Simple listener to determine if the AppBarLayout of a view is collapsed or expanded

AppBarStateChangedListener.java

public abstract class AppBarStateChangedListener implements AppBarLayout.OnOffsetChangedListener {

    public enum State {
        EXPANDED,
        COLLAPSED,
        IDLE
    }

    private State mCurrentState = State.IDLE;

    @Override
    public final void onOffsetChanged(AppBarLayout appBarLayout, int verticalOffset) {
        if (verticalOffset == 0) {
            setCurrentStateAndNotify(appBarLayout, State.EXPANDED);
        } else if (Math.abs(verticalOffset) >= appBarLayout.getTotalScrollRange()) {
            setCurrentStateAndNotify(appBarLayout, State.COLLAPSED);
        } else {
            setCurrentStateAndNotify(appBarLayout, State.IDLE);
        }
    }

    private void setCurrentStateAndNotify(AppBarLayout appBarLayout, State state){
        if (mCurrentState != state) {
            onStateChanged(appBarLayout, state);
        }
        mCurrentState = state;
    }

    public abstract void onStateChanged(AppBarLayout appBarLayout, State state);
}

MainActivity.java

public class MainActivity extends AppCompatActivity {
    
    private AppBarLayout appBarLayout;
    
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        appBarLayout = findViewById(R.id.app_bar);
        appBarLayout.addOnOffsetChangedListener(new AppBarStateChangedListener() {
            @Override
            public void onStateChanged(AppBarLayout appBarLayout, State state) {
                Log.d(getClass().getCanonicalName(), state.name());
            }
        });
    }
}

Thank you for the solution.

It's a really good one. Very useful. Thanks a lot.