// The value is always non-negative.
floatingActionButton: FloatingActionButton(
        elevation: 6.0, // default elevation
        highlightElevation: 12.0, //the elevation when the button is pressed.
        hoverElevation: 8.0, // while they have a pointer hovering over them.
        focusElevation: 8.0, // while they have focus.
        disabledElevation: 0.0, //the elevation when the button is disabled.
        onPressed: (){},
        child: const Icon(Icons.add),
      ),