFragmentTwo"
        android:label="@string/settings_fragment_two" >

        <deepLink
            app:uri="android-app://example.google.app/settings_fragment_two" />
    </fragment>
</navigation>

Then add the following code to the button's onClickListener in the list fragment:

button.setOnClickListener {
    val request = NavDeepLinkRequest.Builder
        .fromUri("android-app://example.google.app/settings_fragment_two".toUri())
        .build()
    findNavController().navigate(request)
}

button.setOnClickListener(new View.OnClickListener() {
    @Override
    public void onClick(View view) {
        NavDeepLinkRequest request = NavDeepLinkRequest.Builder
            .fromUri(Uri.parse("android-app://example.google.app/settings_fragment_two"))
            .build();
        NavHostFragment.findNavController(this).navigate(request);
    }
});

Unlike navigating using action or destination IDs, you can navigate to any URI in any graph, even across modules.

When navigating using URI, the back stack is not reset. This behavior is unlike explicit deep link navigation, where the back stack is replaced when navigating.