Provider: Watch or Read

Part of the State Management section of Coddy's Flutter journey. Lesson 11 of 27.

There are two ways to get a model from Provider, and they do different jobs.

context.watch<StepCounter>() gets the model and subscribes to it: when the model calls notifyListeners(), this widget builds again. Use it in build, for anything the screen shows.

context.read<StepCounter>() gets the model once, with no subscription. Use it in callbacks such as onPressed, where you want to call a method and not to show anything:

Widget build(BuildContext context) {
  final steps = context.watch<StepCounter>().steps;   // shows
  return ElevatedButton(
    onPressed: () => context.read<StepCounter>().walk(),   // acts
    child: Text('$steps steps'),
  );
}

Mix them up and you see it. read in build shows the first value forever. watch inside onPressed is an error, because a tap is not a build and there is nothing to subscribe.

Challenge

Easy

Walk adds 1000 steps to the model, but the screen stays at 0 steps.

Make the steps line follow the model, so each tap shows the new number.
Keep the button's call as it is.

Try it yourself

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

class StepCounter extends ChangeNotifier {
  int _steps = 0;

  int get steps => _steps;

  void walk() {
    _steps += 1000;
    notifyListeners();
  }
}

void main() {
  runApp(
    ChangeNotifierProvider(
      create: (context) => StepCounter(),
      child: const MyApp(),
    ),
  );
}

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

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

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

  @override
  Widget build(BuildContext context) {
    final steps = context.read<StepCounter>().steps;
    return Scaffold(
      appBar: AppBar(title: const Text('Steps')),
      body: Center(
        child: Column(
          mainAxisSize: MainAxisSize.min,
          children: [
            Text('$steps steps', style: const TextStyle(fontSize: 40)),
            const SizedBox(height: 16),
            ElevatedButton(
              onPressed: () {
                context.read<StepCounter>().walk();
              },
              child: const Text('Walk'),
            ),
          ],
        ),
      ),
    );
  }
}

Test yourself

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

All lessons in State Management

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