如何解决在 Flutter 中需要提供程序的帮助
我正在尝试了解 Flutters 中的 Providers ......但是在学习了一些教程之后,我仍然面临一些问题。
错误:在此 MyHomePage 小部件上方找不到正确的提供程序
import 'package:Flutter/material.dart';
import 'package:provider/provider.dart';
import 'package:provider_way/MyHomePageviewmodel.dart';
void main() {
runApp(MyApp());
}
class MyApp extends StatelessWidget {
@override
Widget build(BuildContext context) {
return MaterialApp(
title: 'Flutter Demo',theme: ThemeData(
primarySwatch: Colors.blue,),home: MyHomePage(title: 'Flutter Demo Home Page'),);
}
}
class MyHomePage extends StatefulWidget {
MyHomePage({Key key,this.title}) : super(key: key);
final String title;
@override
_MyHomePageState createState() => _MyHomePageState();
}
class _MyHomePageState extends State<MyHomePage> {
@override
Widget build(BuildContext context) {
return ChangeNotifierProvider(
create: (context) => MyHomePageviewmodel(),child: Scaffold(
appBar: AppBar(
title: Text(widget.title),body: Center(
child: Column(
mainAxisAlignment: MainAxisAlignment.center,children: <Widget>[
Consumer<MyHomePageviewmodel>(
builder: (context,viewmodel,child) {
return Text(viewmodel.text);
},],floatingActionButton: FloatingActionButton(
onpressed: () =>
Provider.of<MyHomePageviewmodel>(context,listen: false)
.onClicked(),tooltip: 'Increment',child: Icon(Icons.add),// This trailing comma makes auto-formatting nicer for build methods.
),);
}
}
import 'package:Flutter/foundation.dart';
class MyHomePageviewmodel extends ChangeNotifier {
String text = 'Initial text';
void onClicked() {
text = 'Something was clicked';
notifyListeners();
}
}
我发现此示例的网站将其用作
Provider.of<Mainviewmodel>(context,listen: false).onClicked(),
但这也行不通...
解决方法
在呈现需要提供程序的小部件之前,您需要在构建页面之前创建该特定提供程序。
在下面查看代码的工作示例。
import 'package:flutter/material.dart';
import 'package:provider/provider.dart';
import 'package:provider_way/MyHomePageViewModel.dart';
void main() {
runApp(MyApp());
}
class MyApp extends StatelessWidget {
@override
Widget build(BuildContext context) {
return ChangeNotifierProvider(
create: (_) => MyHomePageViewModel(),builder: (_,__) => MaterialApp(
title: 'Flutter Demo',theme: ThemeData(
primarySwatch: Colors.blue,),home: MyHomePage(title: 'Flutter Demo Home Page'),);
}
}
class MyHomePage extends StatefulWidget {
MyHomePage({Key key,this.title}) : super(key: key);
final String title;
@override
_MyHomePageState createState() => _MyHomePageState();
}
class _MyHomePageState extends State<MyHomePage> {
@override
Widget build(BuildContext context) {
return Scaffold(
appBar: AppBar(
title: Text(widget.title),body: Center(
child: Column(
mainAxisAlignment: MainAxisAlignment.center,children: <Widget>[
Consumer<MyHomePageViewModel>(
builder: (context,viewModel,child) {
return Text(viewModel.text);
},],floatingActionButton: FloatingActionButton(
onPressed: () =>
Provider.of<MyHomePageViewModel>(context,listen: false)
.onClicked(),tooltip: 'Increment',child: Icon(Icons.add),// This trailing comma makes auto-formatting nicer for build methods.
);
}
}
import 'package:flutter/foundation.dart';
class MyHomePageViewModel extends ChangeNotifier {
String text = 'Initial text';
void onClicked() {
text = 'Something was clicked';
notifyListeners();
}
}
,
在您的提供程序代码中,您是否定义了如下类:
class MyHomePageViewModel extends ChangeNotifier {
// your stuff here,like getter and setters,methods,etc
}
该类将处理您的状态的所有中心化,基本上按照您的希望行事 - 提供者。
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