flutter_design_patterns/lib/main.dart

import 'package:flutter/material.dart';

void main() {
  runApp(const MyApp());
}

class MyApp extends StatelessWidget {
  const MyApp({super.key});

  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      title: 'Flutter Demo',
      theme: ThemeData(
        colorScheme: ColorScheme.fromSeed(seedColor: Colors.deepPurple),
        useMaterial3: true,
      ),
      home: const MyHomePage(),
    );
  }
}

///
/// Shape Builder
///
abstract class ShapeBuilder {
  Widget build(Size size, Widget child) {
    return Container(
      height: size.height,
      width: size.width,
      decoration: BoxDecoration(shape: boxShape, color: shapeColor),
      child: Center(child: child),
    );
  }

  BoxShape get boxShape;
  Color get shapeColor;
}

class RedSquareBuilder extends ShapeBuilder {
  @override
  BoxShape get boxShape => BoxShape.rectangle;

  @override
  Color get shapeColor => Colors.red;
}

class BlueCircleBuilder extends ShapeBuilder {
  @override
  BoxShape get boxShape => BoxShape.circle;

  @override
  Color get shapeColor => Colors.blue;
}

///
/// Message Builder (Writer seemed appropriate)
///
abstract class MessageWriter {
  Text write(String text);
}

class HotMessageWriter extends MessageWriter {
  @override
  Text write(String text) => Text(text, style: const TextStyle(fontWeight: FontWeight.bold, fontSize: 18));
}

class ColdMessageWriter extends MessageWriter {
  @override
  Text write(String text) => Text(text, style: const TextStyle(color: Colors.white, fontSize: 14));
}

final List<ShapeBuilder> builders = [
  RedSquareBuilder(),
  BlueCircleBuilder(),
  RedSquareBuilder(),
];

final List<MessageWriter> writers = [
  ColdMessageWriter(),
  HotMessageWriter(),
  HotMessageWriter(),
];

class MyHomePage extends StatefulWidget {
  const MyHomePage({super.key});

  @override
  State<MyHomePage> createState() => _MyHomePageState();
}

class _MyHomePageState extends State<MyHomePage> {
  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        backgroundColor: Theme.of(context).colorScheme.inversePrimary,
        title: const Text('Builders'),
      ),
      body: Center(
        child: Column(
          mainAxisAlignment: MainAxisAlignment.center,
          children: <Widget>[
            const Text('Builders!!'),
            ...List.generate(
              3,
              (index) => builders[index].build(
                Size(80.0 * (index + 1), 90.0 * (index + 1)),
                writers[index].write('build ${index + 1}'),
              ),
            ),
          ],
        ),
      ),
    );
  }
}