我目前正在阅读 provider 的示例代码包裹:
// 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);
}
}
当用户按下 FloatingRadioButton
在 IncrementCounterButton
内, build()
被称为 CounterLabel
和 IncrementCounterButton
.
它们都依赖于一个更新的继承小部件。 Flutter 是如何发现这种依赖的?
我假设 BuildContext
通过调用 Provider.of<>()
进行修改.
这就是我们添加 IncrementCounterButton
的原因吗? ,它本身没有功能?
只是将调用转移到 Provider.of<>()
在其更大的父小部件之外,重建哪个会更昂贵?
最佳答案
绑定(bind)小部件 InheritedWidget 及其使用者是通过 BuildContext
创建的.
考虑以下 InheritedWidget:
class Foo extends InheritedWidget {}
然后是Foo
的后代可以通过调用订阅它:
BuildContext context
context.inheritFromWidgetOfExactType(Foo);
值得注意的是,小部件可以通过以下方式获取 InheritedWidget 而无需订阅它:
BuildContext context
context.ancestorInheritedElementForWidgetOfExactType(Foo);
此调用通常由 .of(context)
在内部执行模式。
在provider
的情况下, 该订阅是通过调用 Provider.of<T>(context)
完成的.
provider
还公开了一个可选参数以有目的地不订阅继承的小部件:
T value = Provider.of<T>(context, listen: false);
关于 flutter :如何发现对继承的小部件的依赖?,我们在Stack Overflow上找到一个类似的问题: https://stackoverflow.com/questions/56667584/