Riverpod: StateProvider

Part of the State Management section of Coddy's Flutter journey. Lesson 18 of 27.

A Provider holds a value that never changes. For one simple value that does change, a number, a bool, a String, use a StateProvider:

final quantityProvider = StateProvider((ref) => 1);

Read it the same way: ref.watch(quantityProvider) gives the number, and the widget builds again when it changes.

To change it, get its notifier, the object that holds the value, with ref.read, and set its state:

onPressed: () {
  ref.read(quantityProvider.notifier).state++;
},

As in Provider, use ref.watch in build to show a value and ref.read in callbacks to change it. And changing a copy changes nothing: var n = ref.read(quantityProvider); n++; adds 1 to n, not to the provider. Only setting state does.

Challenge

Easy

The pancake order shows its quantity, but the buttons do nothing yet.

The plus adds 1 to quantityProvider.
The minus takes 1 away, but never below 1.

Try it yourself

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

final quantityProvider = StateProvider((ref) => 1);

void main() {
  runApp(const ProviderScope(child: MyApp()));
}

class MyApp extends StatelessWidget {
  const MyApp({super.key});

  @override
  Widget build(BuildContext context) {
    return const MaterialApp(
      debugShowCheckedModeBanner: false,
      home: OrderPage(),
    );
  }
}

class OrderPage extends ConsumerWidget {
  const OrderPage({super.key});

  @override
  Widget build(BuildContext context, WidgetRef ref) {
    final quantity = ref.watch(quantityProvider);
    return Scaffold(
      appBar: AppBar(title: const Text('Pancakes')),
      body: Center(
        child: Row(
          mainAxisSize: MainAxisSize.min,
          children: [
            IconButton(
              icon: const Icon(Icons.remove),
              onPressed: () {},
            ),
            Text('Quantity: $quantity', style: const TextStyle(fontSize: 24)),
            IconButton(
              icon: const Icon(Icons.add),
              onPressed: () {},
            ),
          ],
        ),
      ),
    );
  }
}

Test yourself

This lesson includes a short quiz. Start the lesson to answer it and track your progress.

All lessons in State Management

Practice on your own: Online Flutter compiler