list_view.dart

@override
  Widget build(BuildContext context) {
    final SubTotalBloc subTotalBloc = BlocProvider.getBloc<SubTotalBloc>();

    return BlocProvider(
      blocs: [
        Bloc((i) => SubTotalBloc()),
      ], 
      child: Container(
        child: ListView.builder(
          itemCount: widget.products.length,
          itemBuilder: (context, index) {
            return 
            Padding(
              padding: EdgeInsets.only(top:10, bottom: 10, left: 0, right: 0),
              child: Card(
                elevation: 5,
                child: Container(
                height: 225,
                child: Row(
                  children: <Widget>[
                    
                    Expanded(
                      child: Container(
                        height: double.infinity,
                        child: Padding(
                          padding: EdgeInsets.only(top:2, bottom: 0, left: 10, right: 0),
                          child: Column(
                            children: <Widget>[
                              
                                Padding(
                                  padding: EdgeInsets.only(top: 0, bottom: 10, left: 0, right: 10),
                                  child: 
                                  SizedBox(
                                    width: double.infinity,
                                    height: 60,
                                    child: FlatButton(
                                      child: Text(
                                        "ADICIONAR",
                                        style: TextStyle(
                                          fontSize: 30,
                                          fontWeight: FontWeight.bold
                                        ),
                                      ),
                                      color: Colors.deepOrangeAccent[400],
                                      shape: RoundedRectangleBorder(
                                        borderRadius: BorderRadius.circular(10.0),
                                      ),
                                      onPressed: () {
                                        subTotalBloc.increment(double.parse(widget.products[index].pvenda.replaceAll(",", ".")));
                                      },
                                    )
                                  )
                                )
                                
                              ],
                            ),
                          ),
                        ),
                      )
                    ],
                  ),
                ),
              ),
            );
          },
        )
      ),
    );
  }

subtotal_bloc.dart

import 'dart:async';
import 'package:bloc_pattern/bloc_pattern.dart';
import 'package:rxdart/rxdart.dart';

class SubTotalBloc extends BlocBase {
  SubTotalBloc();

  var _subtotalController = BehaviorSubject<double>.seeded(0.0);

  //saida
  Stream<double> get outSubTotal => _subtotalController.stream;
  //entrada
  Sink<double> get inSubTotal => _subtotalController.sink;

  increment(double value) {
    inSubTotal.add(_subtotalController.value + value);
  }

  decrement(double value) {
    inSubTotal.add(_subtotalController.value - value);
  }

  @override
  void dispose() {
    _subtotalController.close();
    super.dispose();
  }  
}