List is emptied after notifyListeners

I've got a weird one, I've been scratching my head for a while

I have a property on one of my classes using provider I can access and modify just fine except for this particular scenario:

I add a product to the _products list

_products.add(product);
notifyListeners();

At this point product is a class with the following properties

name String;
id String;
ingredients List<String>;

let's say for example product is

final product = Product(name:'milk', id:1, ingredients:['milk']);

This element gets added to the List and I can see in debugging that now _products is indeed a list with 1 item inside and all properties are correct

Once it notifies listeners they get the new value of _products and I can see that they receive the new value just fine except that for some odd reason the ingredients list is empty!

[ Product(name:'milk', id:1, ingredients:[]) ] // _products for representation purposes

I just verified the value of _products before notifying and it had the ingredients property correctly then right after notifying, that property becomes an empty list The other 2 properties id and name are updated correctly

I have checked very thoroughly and I have no code to modify _products else where

Solution 1:

Since we do not know your implementation details for Product and ProductList, I created an example myself in this case.

import 'package:flutter/material.dart';
import 'package:provider/provider.dart';

class Product
{
  final String name;
  final String id;
  final List<String> ingredients;
  
  const Product({
    required this.name,
    required this.id,
    required this.ingredients
  });
  
}

class ProductList extends ChangeNotifier
{
  final _products = <Product>[];
  
  void add(Product product)
  {
    _products.add(product);
    notifyListeners();
  }

  Iterable<Product> get products => _products;
  
}

void main() => runApp(MyApp());

class MyApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return MultiProvider(
      providers: [
        ChangeNotifierProvider(create: (_) => ProductList()),
      ],
      child: MaterialApp(
        title: 'Flutter Demo',
        debugShowCheckedModeBanner: false,
        theme: ThemeData(
          primarySwatch: Colors.blue,
        ),
        home: const MyHomePage(title: 'Flutter Demo Home Page'),
      ),
    );
  }
}

class MyHomePage extends StatefulWidget {
  final String title;

  const MyHomePage({
    Key? key,
    required this.title,
  }) : super(key: key);

  @override
  _MyHomePageState createState() => _MyHomePageState();
}

class _MyHomePageState extends State<MyHomePage> {

  void _incrementCounter() {
    context.read<ProductList>().add(const Product(name: 'Product 1', id: '1234', ingredients: ['Hello', 'There']));
  }
  
  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        title: Text(widget.title),
      ),
      body: const SafeArea(
        child: Products(),
      ),
      floatingActionButton: FloatingActionButton(
        onPressed: _incrementCounter,
        tooltip: 'Increment',
        child: const Icon(Icons.add),
      ),
    );
  }
}

class Products extends StatelessWidget {
  const Products({Key? key}) : super(key: key);

  @override
  Widget build(BuildContext context) {
    return Column(
      mainAxisAlignment: MainAxisAlignment.center,
      children: [
        for(var product in context.watch<ProductList>().products)
          ListTile(
            leading: Text(product.id),
            title: Text(product.name),
            subtitle: Text(product.ingredients.join(', ')),
          ),
      ]
    );
  }
}

You can check it out on Dart Pad. Hope this helps. Please refer to How to create a Minimal, Reproducible Example.