Add click listener of BottomNavigationView using Data Binding

Question

I would like to change toolbar title using Data Binding Library. Everything works correctly except listener of BottomNavigationView, i.e. I have an error:

FAILURE: Build failed with an exception.

* What went wrong:
Execution failed for task ':app:compileDebugJavaWithJavac'.
> java.lang.RuntimeException: Found data binding errors.
****/ data binding error ****msg:Could not resolve viewmodel::onNavigationClick as a listener.
file:D:\Replacements\app\src\main\res\layout\activity_main.xml
loc:85:40 - 85:67
****\ data binding error ****

I think it is caused by wrong name of setter used to associate xml attribute. In xml BottomNavigationView has app:onNavigationItemSelected, but the setter of BottomNavigationView is setOnNavigationItemSelectedListener, not setOnNavigationItemSelected. If I am right, the solution is in Android documentation. But where should I put this annotations? In which class?

In other words, what should I do in order to have code in ViewModel which will allow me to control clicks on BottomNavigationView?

Here are files:

ActivityMain.java

public class ActivityMain extends AppCompatActivity {

    private ActivityMainViewModel viewModel;

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

        // Initiating ViewModel to Activity
        viewModel = ViewModelProviders.of(this).get(ActivityMainViewModel.class);

        // Initiating ContentView in Activity
        setContentView(R.layout.activity_main);

        // Initiating DataBinding for ContentView in Activity
        ActivityMainBinding binding = DataBindingUtil.setContentView(this, R.layout.activity_main);
        binding.setViewmodel(viewModel);
    }
}

ActivityMainViewModel.java

public class ActivityMainViewModel extends ViewModel {

    private MutableLiveData<String> toolbarTitle = new MutableLiveData<>();

    public String getToolbarTitle() {
        return toolbarTitle.getValue();
    }

    public void setToolbarTitle(String toolbarTitle) {
        this.toolbarTitle.setValue(toolbarTitle);
    }

    public boolean onNavigationClick(@NonNull MenuItem item) {
        switch (item.getItemId()) {
            case R.id.navigation_1:
                setToolbarTitle("title 1");
                return true;
            case R.id.navigation_2:
                setToolbarTitle("title 2");
                return true;
            case R.id.navigation_3:
                setToolbarTitle("title 3");
                return true;
        }
        return false;
    }
}

activity_main.xml

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

<data>
    <variable name="viewmodel" type="com.example.example.viewmodel.ActivityMainViewModel"/>
</data>

<android.support.constraint.ConstraintLayout
    ...>

    <android.support.design.widget.AppBarLayout
        ...>

        <android.support.v7.widget.Toolbar
            android:id="@+id/main_toolbar"
            android:title="@{viewmodel.toolbarTitle}"
            android:layout_width="match_parent"
            android:layout_height="?attr/actionBarSize"
            android:background="@color/new_blue_light"
            android:minHeight="?attr/actionBarSize"
            android:theme="@style/ThemeOverlay.AppCompat.Dark.ActionBar"
            app:popupTheme="@style/ThemeOverlay.AppCompat.Light" />

    </android.support.design.widget.AppBarLayout>

    <FrameLayout
        ...>
        ...
    </FrameLayout>

    <android.support.design.widget.BottomNavigationView
        android:id="@+id/navigation"
        android:layout_width="0dp"
        android:layout_height="wrap_content"
        android:background="?android:attr/windowBackground"
        app:itemIconTint="@drawable/navigation_color_state"
        app:itemTextColor="@drawable/navigation_color_state"
        app:layout_constraintBottom_toBottomOf="parent"
        app:layout_constraintLeft_toLeftOf="parent"
        app:layout_constraintRight_toRightOf="parent"
        app:menu="@menu/navigation"
        app:onNavigationItemSelected="@{viewmodel::onNavigationClick}"
        tools:layout_constraintBottom_creator="0"
        tools:layout_constraintLeft_creator="0"
        tools:layout_constraintRight_creator="0" />

</android.support.constraint.ConstraintLayout>

</layout>

The only thing, what I tried to do is to create custom view (app runs, but it doesn't change toolbar title and gives warning: warning: Application namespace for attribute app:onNavigationItemSelected will be ignored.):

@BindingMethods({
        @BindingMethod(
                type = BottomNavigationViewBinding.class,
                attribute = "app:onNavigationItemSelected",
                method = "setOnNavigationItemSelectedListener"
        ),
})
public class BottomNavigationViewBinding extends BottomNavigationView{

    private OnNavigationItemSelectedListener mSelectedListener;

    public BottomNavigationViewBinding(Context context) {
        this(context, null);
    }

    public BottomNavigationViewBinding(Context context, AttributeSet attrs) {
        this(context, attrs, 0);
    }

    public BottomNavigationViewBinding(Context context, AttributeSet attrs, int defStyleAttr) {
        super(context, attrs, defStyleAttr);
    }

    public interface OnNavigationItemSelectedListener {
        boolean onNavigationItemSelected(MenuItem item);
    }

    public void setOnNavigationItemSelectedListener(@Nullable OnNavigationItemSelectedListener listener) {
        mSelectedListener = listener;
    }
}

Answer 1

Replace your NavigationItemViewBinding class with this and it will work:

public class BottomNavigationViewBindingAdapter {
    @BindingAdapter("onNavigationItemSelected")
    public static void setOnNavigationItemSelectedListener(
            BottomNavigationView view, OnNavigationItemSelectedListener listener) {
        view.setOnNavigationItemSelectedListener(listener);
    }
}

The idea is that you want to use a custom setter to incorporate the listener, which requires a little more work than renaming the method that sets the listener.

https://developer.android.com/topic/libraries/data-binding/index.html#custom_setters