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

Question

Anyone please give some information why this is happening?

When I try to add a class AppBarDesign which implements appBar flutter is gi

Answer 1

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);
  }
}