Interact programmatically with the Navigation component

The Navigation component provides ways to programmatically create and interact with certain navigation elements.

Create a NavHostFragment

You can use NavHostFragment.create() to programmatically create a NavHostFragment with a specific graph resource, as shown in the example below:

Kotlin

val finalHost = NavHostFragment.create(R.navigation.example_graph)
supportFragmentManager.beginTransaction()
    .replace(R.id.nav_host, finalHost)
    .setPrimaryNavigationFragment(finalHost) // equivalent to app:defaultNavHost="true"
    .commit()

Java

NavHostFragment finalHost = NavHostFragment.create(R.navigation.example_graph);
getSupportFragmentManager().beginTransaction()
    .replace(R.id.nav_host, finalHost)
    .setPrimaryNavigationFragment(finalHost) // equivalent to app:defaultNavHost="true"
    .commit();

Note that setPrimaryNavigationFragment(finalHost) lets your NavHost intercept system Back button presses. You can also implement this behavior in your NavHost XML by adding app:defaultNavHost="true". If you're implementing custom Back button behavior and don't want your NavHost intercepting Back button presses, you can pass null to setPrimaryNavigationFragment().

Reference a destination using NavBackStackEntry

Starting with Navigation 2.2.0, you can get a reference to the NavBackStackEntry for any destination on the navigation stack by calling NavController.getBackStackEntry(), passing it a destination ID. If the back stack contains more than one instance of the specified destination, getBackStackEntry() returns the topmost instance from the stack.

The returned NavBackStackEntry provides a Lifecycle, a ViewModelStore, and a SavedStateRegistry at the destination level. These objects are valid for the lifetime of the destination on the back stack. When the associated destination is popped off the back stack, the Lifecycle is destroyed, the state is no longer saved, and any ViewModel objects are cleared.

These properties give you a Lifecycle and a store for ViewModel objects and classes that work with saved state no matter what type of destination you use. This is especially useful when working with destination types which do not automatically have an associated Lifecycle, such as custom destinations.

For example, you can observe the Lifecycle of a NavBackStackEntry just as you would observe the Lifecycle of a fragment or activity. In addition, NavBackStackEntry is a LifecycleOwner, which means that you can use it when observing LiveData or with other lifecycle-aware components, as shown in the following example:

Kotlin

myViewModel.liveData.observe(backStackEntry, Observer { myData ->
    // react to live data update
})

Java

myViewModel.getLiveData().observe(backStackEntry, myData -> {
    // react to live data update
});

Lifecycle state automatically updates whenever you call navigate(). Lifecycle states for destinations that are not at the top of the back stack move from RESUMED to STARTED if the destinations are still visible under a FloatingWindow destination, such as a dialog destination, or to STOPPED otherwise.

Returning a result to the previous Destination

In Navigation 2.3.0-alpha02 and higher, NavBackStackEntry gives access to a SavedStateHandle. A SavedStateHandle is a key-value map that can be used to store and retrieve data. These values persist through process death, including configuration changes, and remain available through the same object. By using the given SavedStateHandle, you can access and pass data between destinations. This is especially useful as a mechanism to get data back from a destination after it is popped off the stack.

To pass data back to Destination A from Destination B, first set up Destination A to listen for a result on its SavedStateHandle. To do so, retrieve the NavBackStackEntry by using the getCurrentBackStackEntry() API and then observe the LiveData provided by SavedStateHandle.

Kotlin

override fun onViewCreated(view: View, savedInstanceState: Bundle?) {
    val navController = findNavController();
    // We use a String here, but any type that can be put in a Bundle is supported
    navController.currentBackStackEntry?.savedStateHandle?.getLiveData<String>("key")?.observe(
        viewLifecycleOwner) { result ->
        // Do something with the result.
    }
}

Java

@Override
public void onViewCreated(@NonNull View view, @Nullable Bundle savedInstanceState) {
    NavController navController = NavHostFragment.findNavController(this);
    // We use a String here, but any type that can be put in a Bundle is supported
    MutableLiveData<String> liveData = navController.getCurrentBackStackEntry()
            .getSavedStateHandle()
            .getLiveData("key");
    liveData.observe(getViewLifecycleOwner(), new Observer<String>() {
        @Override
        public void onChanged(String s) {
            // Do something with the result.
        }
    });
}

Figure 1. Destination B can use getPreviousBackStackEntry() to access the NavBackStackEntry for the previous destination A.

In Destination B, you must set the result on the SavedStateHandle of Destination A by using the getPreviousBackStackEntry() API.

Kotlin

navController.previousBackStackEntry?.savedStateHandle?.set("key", result)

Java

navController.getPreviousBackStackEntry().getSavedStateHandle().set("key", result);

Note: Since NavBackStackEntry uses SavedStateHandle, all results must be of a type that you can place in a Bundle. To save results of custom types that are not Parcelable or Serializable, create a ViewModel using the ViewModelStore from the previous NavBackStackEntry. For instructions on using ViewModels see the ViewModel guide.

If you’d only like to handle a result only once, you must call remove() on the SavedStateHandle to clear the result. If you do not remove the result, the LiveData will continue to return the last result to any new Observer instances.

Considerations when using dialog destinations

When you navigate to a destination that takes the full view of the NavHost (such as a <fragment> destination), the previous destination has its lifecycle stopped, preventing any callbacks to the LiveData provided by SavedStateHandle.

However, when navigating to a dialog destination, the previous destination is also visible on the screen and is therefore also STARTED despite not being the current destination. This means that calls to getCurrentBackStackEntry() from within lifecycle methods such as onViewCreated() will return the NavBackStackEntry of the dialog destination after a configuration change or process death and recreation (since the dialog is restored above the other destination). Therefore you should use getBackStackEntry() with the ID of your destination to ensure that you always use the correct NavBackStackEntry.

This also means that any Observer you set on the result LiveData will be triggered even while the dialog destinations is still on the screen. If you only want to check the result when the dialog destination is closed and the underlying destination becomes the current destination, you can observe the Lifecycle associated with the NavBackStackEntry and retrieve the result only when it becomes RESUMED.

Kotlin

override fun onViewCreated(view: View, savedInstanceState: Bundle?) {
    super.onViewCreated(view, savedInstanceState)
    val navController = findNavController();
    // After a configuration change or process death, the currentBackStackEntry
    // points to the dialog destination, so you must use getBackStackEntry()
    // with the specific ID of your destination to ensure we always
    // get the right NavBackStackEntry
    val navBackStackEntry = navController.getBackStackEntry(R.id.your_fragment)

    // Create our observer and add it to the NavBackStackEntry's lifecycle
    val observer = LifecycleEventObserver { _, event ->
        if (event == Lifecycle.Event.ON_RESUME
            && navBackStackEntry.savedStateHandle.contains("key")) {
            val result = navBackStackEntry.savedStateHandle.get<String>("key");
            // Do something with the result
        }
    }
    navBackStackEntry.lifecycle.addObserver(observer)

    // As addObserver() does not automatically remove the observer, we
    // call removeObserver() manually when the view lifecycle is destroyed
    viewLifecycleOwner.lifecycle.addObserver(LifecycleEventObserver { _, event ->
        if (event == Lifecycle.Event.ON_DESTROY) {
            navBackStackEntry.lifecycle.removeObserver(observer)
        }
    })
}

Java

@Override
public void onViewCreated(@NonNull View view, @Nullable Bundle savedInstanceState) {
    super.onViewCreated(view, savedInstanceState);
    NavController navController = NavHostFragment.findNavController(this);
    // After a configuration change or process death, the currentBackStackEntry
    // points to the dialog destination, so you must use getBackStackEntry()
    // with the specific ID of your destination to ensure we always
    // get the right NavBackStackEntry
    final NavBackStackEntry navBackStackEntry = navController.getBackStackEntry(R.id.your_fragment);

    // Create our observer and add it to the NavBackStackEntry's lifecycle
    final LifecycleEventObserver observer = new LifecycleEventObserver() {
        @Override
        public void onStateChanged(@NonNull LifecycleOwner source, @NonNull Lifecycle.Event event) {
            if (event.equals(Lifecycle.Event.ON_RESUME)
                && navBackStackEntry.getSavedStateHandle().contains("key")) {
                String result = navBackStackEntry.getSavedStateHandle().get("key");
                // Do something with the result
            }
        }
    };
    navBackStackEntry.getLifecycle().addObserver(observer);

    // As addObserver() does not automatically remove the observer, we
    // call removeObserver() manually when the view lifecycle is destroyed
    getViewLifecycleOwner().getLifecycle().addObserver(new LifecycleEventObserver() {
        @Override
        public void onStateChanged(@NonNull LifecycleOwner source, @NonNull Lifecycle.Event event) {
            if (event.equals(Lifecycle.Event.ON_DESTROY)) {
                navBackStackEntry.getLifecycle().removeObserver(observer)
            }
        }
    });
}

Note: it is important that you use the Lifecycle object provided by NavBackStackEntry rather than the fragment’s lifecycle since only the NavBackStackEntry lifecycle transitions between STARTED and RESUMED when a dialog destination overlaps your fragment. The fragment lifecycle will always be RESUMED.

Share UI-related data between destinations with ViewModel

The Navigation back stack stores a NavBackStackEntry not only for each individual destination, but also for each parent navigation graph that contains the individual destination. This allows you to retrieve a NavBackStackEntry that is scoped to a navigation graph. A navigation graph-scoped NavBackStackEntry provides a way to create a ViewModel that's scoped to a navigation graph, enabling you to share UI-related data between the graph's destinations. Any ViewModel objects created in this way live until the associated NavHost and its ViewModelStore are cleared or until the navigation graph is popped from the back stack.

The following example shows how to retrieve a ViewModel that's scoped to a navigation graph:

Kotlin

val viewModel: MyViewModel
        by navGraphViewModels(R.id.my_graph)

Java

NavBackStackEntry backStackEntry = navController.getBackStackEntry(R.id.my_graph);
MyViewModel viewModel = new ViewModelProvider(backStackEntry).get(MyViewModel.class);

If you're using Navigation 2.2.0 or earlier, you need to provide your own factory to use Saved State with ViewModels, as shown in the following example:

Kotlin

val viewModel: MyViewModel by navGraphViewModels(R.id.my_graph) {
    SavedStateViewModelFactory(requireActivity().application, requireParentFragment())
}

Java

NavBackStackEntry backStackEntry = navController.getBackStackEntry(R.id.my_graph);

ViewModelProvider viewModelProvider = new ViewModelProvider(
        backStackEntry.getViewModelStore(),
        new SavedStateViewModelFactory(
                requireActivity().getApplication(), requireParentFragment()));

MyViewModel myViewModel = provider.get(myViewModel.getClass());

For more information about ViewModel, see ViewModel Overview.