• The argument type ‘Null’ can’t be assigned to the parameter type ‘Key’.

The argument type ‘Null’ can’t be assigned to the parameter type ‘Key’.

import 'dart:math';

import 'package:flutter/material.dart';

class MyApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      title: 'Flutter Demo',
      theme: ThemeData(
        primarySwatch: Colors.blue,
      ),
      home: MyHomePage(title: 'Flutter Demo'),
    );
  }
}

class MyHomePage extends StatefulWidget {
  MyHomePage({required Key? key, required this.title}) : super(key: key);
  final String title;

  @override
  _MyHomePageState createState() => _MyHomePageState();
}

class _MyHomePageState extends State<MyHomePage> {
  int _number = 0;
  void _incrementCounter() {
    _number = Random().nextInt(100);
    setState(() {});
  }

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        title: Text(widget.title),
      ),
      body: Center(
        child: InfoWidget(
          number: _number,
          child: Center(
            child: Column(
              mainAxisAlignment: MainAxisAlignment.center,
              children: [
                InfoChildWidget(),
                InfoChildWidget(),
                InfoChildWidget(),
              ],
            ),
          ),
        ),
      ),
      floatingActionButton: FloatingActionButton(
        onPressed: _incrementCounter,
        tooltip: 'Increment',
        child: Icon(Icons.add),
      ),
    );
  }
}

// <!-- InfoWidget -->
class InfoWidget extends InheritedWidget {
  final int number;

  InfoWidget({required Key key, required this.number, @required child})
      : super(key: key, child: child);

  @override
  bool updateShouldNotify(InfoWidget oldWidget) {
    return number != oldWidget.number;
  }

  static InfoWidget? of(BuildContext context) {
    return context.dependOnInheritedWidgetOfExactType();
  }
}

// <!-- InfoChildWidget -->
class InfoChildWidget extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    final int? number = InfoWidget.of(context)?.number;
    return Text("$number", style: TextStyle(color: Colors.amber, fontSize: 40));
  }
}

第13行 MyHomePage 报错：The named parameter 'key' is required, but there's no corresponding argument.

第40行 InfoWidget 报错：The named parameter 'key' is required, but there's no corresponding argument. （命名参数 key 是必须的，但是没有对应的参数。）

这个错误的出现是因为用了required关键字。如果想传递一个空值作为 key，可以像这样改造构造函数：

MyWidget({Key? key}) : super(key: key);

第 19 行 改成：MyHomePage({Key? key, required this.title}) : super(key: key);；

第 67、68 行 改成：InfoWidget({Key? key, required this.number, @required child}) : super(key: key, child: child);。