Simple Riverpod example beyond the counter app: sum of two cells - flutter

Question

I'm learning Flutter and I'm stuck on state management. I took a look at Riverpod and it looks promising, but I have a hard time to go beyond the counter app to something more complicated.
For example, I want to have two TextFields that collect numbers, and another Text widget to display the sum of the two TextField values. Here's what I have.
import 'package:flutter/material.dart';
import 'package:flutter/services.dart';
import 'package:hooks_riverpod/hooks_riverpod.dart';
import 'package:flutter_hooks/flutter_hooks.dart';
void main() {
runApp(ProviderScope(
child: MyApp(),
));
}
class MyApp extends HookWidget {
#override
Widget build(BuildContext context) {
return MaterialApp(
home: MyHomePage(title: 'Adding two cells'),
);
}
}
final cellProvider = StateProvider((_) => <int>[0, 0]);
class MyHomePage extends HookWidget {
MyHomePage({Key key, this.title}) : super(key: key);
final String title;
#override
Widget build(BuildContext context) {
return Scaffold(
appBar: AppBar(
title: Text(title),
),
body: Consumer(builder: (context, watch, _) {
print(watch(cellProvider).state);
num _sum = watch(cellProvider).state[0] + watch(cellProvider).state[1];
return Center(
child: Column(
children: [
Row(
mainAxisAlignment: MainAxisAlignment.center,
children: [
Cell(0),
Cell(1),
],
),
SizedBox(
height: 100,
),
Row(
mainAxisAlignment: MainAxisAlignment.center,
children: <Widget>[
Text('Sum: ${_sum.toString()}'),
],
),
],
),
);
}),
);
}
}
class Cell extends HookWidget {
Cell(this.index);
final int index;
#override
Widget build(BuildContext context) {
return Container(
width: 150,
padding: EdgeInsets.all(30),
child: TextField(
inputFormatters: [FilteringTextInputFormatter.digitsOnly],
onChanged: (value) {
context.read(cellProvider).state[index] = num.tryParse(value);
},
),
);
}
}
The Text widget does not update. Any suggestion?
Thank you very much,
Tony

Answer 1

The provider only updates when the object it provides changes, just as a Stream returns a final value you need to update the whole object (List<int>) so the consumer updates properly, changing inner values of an iterable won't trigger an update
onChanged: (value) {
final List<int> myList = context.read(cellProvider).state;
myList[index] = num.tryParse(value);
context.read(cellProvider).state = myList; //update the state with a new list
},