在 Flutter 中需要提供程序的帮助

问题描述

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。

当我尝试运行此代码时，它给了我一个错误

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。

provider provider