add_button_widget.dart

class _AddButton extends StatelessWidget {
  final Item item;
  const _AddButton({Key key, @required this.item}) : super(key: key);

  @override
  Widget build(BuildContext context) {
    // Get the registered cartState ReactiveModel
    final cartRM = RM.get<CartState>();
    return FlatButton(
      onPressed: cartRM.state.items.contains(item)
          ? null
          : () async {
              await cartRM.setState(
                (currentState) => CartState.add(
                  currentState,
                  item,
                ),
                //cartRM has no observer yet
                // silent no-observer exception
                silent: true,
              );
              //Notify the global CatalogState ReactiveModel to rebuild.
              RM.get<CatalogState>().notify();
            },
      splashColor: Theme.of(context).primaryColor,
      child: cartRM.state.items.contains(item)
          ? Icon(Icons.check, semanticLabel: 'ADDED')
          : Text('ADD'),
    );
  }
}