flutter提供者与“上下文上下文"和“上下文"有什么区别 [英] flutter provider what is the difference with 'Context context' and 'context '

问题描述

```
import 'package:a_class_flutter/provider/ac_score/ac_score_provider.dart';
import 'package:flutter/material.dart';
import 'package:flutter_screenutil/flutter_screenutil.dart';
import 'package:provider/provider.dart';

class ACScoreView extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return ChangeNotifierProvider(
      create: (context) => ACScoreProvider(),
      builder: (context, child) {
        return Stack(
          children: <Widget>[
            _topMenu(context),
          ],
        );
      },
    );
  }
}
// Important!
Widget _topMenu(BuildContext context) {
  return GestureDetector(
    onTap: () {
      context.read<ACScoreProvider>().showTopMenueStatus();
    },
    child: Container(
      width: double.infinity,
      height: double.infinity,
      color: Colors.red,
      child: Stack(
        children: <Widget>[
          Positioned(
            child: AnimatedOpacity(
              duration: Duration(microseconds: 100),
              opacity:
                  context.watch<ACScoreProvider>().needShowTopMenue ? 1.0 : 0.0,
              child: Container(
                width: double.infinity,
                height: ScreenUtil().setHeight(60),
                color: Colors.black,
              ),
            ),
          )
        ],
      ),
    ),
  );
}
```

这是我的代码，可以正常工作，但是当我尝试将 Widget _topMenu(BuildContext context)更改为 Widget _topMenu(context)时，Flutter给了我一个错误这是:

This is my code which is working well, but when I try to change Widget _topMenu(BuildContext context) to Widget _topMenu(context), Flutter gives me an error which is:

The following NoSuchMethodError was thrown building Builder(dirty, dependencies: [_InheritedProviderScope<ACScoreProvider>]): Class 'StatelessElement' has no instance method 'watch'. Receiver: Instance of 'StatelessElement'. Tried calling: watch<ACScoreProvider>().The relevant error-causing widget was ChangeNotifierProvider<ACScoreProvider> package:a_class_flutter/…/ac_score/ac_score.dart:9.

为什么缺少 BuildContext 声明可能导致此错误?我需要将每个上下文都放到BuildContext吗?

Why missing BuildContext declaration could cause this error? Do I need to put every context with BuildContext?

推荐答案

来自提供商文档:

读取值的最简单方法是使用[BuildContext]上的扩展方法:

The easiest way to read a value is by using the extension methods on [BuildContext]:

• context.read< T>()，它会返回T，而不会监听

• context.read<T>(), which returns T without listening to it

仅编写 Widget _topMenu(context)，将 context 参数推断为动态上下文，而不是 BuildContext上下文，因此您没有要调用的扩展方法 .read< T>().

Writing just Widget _topMenu(context), the context parameter is inferred as a dynamic context not BuildContext context and hence you it does not have the extension method .read<T>() you are trying to call.

如果它在 StatefulWidget 内部，则不会有问题，因为它的 State 类具有实例

If it were inside a StatefulWidget, this wouldn't have been a problem because it's State class has a instance context (type BuildContext) property and so the extension method .read<T>() can be called on it.

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