The onTap parameter will be implemented only on step 7. The codelab is mdc-104-flutter **Copy-pasted from step 6:** ```dart return Stack( key: _backdropKey, children: <Widget>[ widget.backLayer, PositionedTransition( rect: layerAnimation, child: _FrontLayer( onTap: _toggleBackdropLayerVisibility, child: widget.frontLayer, ), ), ], ); ```