A simple base ui class in flutter and how to implement it.

base_screen.dart

import 'package:flutter/material.dart';

abstract class BaseScreen extends StatefulWidget {
  const BaseScreen({Key? key}) : super(key: key);
}

base_screen_state_mixin.dart

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

abstract class BaseScreenState<Screen extends BaseScreen>
    extends State<Screen> {
  String appBarTitle();

  bool _isBack = true;

  void isBackButtonShown(bool isBack) {
    _isBack = isBack;
  }
}

mixin BaseScreenMixin<Page extends BaseScreen> on BaseScreenState<Page> {
  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        title: Text(
          appBarTitle(),
          style: const TextStyle(
              color: Colors.black, fontSize: 20, fontWeight: FontWeight.bold),
        ),
        leading: _isBack
            ? IconButton(
                icon: const Icon(
                  Icons.arrow_back_ios,
                  color: Colors.black,
                ),
                onPressed: () {
                  onClickBackButton();
                },
              )
            : const SizedBox(),
      ),
      body: Container(
        height: MediaQuery.of(context).size.height,
        width: MediaQuery.of(context).size.width,
        child: body(),
        color: Colors.white,
      ),
    );
  }

  Widget body();
}

home_screen.dart

import 'package:flutter/src/widgets/framework.dart';
import '../base_screen.dart';
import '../base_screen_state.dart';

class HomeScreen extends BaseScreen {
  const HomeScreen({Key? key}) : super(key: key);

  @override
  _HomeScreenState createState() => _HomeScreenState();
}

class _HomeScreenState extends BaseScreenState<HomeScreen>
    with BaseScreenMixin {
  @override
  void initState() {
    isBackButtonShown(false);
    super.initState();
  }

  @override
  String appBarTitle() {
    return "Home";
  }

  @override
  void isBackButtonShown(bool isBack) {
    super.isBackButtonShown(isBack);
  }

  @override
  Widget body() {
    // TODO: implement body
    throw UnimplementedError();
  }
}