CustomNavDrawer1.3

activity_main.xml

<!--  
Basically, we have to add whole of our content view along with the toolbars inside a temporary _holder_ . 
And as our drawer slides,
we will be scaling down the height of this holder in a relative factor, by overriding 'on drawer slide(){…}'
-->

<androidx.drawerlayout.widget.DrawerLayout xmlns:android="http://schemas.android.com/apk/res/android"
    ...>

    <LinearLayout                                           <--------holder layout
        android:id="@+id/holder"
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:orientation="vertical"
        >
        <include
            layout="@layout/app_bar_main"
            android:layout_width="match_parent"
            android:layout_height="match_parent" />

    </LinearLayout>
<...>

</...>

MainActivity.java

public class MainActivity extends AppCompatActivity
        implements NavigationView.OnNavigationItemSelectedListener {

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        ...
        ...
        DrawerLayout drawer = (DrawerLayout) findViewById(R.id.drawer_layout);
        final LinearLayout holder=findViewById(R.id.holder);
        ActionBarDrawerToggle toggle = new ActionBarDrawerToggle(
                this, drawer, toolbar, R.string.navigation_drawer_open, R.string.navigation_drawer_close)
        {
            @Override
            public void onDrawerSlide(View drawerView, float slideOffset) 
            {

                //this code-block is the real player behind this beautiful ui
                // basically, it's a mathemetical calculation which handles the shrinking of 
                // our content view.

                float scaleFactor = 7f;
                float slideX = drawerView.getWidth() * slideOffset;

                holder.setTranslationX(slideX);
                holder.setScaleX(1 - (slideOffset / scaleFactor));
                holder.setScaleY(1 - (slideOffset / scaleFactor));

                super.onDrawerSlide(drawerView, slideOffset);
            }
        };
        ...
    }
}