Snippet showing a custom app bar

main.dart

import 'package:flutter/material.dart';
import 'package:flutter/services.dart';

void main() {
  runApp(
    MaterialApp(
      debugShowCheckedModeBanner: false,
      home: CustomAppBarDemo(),
    ),
  );
}

class CustomAppBarDemo extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        backgroundColor: Colors.yellow.shade300,
        leading: Icon(Icons.arrow_back),
        title: 'Geography',
        description:
            'Lorem ipsum dolor sit amet, consectetur \ndipiscing elit donec consectetur semper',
      ),
      body: Center(
        child: Text(
          'Body Here',
          style: Theme.of(context).textTheme.headline6,
        ),
      ),
    );
  }
}

class AppBar extends StatelessWidget implements PreferredSizeWidget {
  final Widget leading;
  final String title;
  final String description;

  /// Default [Color] is [White]
  final Color backgroundColor;

  @override
  final Size preferredSize;

  const AppBar({
    Key key,
    @required this.leading,
    @required this.title,
    @required this.description,
    this.backgroundColor = Colors.white,
  })  : preferredSize = const Size.fromHeight(200.0),
        super(key: key);

  @override
  Widget build(BuildContext context) {
    return AnnotatedRegion(
      value: SystemUiOverlayStyle(
        statusBarColor: Colors.transparent,
        statusBarIconBrightness: Brightness.dark,
      ),
      child: Container(
        color: backgroundColor,
        height: preferredSize.height,
        padding: EdgeInsets.only(
          right: 16.0,
          left: 16.0,
          top: MediaQuery.of(context).padding.top + 16.0,
        ),
        child: Column(
          crossAxisAlignment: CrossAxisAlignment.start,
          children: <Widget>[
            leading,
            SizedBox(height: 16),
            Text(
              title,
              style: TextStyle(
                fontSize: 24,
                fontWeight: FontWeight.bold,
              ),
            ),
            SizedBox(height: 16),
            Text(
              description,
              style: TextStyle(height: 1.5),
            ),
          ],
        ),
      ),
    );
  }
}