flutter学习之NULL问题解决

flutter学习之NULL问题解决

在flutter实战的第二章计数器实例学习和应用的过程中遇到两个null问题，这是直接复用原来的代码产生的，应该是后续的flutter版本升级对相关调用类构造方法添加了空判断导致的。

计数器实例代码

import package:flutter/material.dart;

class Study extends StatelessWidget {
          
   
  @override
  Widget build(BuildContext context) {
          
   
    return new MaterialApp(
      title: "The First Flutter App",
      theme: new ThemeData(
        primaryColor: Colors.red,
      ),
      home: new MyHomePage(title: "This is MyHomePage"),
    );
  }
}

class MyHomePage extends StatefulWidget {
          
   
  //此处报错
  //此处报错 
  //此处报错
  MyHomePage({
          
   Key key, this.title}) : super(key: key);

  final String title;

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

class _MyHomePageState extends State<MyHomePage> {
          
   
  int _counter = 0;

  @override
  Widget build(BuildContext context) {
          
   
    return new Scaffold(
      appBar: new AppBar(
        title: new Text(widget.title),
      ),
      body: new Center(
        child: new Column(
          mainAxisAlignment: MainAxisAlignment.center,
          children: <Widget>[
            new Text("please click the button"),
            new Text(
              $_counter,
              style: Theme.of(context).textTheme.headline4,
            ),
          ],
        ),
      ),
      floatingActionButton: new FloatingActionButton(
        onPressed: _incrementCounter,
        tooltip: "增加",
        child: new Icon(Icons.add),
      ),
    );
  }

  void _incrementCounter() {
          
   
    setState(() {
          
   
      _counter++;
    });
  }
}

报错内容

Error: The parameter key cant have a value of null because of its type Key, 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);
                  ^^^
Error: The parameter title cant 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);

报错修改

  我们可以用官方推荐的修改方式修改即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);

运行效果