关于无状态组件的记录

自定义组件的情况下， 如果组件是继承 StatelessWidget ，那么该组件内就不应该有可变的属性，例如：

import 'package:flutter/material.dart';
import 'data/newList.dart';
void main() => runApp(MyApp());
class MyApp extends StatelessWidget {
    @override
    Widget build(BuildContext context) {
        return MaterialApp(
            home: Scaffold(
                appBar: AppBar(
                    title: Text('Welcome to Flutter'),
                ),
                body: HomeContent(),
            ),
            theme: ThemeData(
                primaryColor: Colors.pink,
            ),
        );
    }
}
class HomeContent extends StatelessWidget {
    @override
    Widget build(BuildContext context) {
        return Row(
            children: [
                IconWidget(Icons.home_filled, color: Colors.pink, size: 30),
                IconWidget(Icons.search_off, color: Colors.blue, size: 30),
                IconWidget(Icons.access_alarm, color: Colors.redAccent, size: 30),
            ],
        );
    }
}
class IconWidget extends StatelessWidget {
    IconWidget(this.icon, {this.color = Colors.white70, this.size = 32});
    // 下面属性不应该在这里初始化， 而是在构造函数内设置默认的值，并且要 使用 final 关键字防止它改变， 否则会报警告
    final double size;
    final IconData icon;
    final Color color;
    /*
        // 这种写法会报警告： This class (or a class that this class inherits from) is marked as '@immutable', but one or more of its instance fields aren't final
        double size  = 32;
        IconData icon;
        Color color = Colors.white70 ;
        IconWidget(this.icon, {this.color , this.size});
    */
    @override
    Widget build(BuildContext context) {
        return Container(
            color: this.color,
            width: 100,
            height: 100,
            child: Center(
                child: Icon(this.icon, size: this.size, color: Colors.black),
            ),
        );
    }
}