颤振提供者与“上下文上下文”和“上下文”有什么区别

Question

```
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,
              ),
            ),
          )
        ],
      ),
    ),
  );
}
```

这是我运行良好的代码，但是当我尝试将 Widget _topMenu(BuildContext context) 更改为 Widget _topMenu(context) 时，Flutter 给我一个错误：

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 声明会导致此错误？我需要将每个上下文都放在 BuildContext 中吗？

Answer 1

来自提供商docs：

读取值的最简单方法是使用 [BuildContext] 上的扩展方法：

• context.read<T>()，不听就返回 T

只写Widget _topMenu(context)，context 参数被推断为dynamic context 而不是BuildContext context，因此它没有您尝试调用的扩展方法.read<T>()。

如果它在StatefulWidget 中，这不会有问题，因为它的State 类有一个实例context（类型BuildContext）属性，因此可以调用扩展方法.read<T>()就可以了。