Solved: The AppBarDesign can't be assigned to the parameter type 'PreferredSizeWidget'

Question

Asked by Rajath on October 06, 2018 (source).

Anyone please give some information why this is happening?

When I try to add a class AppBarDesign which implements appBar flutter is giving the below error.

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

Question answered by Rémi R (source).

Scaffold requires as appbar a class that implements PreferredSizeWidget

Either wrap your custom appbar into a PreferredSize

Scaffold(
  appBar: PreferredSize(
    preferredSize: const Size.fromHeight(100),
    child: Container(color: Colors.red),
  ),
)

or implement PreferredSizeWidget:

Scaffold(
  appBar: MyAppBar(),
)

...

class MyAppBar extends StatelessWidget implements PreferredSizeWidget {
  @override
  Size get preferredSize => const Size.fromHeight(100);

  @override
  Widget build(BuildContext context) {
    return Container(color: Colors.red);
  }
}
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