无法在此小部件上方找到正确的提供者 [英] Could not find the correct provider above this widget

问题描述

我在使用 Flutter Provider 时遇到问题...我的流程是这样的:登录用户 ID 传递给新小部件后 -> 从那里执行保存到数据库，然后重定向到新小部件(仪表板).

这是登录后小部件的代码:

返回 MaterialApp(标题:标题，主页: 脚手架(应用栏:应用栏(标题:文本(标题)，),正文:列表视图(孩子们:<小部件>[容器(边距:EdgeInsets.all(8.0),孩子:卡片(形状:RoundedRectangleBorder(borderRadius: BorderRadius.all(Radius.circular(8.0))),孩子:InkWell(onTap: () {var user = Provider.of(上下文)；user.savePreference(user.user.id, "Something");user.navigateToNewPage(Dashboard(), context);打印(user.user.id)；},

这有效:

user.savePreference(user.user.id, "Something");

但这会导致一个问题:

user.navigateToNewPage(Dashboard(), context);

在仪表板小部件中，我正在创建:

 小部件构建(BuildContext 上下文){var user = Provider.of(上下文)；

在 UserRepository 中我有这个:

 类 UserRepository 和 ChangeNotifier {用户用户；状态_status = Status.Uninitialized;状态获取状态=>_地位;用户获取 getUser =>用户;UserRepository.instance();未来<void>导航到新页面(小部件页面，BuildContext 上下文){Navigator.push(context, MaterialPageRoute(builder: (context) => page));}

我知道这个话题已经解决了问题，但没有找到适合我的问题.

解决方案

Provider Scope

MaterialApp>提供者(屏幕 A)>屏幕 B

如果 Provider 在屏幕 A 中实例化，则在从 A → B 执行 Navigator.push 后，它将无法在屏幕 B 中访问.

为什么?

因为 Provider 是一个 InheritedWidget 并且 Navigator 在它的 Screen A context<之外使用 MaterialApp context/代码> 范围.(请参阅下面的详细信息.)

修复

将 Provider 移动到一个共同的父级，MaterialApp 上下文，允许屏幕 A 和 B 继承其状态/上下文.

provider(MaterialApp)>屏幕A>屏幕 B

示例

void main() {运行应用程序(我的应用程序())；}class MyApp 扩展 StatelessWidget {@覆盖小部件构建(BuildContext 上下文){///将 MaterialApp 包装在 Provider 小部件中返回 ChangeNotifierProvider(创建:(上下文)=>ColorModel(),//← 创建/初始化你的状态模型孩子:材料应用程序(主页:屏幕A()),);}}

详情

供应商

• Provider 基于 InheritedWidget.只有子小部件可以继承父小部件的状态.

  • Provider 需要是任何想要访问提供"的小部件树的根小部件.状态对象.

导航器

• Navigator.push(context) 屏幕 A 不使用屏幕 A 的 context.
  • 它使用来自 MaterialApp 的 context.
• Navigator.push(context) 其实就是Navigator.of(context).push
• Navigator.of(context) 意味着:搜索这个上下文层次结构，直到找到一个实例化导航器的上下文
  • 默认的 Navigator 在 MaterialApp 中实例化.
  • 除非您明确创建/使用不同的Navigator，否则您使用的是默认值.
  • Navigator 的context 就是MaterialApp 的context.
• 屏幕 B 将获得那个上下文(MaterialApp)，不是屏幕 A 的上下文.
  • B 是 A 的兄弟姐妹，而不是它的孩子.
  • B 不从 A 继承，即使它看起来是实例化的"context 内 A.
  • 屏幕 B 是MaterialApp context 的子，而不是屏幕 A context 的子.
  • Provider context 范围，如果在屏幕 A 中定义，则不覆盖屏幕 B

屏幕 A → B

Navigator.push(context, MaterialPageRoute(builder: (context) => ScreenB()))

实际上是:

Navigator.of(context).push(MaterialPageRoute(builder: (context) => ScreenB()))

就像:

Navigator.of(MaterialApp).push(MaterialPageRoute(builder: (MaterialAppContext) => ScreenB()))

所以屏幕 B 在 MaterialApp context 下，不是在屏幕 A context 下，因此无法访问屏幕 A Provider 和它的 context.

有关Provider 的类似问题，请参阅此代码示例答案.>

I have a problem using Flutter Provider... My flow is like this: After login user id is passed to new widget -> from there it preforms save to db and then it redirects to new widget (Dashboard).

And this is a code of a widget after Login:

return MaterialApp(
      title: title,
      home: Scaffold(
          appBar: AppBar(
            title: Text(title),
          ),
          body: ListView(
            children: <Widget>[
              Container(
                margin: EdgeInsets.all(8.0),
                child: Card(
                  shape: RoundedRectangleBorder(
                      borderRadius: BorderRadius.all(Radius.circular(8.0))),
                  child: InkWell(
                    onTap: () {
                      var user = Provider.of<UserRepository>(context);
                      user.savePreference(user.user.id, "Something");
                      user.navigateToNewPage(Dashboard(), context);
                      print(user.user.id);
                    },

This works:

user.savePreference(user.user.id, "Something");

But this is causing a problem:

user.navigateToNewPage(Dashboard(), context);

In Dashboard widget I am creating this:

 Widget build(BuildContext context) {
    var user = Provider.of<UserRepository>(context);

And in UserRepository I have this:

class UserRepository with ChangeNotifier {
  User user;
  Status _status = Status.Uninitialized;

  Status get status => _status;
  User get getUser => user;

  UserRepository.instance();

Future<void> navigateToNewPage(Widget page, BuildContext context) {
    Navigator.push(context, MaterialPageRoute(builder: (context) => page));
  }

I know this topic has already solved questions, but could not find anything suitable to my problem.

解决方案

Provider Scope

MaterialApp
 > provider(Screen A)
 > Screen B

If Provider is instantiated in Screen A, it won't be accessible in Screen B after a Navigator.push from A → B.

Why?

Because Provider is an InheritedWidget and Navigator uses MaterialApp context outside its Screen A context scope. (See Details below.)

Fix

Moving Provider up to a common-parent, MaterialApp context, allows both Screen A and B to inherit its state/context.

provider(MaterialApp)
 > Screen A
 > Screen B

Example

void main() {
  runApp(MyApp());
}

class MyApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    /// wrap MaterialApp in Provider widget
    return ChangeNotifierProvider(
      create: (context) => ColorModel(), // ← create/init your state model
      child: MaterialApp(
          home: ScreenA()
      ),
    );
  }
}

Details

Provider

• Provider is based on InheritedWidget. Only child widgets can inherit parent widget's state.

  • Provider needs to be the root widget for any widget tree that wants access to your "provided" state object.

Navigator

• Navigator.push(context) on Screen A doesn't use the context from Screen A.
  • It uses context from MaterialApp.
• Navigator.push(context) is actually Navigator.of(context).push
• Navigator.of(context) means: search up this context hierarchy until you find a context that instantiated a Navigator
  • A default Navigator is instantiated in MaterialApp.
  • Unless you explicitly create/use a different Navigator, you're using the default.
  • Navigator's context is that of MaterialApp.
• Screen B will get that context (of MaterialApp), not the context of Screen A.
  • B is a sibling of A, not its child.
  • B does not inherit from A, even though it appears "instantiated" inside context A.
  • Screen B is a child of MaterialApp context, not of Screen A context.
  • Provider context scope, if defined in Screen A, doesn't cover Screen B

Screen A → B

Navigator.push(context, MaterialPageRoute(builder: (context) => ScreenB()))

is actually:

Navigator.of(context).push(MaterialPageRoute(builder: (context) => ScreenB()))

which is like:

Navigator.of(MaterialApp).push(
  MaterialPageRoute(builder: (MaterialAppContext) => ScreenB())
)

So Screen B is under MaterialApp context, not under Screen A context and therefore has no access to Screen A Provider and its context.

See this answer for a code sample to a similar question about Provider.

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