Handling back button in Android Navigation Component

For anyone looking for a Kotlin implementation see below.

Note that the OnBackPressedCallback only seems to work for providing custom back behavior to the built-in software/hardware back button and not the back arrow button/home as up button within the actionbar/toolbar. To also override the behavior for the actionbar/toolbar back button I'm providing the solution that's working for me. If this is a bug or you are aware of a better solution for that case please comment.

build.gradle

...
implementation "androidx.appcompat:appcompat:1.1.0-rc01"
implementation "androidx.navigation:navigation-fragment-ktx:2.0.0"
implementation "androidx.navigation:navigation-ui-ktx:2.0.0"
...

MainActivity.kt

...
import androidx.appcompat.app.AppCompatActivity
...

class MainActivity : AppCompatActivity() {
    override fun onCreate(savedInstanceState: Bundle?) {
        setContentView(R.layout.activity_main)

        ...

        val navController = findNavController(R.id.nav_host_fragment)
        val appBarConfiguration = AppBarConfiguration(navController.graph)

        // This line is only necessary if using the default action bar.
        setupActionBarWithNavController(navController, appBarConfiguration)

        // This remaining block is only necessary if using a Toolbar from your layout.
        val toolbar = findViewById<Toolbar>(R.id.toolbar)
        toolbar.setupWithNavController(navController, appBarConfiguration)
        // This will handle back actions initiated by the the back arrow 
        // at the start of the toolbar.
        toolbar.setNavigationOnClickListener {
            // Handle the back button event and return to override 
            // the default behavior the same way as the OnBackPressedCallback.
            // TODO(reason: handle custom back behavior here if desired.)

            // If no custom behavior was handled perform the default action.
            navController.navigateUp(appBarConfiguration) || super.onSupportNavigateUp()
        }
    }

    /**
     * If using the default action bar this must be overridden.
     * This will handle back actions initiated by the the back arrow 
     * at the start of the action bar.
     */
    override fun onSupportNavigateUp(): Boolean {
        // Handle the back button event and return true to override 
        // the default behavior the same way as the OnBackPressedCallback.
        // TODO(reason: handle custom back behavior here if desired.)

        // If no custom behavior was handled perform the default action.
        val navController = findNavController(R.id.nav_host_fragment)
        return navController.navigateUp(appBarConfiguration) || super.onSupportNavigateUp()
    }
}

MyFragment.kt

...
import androidx.activity.OnBackPressedCallback
import androidx.fragment.app.Fragment
...

class MyFragment : Fragment() {
    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        val onBackPressedCallback = object : OnBackPressedCallback(true) {
            override fun handleOnBackPressed() {
                // Handle the back button event
            }
        }
        requireActivity().getOnBackPressedDispatcher().addCallback(this, onBackPressedCallback)
    }
}

The official documentation can be viewed at https://developer.android.com/guide/navigation/navigation-custom-back

Newest Update - April 25th, 2019

New release androidx.activity ver. 1.0.0-alpha07 brings some changes

More explanations in android official guide: Provide custom back navigation

Example:

public class MyFragment extends Fragment {

    @Override
    public void onCreate(@Nullable Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);

        // This callback will only be called when MyFragment is at least Started.
        OnBackPressedCallback callback = new OnBackPressedCallback(true /* enabled by default */) {
            @Override
            public void handleOnBackPressed() {
                // Handle the back button event
            }
        };
        requireActivity().getOnBackPressedDispatcher().addCallback(this, callback);

        // The callback can be enabled or disabled here or in handleOnBackPressed()
    }
    ...
}

Old Updates

UPD: April 3rd, 2019

Now its simplified. More info here

Example:

requireActivity().getOnBackPressedDispatcher().addCallback(getViewLifecycleOwner(), this);

@Override
public boolean handleOnBackPressed() {
    //Do your job here
    //use next line if you just need navigate up
    //NavHostFragment.findNavController(this).navigateUp(); 
    //Log.e(getClass().getSimpleName(), "handleOnBackPressed");
    return true;
    }

Deprecated (since Version 1.0.0-alpha06 April 3rd, 2019) :

Since this, it can be implemented just using JetPack implementation OnBackPressedCallback in your fragment and add it to activity: getActivity().addOnBackPressedCallback(getViewLifecycleOwner(),this);

Your fragment should looks like this:

public MyFragment extends Fragment implements OnBackPressedCallback {

    @Override
    public void onActivityCreated(@Nullable Bundle savedInstanceState) {
        super.onActivityCreated(savedInstanceState);
        
        getActivity().addOnBackPressedCallback(getViewLifecycleOwner(),this);
}
    
    @Override
    public boolean handleOnBackPressed() {
        //Do your job here
        //use next line if you just need navigate up
        //NavHostFragment.findNavController(this).navigateUp(); 
        //Log.e(getClass().getSimpleName(), "handleOnBackPressed");
        return true;
    }

    @Override
    public void onDestroyView() {
        super.onDestroyView();
        getActivity().removeOnBackPressedCallback(this);
    }
}

UPD: Your activity should extends AppCompatActivityor FragmentActivity and in Gradle file:

 implementation 'androidx.appcompat:appcompat:{lastVersion}'

So, I created an interface

public interface OnBackPressedListener {
    void onBackPressed();
}

And implemented it by all fragments that need to handle back button. In main activity I overrided onBackPressed() method:

@Override
public void onBackPressed() {
    final Fragment currentFragment = mNavHostFragment.getChildFragmentManager().getFragments().get(0);
    final NavController controller = Navigation.findNavController(this, R.id.nav_host_fragment);
    if (currentFragment instanceof OnBackPressedListener)
        ((OnBackPressedListener) currentFragment).onBackPressed();
    else if (!controller.popBackStack())
        finish();

}

So, If the top fragment of my Navigation host implements OnBackPressedListener interface, I call its onBackPressed() method, elsewhere I simply pop back stack and close application if the back stack is empty.