The parameter 'title' can't have

官方demo如下

import 'package:flutter/material.dart';

void main() => runApp(new MyApp());

class MyApp extends StatelessWidget {

  @override

  Widget build(BuildContext context) {

    final title = 'InkWell Demo';

    return new MaterialApp(

      title: title,

      home: new MyHomePage(title: title),

    );

  }

}

class MyHomePage extends StatelessWidget {

  final String title;

  MyHomePage({Key key, this.title}) : super(key: key);

  @override

  Widget build(BuildContext context) {

    return new Scaffold(

      appBar: new AppBar(

        title: new Text(title),

      ),

      body: new Center(child: new MyButton()),

    );

  }

}

class MyButton extends StatelessWidget {

  @override

  Widget build(BuildContext context) {

    // The InkWell Wraps our custom flat button Widget

    return new InkWell(

      // When the user taps the button, show a snackbar

      onTap: () {

        Scaffold.of(context).showSnackBar(new SnackBar(

          content: new Text('Tap'),

        ));

      },

      child: new Container(

        padding: new EdgeInsets.all(12.0),

        child: new Text('Flat Button'),

      ),

    );

  }

}

运行报错

Xcode's output:

↳

    lib/main.dart:20:19: Error: The parameter 'key' can't have a value of 'null' because of its type 'Key', but the implicit default value is 'null'.

    - 'Key' is from 'package:flutter/src/foundation/key.dart' ('../../flutter/packages/flutter/lib/src/foundation/key.dart').

    Try adding either an explicit non-'null' default value or the 'required' modifier.

      MyHomePage({Key key, this.title}) : super(key: key);

                      ^^^

    lib/main.dart:20:29: Error: The parameter 'title' can't have a value of 'null' because of its type 'String', but the implicit default value is 'null'.

    Try adding either an explicit non-'null' default value or the 'required' modifier.

      MyHomePage({Key key, this.title}) : super(key: key);

                                ^^^^^

    Failed to package /Users/wangweiwei/Downloads/readme.

    Command PhaseScriptExecution failed with a nonzero exit code

    note: Using new build system

    note: Planning

    note: Build preparation complete

    note: Building targets in parallel

原因：

demo的flutter版本过老，新的flutter版本升级对相关调用类构造方法添加了空判断导致

解决：

根据报错提示进行调整。可以用Key ？key来表示可空，又因为title是final修饰量，final修饰的常量必须在声明进初始化或者在构造函数中初始化，它的值可以动态计算。 所以可以添加required修饰要求必须作为参数填入。

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