import 'package:flutter/material.dart';
void main() => runApp(MyApp());
class MyApp extends StatelessWidget {
@override
Widget build(BuildContext context) {
return MaterialApp(
home: CounterPage(),
);
}
}
class Counter with ChangeNotifier {
int _count = 0;
int get count => _count;
void increment() {
_count++;
notifyListeners();
}
}
class CounterPage extends StatefulWidget {
@override
_CounterPageState createState() => _CounterPageState();
}
class _CounterPageState extends State<CounterPage> {
Counter _counter = Counter();
@override
Widget build(BuildContext context) {
return Scaffold(
body: Column(
mainAxisAlignment: MainAxisAlignment.center,
children: <Widget>[
Text('You have pushed the button this many times:'),
CounterText(),
RaisedButton(
child: Text('Increment'),
onPressed: _counter.increment,
),
],
),
);
}
@override
void dispose() {
_counter.dispose();
super.dispose();
}
}
class CounterText extends StatelessWidget {
@override
Widget build(BuildContext context) {
return Text(
'${context.dependOnInheritedWidgetOfExactType<_CounterModel>().counter.count}',
style: Theme.of(context).textTheme.display1,
);
}
}
class _CounterModel extends InheritedWidget {
final Counter counter;
_CounterModel({Key key, @required this.counter, Widget child})
: super(key: key, child: child);
@override
bool updateShouldNotify(_CounterModel old) =>
counter.count != old.counter.count;
}这个代码示例展示了如何使用InheritedWidget来管理状态，并且在状态发生变化时通知依赖于它的widgets。这种方式避免了使用全局变量，并且保持了widget树的高效更新。在这个例子中，我们创建了一个名为Counter的ChangeNotifier类，它包含了一个计数器和一个用于增加计数器值的方法。CounterPage是一个StatefulWidget，它在状态中创建了Counter的实例。在build方法中，它包含了CounterText widget，后者通过InheritedWidget的方式来访问计数器的值。当计数器值发生变化时，Counter通过调用notifyListeners来通知依赖于它的widgets更新。这样，我们避免了使用任何全局状态，使应用程序更容易理解和维护。