samples/provider_counter/lib/main.dart

// Copyright 2019 The Chromium Authors. All rights reserved.
// Use of this source code is governed by a BSD-style license that can be
// found in the LICENSE file.

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

void main() {
  runApp(
    // Provide the model to all widgets within the app. We're using
    // ChangeNotifierProvider because that's a simple way to rebuild
    // widgets when a model changes. We could also just use
    // Provider, but then we would have to listen to Counter ourselves.
    //
    // Read Provider's docs to learn about all the available providers.
    ChangeNotifierProvider(
      // Initialize the model in the builder. That way, Provider
      // can own Counter's lifecycle, making sure to call `dispose`
      // when not needed anymore.
      create: (context) => Counter(),
      child: MyApp(),
    ),
  );
}

/// Simplest possible model, with just one field.
///
/// [ChangeNotifier] is a class in `flutter:foundation`. [Counter] does
/// _not_ depend on Provider.
class Counter with ChangeNotifier {
  int value = 0;

  void increment() {
    value += 1;
    notifyListeners();
  }
}

class MyApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      title: 'Flutter Demo',
      theme: ThemeData(
        primarySwatch: Colors.blue,
      ),
      home: MyHomePage(),
    );
  }
}

class MyHomePage extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        title: Text('Flutter Demo Home Page'),
      ),
      body: Center(
        child: Column(
          mainAxisAlignment: MainAxisAlignment.center,
          children: <Widget>[
            Text('You have pushed the button this many times:'),
            // Consumer looks for an ancestor Provider widget
            // and retrieves its model (Counter, in this case).
            // Then it uses that model to build widgets, and will trigger
            // rebuilds if the model is updated.
            Consumer<Counter>(
              builder: (context, counter, child) => Text(
                '${counter.value}',
                style: Theme.of(context).textTheme.headline4,
              ),
            ),
          ],
        ),
      ),
      floatingActionButton: FloatingActionButton(
        onPressed: () {
          // You can access your providers anywhere you have access
          // to the context. One way is to use Provider<Counter>.of(context).
          //
          // The provider package also defines extension methods on context
          // itself. You can call context.watch<Counter>() in a build method
          // of any widget to access the current state of Counter, and to ask
          // Flutter to rebuild your widget anytime Counter changes.
          //
          // You can't use context.watch() outside build methods, because that
          // often leads to subtle bugs. Instead, you should use
          // context.read<Counter>(), which gets the current state
          // but doesn't ask Flutter for future rebuilds.
          //
          // Since we're in a callback that will be called whenever the user
          // taps the FloatingActionButton, we are not in the build method here.
          // We should use context.read().
          var counter = context.read<Counter>();
          counter.increment();
        },
        tooltip: 'Increment',
        child: Icon(Icons.add),
      ),
    );
  }
}