flutter - AppBarDesign 无法分配给参数类型 'PreferredSizeWidget'

Question

任何人请提供一些信息为什么会发生这种情况？

当我尝试添加实现 appBar 的类 AppBarDesign 时， flutter 会出现以下错误。

error: The argument type 'AppBarDesign' can't be assigned to the parameter type 'PreferredSizeWidget'. (argument_type_not_assignable at [flutterbyrajath] lib\main.dart:27)

    import 'package:flutter/material.dart';

    main() {
      runApp(new MyApp());
    }

    class MyApp extends StatelessWidget {
      @override
      Widget build(BuildContext context) {
        return MaterialApp(
          title: 'Rajath\'s design ',
          debugShowCheckedModeBanner: false,
          theme: new ThemeData(primarySwatch: Colors.amber),
          home: new MyHomePage(key, 'App is Fun'),
        );
      }
    }

    class MyHomePage extends StatelessWidget {
      MyHomePage(Key key, this.title) : super(key: key);

      final title;

      @override
      Widget build(BuildContext context) {
        return new Scaffold(
          appBar: new AppBarDesign(key, title),
        );
      }
    }

    class AppBarDesign extends StatelessWidget {
      AppBarDesign(Key key, this.title) : super(key: key);

      final title;

      @override
      Widget build(BuildContext context) {
        return new AppBar(
          title: new Text(title),
        );
      }
    }

Answer 1

在不搜索任何其他主题的情况下实现该功能的有用提示:

class ApplicationToolbar extends StatelessWidget with PreferredSizeWidget{
  @override
  Widget build(BuildContext context) {
    return AppBar( ... );
  }

  @override
  Size get preferredSize => Size.fromHeight(kToolbarHeight);
}