flutter: how are dependencies on inherited widgets discovered? - flutter

Question

I'm currently reading the example code of the provider package:
// ignore_for_file: public_member_api_docs
import 'package:flutter/foundation.dart';
import 'package:flutter/material.dart';
import 'package:provider/provider.dart';
void main() => runApp(MyApp());
class Counter with ChangeNotifier {
int _count = 0;
int get count => _count;
void increment() {
_count++;
notifyListeners();
}
}
class MyApp extends StatelessWidget {
#override
Widget build(BuildContext context) {
return MultiProvider(
providers: [
ChangeNotifierProvider(builder: (_) => Counter()),
],
child: Consumer<Counter>(
builder: (context, counter, _) {
return MaterialApp(
supportedLocales: const [Locale('en')],
localizationsDelegates: [
DefaultMaterialLocalizations.delegate,
DefaultWidgetsLocalizations.delegate,
_ExampleLocalizationsDelegate(counter.count),
],
home: const MyHomePage(),
);
},
),
);
}
}
class ExampleLocalizations {
static ExampleLocalizations of(BuildContext context) =>
Localizations.of<ExampleLocalizations>(context, ExampleLocalizations);
const ExampleLocalizations(this._count);
final int _count;
String get title => 'Tapped $_count times';
}
class _ExampleLocalizationsDelegate
extends LocalizationsDelegate<ExampleLocalizations> {
const _ExampleLocalizationsDelegate(this.count);
final int count;
#override
bool isSupported(Locale locale) => locale.languageCode == 'en';
#override
Future<ExampleLocalizations> load(Locale locale) =>
SynchronousFuture(ExampleLocalizations(count));
#override
bool shouldReload(_ExampleLocalizationsDelegate old) => old.count != count;
}
class MyHomePage extends StatelessWidget {
const MyHomePage({Key key}) : super(key: key);
#override
Widget build(BuildContext context) {
return Scaffold(
appBar: AppBar(title: const Title()),
body: const Center(child: CounterLabel()),
floatingActionButton: const IncrementCounterButton(),
);
}
}
class IncrementCounterButton extends StatelessWidget {
const IncrementCounterButton({Key key}) : super(key: key);
#override
Widget build(BuildContext context) {
return FloatingActionButton(
onPressed: Provider.of<Counter>(context).increment,
tooltip: 'Increment',
child: const Icon(Icons.add),
);
}
}
class CounterLabel extends StatelessWidget {
const CounterLabel({Key key}) : super(key: key);
#override
Widget build(BuildContext context) {
final counter = Provider.of<Counter>(context);
return Column(
mainAxisSize: MainAxisSize.min,
mainAxisAlignment: MainAxisAlignment.center,
children: <Widget>[
const Text(
'You have pushed the button this many times:',
),
Text(
'${counter.count}',
style: Theme.of(context).textTheme.display1,
),
],
);
}
}
class Title extends StatelessWidget {
const Title({Key key}) : super(key: key);
#override
Widget build(BuildContext context) {
return Text(ExampleLocalizations.of(context).title);
}
}
When the user presses the FloatingRadioButton within IncrementCounterButton, build() is called on CounterLabel and IncrementCounterButton.
They both depend on an inherited widget, which is updated.
How does flutter discover this dependency?
I assume that the BuildContext is modified by the call to Provider.of<>().
Is this why we add the IncrementCounterButton, which has no functionality on its own?
Just to move the call to Provider.of<>() outside of its bigger parent widget, which would be more expensive to rebuild?

Answer 1

The binding widget an InheritedWidget and its consumers is created through BuildContext.
Consider the following InheritedWidget:
class Foo extends InheritedWidget {}
Then the descendants of Foo can subscribe to it by calling:
BuildContext context
context.inheritFromWidgetOfExactType(Foo);
It's worth noting that a widget can obtain the InheritedWidget without subscribing to it, by instead doing:
BuildContext context
context.ancestorInheritedElementForWidgetOfExactType(Foo);
This call is usually performed internally by the .of(context) pattern.
In the case of provider, that subscription is done by calling Provider.of<T>(context).
provider also exposes an optional argument to purposefully not subscribe to the inherited widget:
T value = Provider.of<T>(context, listen: false);