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(),
    );
  }
}

abstract class ShapeFactory {
  Widget build(Size size, Color color);
}

class SquareFactory extends ShapeFactory {
  @override
  Widget build(Size size, Color color) {
    return SizedBox.fromSize(
      size: size,
      child: Container(color: color),
    );
  }
}

class CircleFactory extends ShapeFactory {
  @override
  Widget build(Size size, Color color) {
    return SizedBox.fromSize(
      size: size,
      child: Container(
        decoration: BoxDecoration(shape: BoxShape.circle, color: color),
      ),
    );
  }
}

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

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

class _MyHomePageState extends State<MyHomePage> {
  final List<ShapeFactory> shapeFactories = [CircleFactory(), SquareFactory()];
  int factoryIndex = 0;
  ShapeFactory get factory => shapeFactories[factoryIndex];

  void swapFactories() {
    if (factoryIndex == 0) {
      setState(() => factoryIndex = 1);
      return;
    }
    setState(() => factoryIndex = 0);
  }

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        backgroundColor: Theme.of(context).colorScheme.inversePrimary,
        title: const Text('Abstract Factory'),
      ),
      body: Center(
        child: Column(
          mainAxisAlignment: MainAxisAlignment.center,
          children: <Widget>[
            const Text('Abstract Factories!!'),
            Text('Using ${shapeFactories[factoryIndex].runtimeType.toString()}'),
            ...List.generate(
                3,
                (index) => factory.build(
                      Size(20.0 * (index + 1), 30.0 * (index + 1)),
                      [Colors.red, Colors.green, Colors.blue][index],
                    )),
            ElevatedButton(
              onPressed: () => swapFactories(),
              child: const Text('Swap Factories'),
            ),
          ],
        ),
      ),
    );
  }
}