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samples/provider_counter/lib/main.dart

104 lines
3.4 KiB

// 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: const Text('Flutter Demo Home Page'),
),
body: Center(
child: Column(
mainAxisAlignment: MainAxisAlignment.center,
children: <Widget>[
const 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.of<Counter>(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: const Icon(Icons.add),
),
);
}
}