我正在尝试学习如何使用提供程序包，版本 4.0+。我有颤振团队提供的复制粘贴解决方案并在我的模拟器上运行它。问题是，当我第一次运行它（冷启动）时，我收到以下错误消息：
当我刷新应用程序时（意思是，当我单击 VS Code 中重新构建应用程序的刷新箭头时），它突然开始工作。这意味着，该问题仅发生在第一次（冷）启动期间。
我真的很想学习如何使用这个包；但是由于未知原因，它会引发此错误。正如我阅读其他文章一样，人们试图在构建方法期间修改实现 ChangeNotifier 的类的值；但是在这种情况下，情况不应如此。
对此问题的任何帮助将不胜感激。
PS这里是我正在使用的实际源代码：
// Copyright 2019 The Chromium Authors. All rights reserved.
// Use of this source code is governed by a BSD-style license that can be
// found in the LICENSE file.
import 'package:flutter/material.dart';
import 'package:provider/provider.dart';
void main() {
runApp(
// Provide the model to all widgets within the app. We're using
// ChangeNotifierProvider because that's a simple way to rebuild
// widgets when a model changes. We could also just use
// Provider, but then we would have to listen to Counter ourselves.
//
// Read Provider's docs to learn about all the available providers.
ChangeNotifierProvider(
// Initialize the model in the builder. That way, Provider
// can own Counter's lifecycle, making sure to call `dispose`
// when not needed anymore.
create: (context) => Counter(),
child: MyApp(),
),
);
}
/// Simplest possible model, with just one field.
///
/// [ChangeNotifier] is a class in `flutter:foundation`. [Counter] does
/// _not_ depend on Provider.
class Counter with ChangeNotifier {
int value = 0;
void increment() {
value += 1;
notifyListeners();
}
}
class MyApp extends StatelessWidget {
@override
Widget build(BuildContext context) {
return MaterialApp(
title: 'Flutter Demo',
theme: ThemeData(
primarySwatch: Colors.blue,
),
home: MyHomePage(),
);
}
}
class MyHomePage extends StatelessWidget {
@override
Widget build(BuildContext context) {
return Scaffold(
appBar: AppBar(
title: Text('Flutter Demo Home Page'),
),
body: Center(
child: Column(
mainAxisAlignment: MainAxisAlignment.center,
children: <Widget>[
Text('You have pushed the button this many times:'),
// Consumer looks for an ancestor Provider widget
// and retrieves its model (Counter, in this case).
// Then it uses that model to build widgets, and will trigger
// rebuilds if the model is updated.
Consumer<Counter>(
builder: (context, counter, child) => Text(
'${counter.value}',
style: Theme.of(context).textTheme.display1,
),
),
],
),
),
floatingActionButton: FloatingActionButton(
// Provider.of is another way to access the model object held
// by an ancestor Provider. By default, even this listens to
// changes in the model, and rebuilds the whole encompassing widget
// when notified.
//
// By using `listen: false` below, we are disabling that
// behavior. We are only calling a function here, and so we don't care
// about the current value. Without `listen: false`, we'd be rebuilding
// the whole MyHomePage whenever Counter notifies listeners.
onPressed: () =>
Provider.of<Counter>(context, listen: false).increment(),
tooltip: 'Increment',
child: Icon(Icons.add),
),
);
}
}